heapam.c

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/*-------------------------------------------------------------------------
 *
 * heapam.c
 *    heap access method code
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/heap/heapam.c
 *
 *
 * INTERFACE ROUTINES
 *      relation_open   - open any relation by relation OID
 *      relation_openrv - open any relation specified by a RangeVar
 *      relation_close  - close any relation
 *      heap_open       - open a heap relation by relation OID
 *      heap_openrv     - open a heap relation specified by a RangeVar
 *      heap_close      - (now just a macro for relation_close)
 *      heap_beginscan  - begin relation scan
 *      heap_rescan     - restart a relation scan
 *      heap_endscan    - end relation scan
 *      heap_getnext    - retrieve next tuple in scan
 *      heap_fetch      - retrieve tuple with given tid
 *      heap_insert     - insert tuple into a relation
 *      heap_multi_insert - insert multiple tuples into a relation
 *      heap_delete     - delete a tuple from a relation
 *      heap_update     - replace a tuple in a relation with another tuple
 *      heap_markpos    - mark scan position
 *      heap_restrpos   - restore position to marked location
 *      heap_sync       - sync heap, for when no WAL has been written
 *
 * NOTES
 *    This file contains the heap_ routines which implement
 *    the POSTGRES heap access method used for all POSTGRES
 *    relations.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/heapam.h"
#include "access/heapam_xlog.h"
#include "access/hio.h"
#include "access/multixact.h"
#include "access/relscan.h"
#include "access/sysattr.h"
#include "access/transam.h"
#include "access/tuptoaster.h"
#include "access/valid.h"
#include "access/visibilitymap.h"
#include "access/xact.h"
#include "access/xlogutils.h"
#include "catalog/catalog.h"
#include "catalog/namespace.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/lmgr.h"
#include "storage/predicate.h"
#include "storage/procarray.h"
#include "storage/smgr.h"
#include "storage/standby.h"
#include "utils/datum.h"
#include "utils/inval.h"
#include "utils/lsyscache.h"
#include "utils/relcache.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"
#include "utils/tqual.h"


/* GUC variable */
bool        synchronize_seqscans = true;


static HeapScanDesc heap_beginscan_internal(Relation relation,
                        Snapshot snapshot,
                        int nkeys, ScanKey key,
                        bool allow_strat, bool allow_sync,
                        bool is_bitmapscan);
static HeapTuple heap_prepare_insert(Relation relation, HeapTuple tup,
                    TransactionId xid, CommandId cid, int options);
static XLogRecPtr log_heap_update(Relation reln, Buffer oldbuf,
                Buffer newbuf, HeapTuple oldtup,
                HeapTuple newtup, bool all_visible_cleared,
                bool new_all_visible_cleared);
static void HeapSatisfiesHOTandKeyUpdate(Relation relation,
                             Bitmapset *hot_attrs, Bitmapset *key_attrs,
                             bool *satisfies_hot, bool *satisfies_key,
                             HeapTuple oldtup, HeapTuple newtup);
static void compute_new_xmax_infomask(TransactionId xmax, uint16 old_infomask,
                          uint16 old_infomask2, TransactionId add_to_xmax,
                          LockTupleMode mode, bool is_update,
                          TransactionId *result_xmax, uint16 *result_infomask,
                          uint16 *result_infomask2);
static HTSU_Result heap_lock_updated_tuple(Relation rel, HeapTuple tuple,
                        ItemPointer ctid, TransactionId xid,
                        LockTupleMode mode);
static void GetMultiXactIdHintBits(MultiXactId multi, uint16 *new_infomask,
                       uint16 *new_infomask2);
static TransactionId MultiXactIdGetUpdateXid(TransactionId xmax,
                        uint16 t_infomask);
static void MultiXactIdWait(MultiXactId multi, MultiXactStatus status,
                int *remaining, uint16 infomask);
static bool ConditionalMultiXactIdWait(MultiXactId multi,
                           MultiXactStatus status, int *remaining,
                           uint16 infomask);


/*
 * Each tuple lock mode has a corresponding heavyweight lock, and one or two
 * corresponding MultiXactStatuses (one to merely lock tuples, another one to
 * update them).  This table (and the macros below) helps us determine the
 * heavyweight lock mode and MultiXactStatus values to use for any particular
 * tuple lock strength.
 */
static const struct
{
    LOCKMODE    hwlock;
    MultiXactStatus lockstatus;
    MultiXactStatus updstatus;
}
tupleLockExtraInfo[MaxLockTupleMode + 1] =
{
    {   /* LockTupleKeyShare */
        AccessShareLock,
        MultiXactStatusForKeyShare,
        -1  /* KeyShare does not allow updating tuples */
    },
    {   /* LockTupleShare */
        RowShareLock,
        MultiXactStatusForShare,
        -1  /* Share does not allow updating tuples */
    },
    {   /* LockTupleNoKeyExclusive */
        ExclusiveLock,
        MultiXactStatusForNoKeyUpdate,
        MultiXactStatusNoKeyUpdate
    },
    {   /* LockTupleExclusive */
        AccessExclusiveLock,
        MultiXactStatusForUpdate,
        MultiXactStatusUpdate
    }
};
/* Get the LOCKMODE for a given MultiXactStatus */
#define LOCKMODE_from_mxstatus(status) \
            (tupleLockExtraInfo[TUPLOCK_from_mxstatus((status))].hwlock)

/*
 * Acquire heavyweight locks on tuples, using a LockTupleMode strength value.
 * This is more readable than having every caller translate it to lock.h's
 * LOCKMODE.
 */
#define LockTupleTuplock(rel, tup, mode) \
    LockTuple((rel), (tup), tupleLockExtraInfo[mode].hwlock)
#define UnlockTupleTuplock(rel, tup, mode) \
    UnlockTuple((rel), (tup), tupleLockExtraInfo[mode].hwlock)
#define ConditionalLockTupleTuplock(rel, tup, mode) \
    ConditionalLockTuple((rel), (tup), tupleLockExtraInfo[mode].hwlock)

/*
 * This table maps tuple lock strength values for each particular
 * MultiXactStatus value.
 */
static const int MultiXactStatusLock[MaxMultiXactStatus + 1] =
{
    LockTupleKeyShare,      /* ForKeyShare */
    LockTupleShare,         /* ForShare */
    LockTupleNoKeyExclusive,        /* ForNoKeyUpdate */
    LockTupleExclusive,     /* ForUpdate */
    LockTupleNoKeyExclusive,        /* NoKeyUpdate */
    LockTupleExclusive      /* Update */
};

/* Get the LockTupleMode for a given MultiXactStatus */
#define TUPLOCK_from_mxstatus(status) \
            (MultiXactStatusLock[(status)])
/* Get the is_update bit for a given MultiXactStatus */
#define ISUPDATE_from_mxstatus(status) \
            ((status) > MultiXactStatusForUpdate)

/* ----------------------------------------------------------------
 *                       heap support routines
 * ----------------------------------------------------------------
 */

/* ----------------
 *      initscan - scan code common to heap_beginscan and heap_rescan
 * ----------------
 */
static void
initscan(HeapScanDesc scan, ScanKey key, bool is_rescan)
{
    bool        allow_strat;
    bool        allow_sync;

    /*
     * Determine the number of blocks we have to scan.
     *
     * It is sufficient to do this once at scan start, since any tuples added
     * while the scan is in progress will be invisible to my snapshot anyway.
     * (That is not true when using a non-MVCC snapshot.  However, we couldn't
     * guarantee to return tuples added after scan start anyway, since they
     * might go into pages we already scanned.  To guarantee consistent
     * results for a non-MVCC snapshot, the caller must hold some higher-level
     * lock that ensures the interesting tuple(s) won't change.)
     */
    scan->rs_nblocks = RelationGetNumberOfBlocks(scan->rs_rd);

    /*
     * If the table is large relative to NBuffers, use a bulk-read access
     * strategy and enable synchronized scanning (see syncscan.c).  Although
     * the thresholds for these features could be different, we make them the
     * same so that there are only two behaviors to tune rather than four.
     * (However, some callers need to be able to disable one or both of these
     * behaviors, independently of the size of the table; also there is a GUC
     * variable that can disable synchronized scanning.)
     *
     * During a rescan, don't make a new strategy object if we don't have to.
     */
    if (!RelationUsesLocalBuffers(scan->rs_rd) &&
        scan->rs_nblocks > NBuffers / 4)
    {
        allow_strat = scan->rs_allow_strat;
        allow_sync = scan->rs_allow_sync;
    }
    else
        allow_strat = allow_sync = false;

    if (allow_strat)
    {
        if (scan->rs_strategy == NULL)
            scan->rs_strategy = GetAccessStrategy(BAS_BULKREAD);
    }
    else
    {
        if (scan->rs_strategy != NULL)
            FreeAccessStrategy(scan->rs_strategy);
        scan->rs_strategy = NULL;
    }

    if (is_rescan)
    {
        /*
         * If rescan, keep the previous startblock setting so that rewinding a
         * cursor doesn't generate surprising results.  Reset the syncscan
         * setting, though.
         */
        scan->rs_syncscan = (allow_sync && synchronize_seqscans);
    }
    else if (allow_sync && synchronize_seqscans)
    {
        scan->rs_syncscan = true;
        scan->rs_startblock = ss_get_location(scan->rs_rd, scan->rs_nblocks);
    }
    else
    {
        scan->rs_syncscan = false;
        scan->rs_startblock = 0;
    }

    scan->rs_inited = false;
    scan->rs_ctup.t_data = NULL;
    ItemPointerSetInvalid(&scan->rs_ctup.t_self);
    scan->rs_cbuf = InvalidBuffer;
    scan->rs_cblock = InvalidBlockNumber;

    /* we don't have a marked position... */
    ItemPointerSetInvalid(&(scan->rs_mctid));

    /* page-at-a-time fields are always invalid when not rs_inited */

    /*
     * copy the scan key, if appropriate
     */
    if (key != NULL)
        memcpy(scan->rs_key, key, scan->rs_nkeys * sizeof(ScanKeyData));

    /*
     * Currently, we don't have a stats counter for bitmap heap scans (but the
     * underlying bitmap index scans will be counted).
     */
    if (!scan->rs_bitmapscan)
        pgstat_count_heap_scan(scan->rs_rd);
}

/*
 * heapgetpage - subroutine for heapgettup()
 *
 * This routine reads and pins the specified page of the relation.
 * In page-at-a-time mode it performs additional work, namely determining
 * which tuples on the page are visible.
 */
static void
heapgetpage(HeapScanDesc scan, BlockNumber page)
{
    Buffer      buffer;
    Snapshot    snapshot;
    Page        dp;
    int         lines;
    int         ntup;
    OffsetNumber lineoff;
    ItemId      lpp;
    bool        all_visible;

    Assert(page < scan->rs_nblocks);

    /* release previous scan buffer, if any */
    if (BufferIsValid(scan->rs_cbuf))
    {
        ReleaseBuffer(scan->rs_cbuf);
        scan->rs_cbuf = InvalidBuffer;
    }

    /*
     * Be sure to check for interrupts at least once per page.  Checks at
     * higher code levels won't be able to stop a seqscan that encounters many
     * pages' worth of consecutive dead tuples.
     */
    CHECK_FOR_INTERRUPTS();

    /* read page using selected strategy */
    scan->rs_cbuf = ReadBufferExtended(scan->rs_rd, MAIN_FORKNUM, page,
                                       RBM_NORMAL, scan->rs_strategy);
    scan->rs_cblock = page;

    if (!scan->rs_pageatatime)
        return;

    buffer = scan->rs_cbuf;
    snapshot = scan->rs_snapshot;

    /*
     * Prune and repair fragmentation for the whole page, if possible.
     */
    Assert(TransactionIdIsValid(RecentGlobalXmin));
    heap_page_prune_opt(scan->rs_rd, buffer, RecentGlobalXmin);

    /*
     * We must hold share lock on the buffer content while examining tuple
     * visibility.  Afterwards, however, the tuples we have found to be
     * visible are guaranteed good as long as we hold the buffer pin.
     */
    LockBuffer(buffer, BUFFER_LOCK_SHARE);

    dp = (Page) BufferGetPage(buffer);
    lines = PageGetMaxOffsetNumber(dp);
    ntup = 0;

    /*
     * If the all-visible flag indicates that all tuples on the page are
     * visible to everyone, we can skip the per-tuple visibility tests.
     *
     * Note: In hot standby, a tuple that's already visible to all
     * transactions in the master might still be invisible to a read-only
     * transaction in the standby. We partly handle this problem by tracking
     * the minimum xmin of visible tuples as the cut-off XID while marking a
     * page all-visible on master and WAL log that along with the visibility
     * map SET operation. In hot standby, we wait for (or abort) all
     * transactions that can potentially may not see one or more tuples on the
     * page. That's how index-only scans work fine in hot standby. A crucial
     * difference between index-only scans and heap scans is that the
     * index-only scan completely relies on the visibility map where as heap
     * scan looks at the page-level PD_ALL_VISIBLE flag. We are not sure if the
     * page-level flag can be trusted in the same way, because it might get
     * propagated somehow without being explicitly WAL-logged, e.g. via a full
     * page write. Until we can prove that beyond doubt, let's check each
     * tuple for visibility the hard way.
     */
    all_visible = PageIsAllVisible(dp) && !snapshot->takenDuringRecovery;

    for (lineoff = FirstOffsetNumber, lpp = PageGetItemId(dp, lineoff);
         lineoff <= lines;
         lineoff++, lpp++)
    {
        if (ItemIdIsNormal(lpp))
        {
            HeapTupleData loctup;
            bool        valid;

            loctup.t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
            loctup.t_len = ItemIdGetLength(lpp);
            ItemPointerSet(&(loctup.t_self), page, lineoff);

            if (all_visible)
                valid = true;
            else
                valid = HeapTupleSatisfiesVisibility(&loctup, snapshot, buffer);

            CheckForSerializableConflictOut(valid, scan->rs_rd, &loctup,
                                            buffer, snapshot);

            if (valid)
                scan->rs_vistuples[ntup++] = lineoff;
        }
    }

    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

    Assert(ntup <= MaxHeapTuplesPerPage);
    scan->rs_ntuples = ntup;
}

/* ----------------
 *      heapgettup - fetch next heap tuple
 *
 *      Initialize the scan if not already done; then advance to the next
 *      tuple as indicated by "dir"; return the next tuple in scan->rs_ctup,
 *      or set scan->rs_ctup.t_data = NULL if no more tuples.
 *
 * dir == NoMovementScanDirection means "re-fetch the tuple indicated
 * by scan->rs_ctup".
 *
 * Note: the reason nkeys/key are passed separately, even though they are
 * kept in the scan descriptor, is that the caller may not want us to check
 * the scankeys.
 *
 * Note: when we fall off the end of the scan in either direction, we
 * reset rs_inited.  This means that a further request with the same
 * scan direction will restart the scan, which is a bit odd, but a
 * request with the opposite scan direction will start a fresh scan
 * in the proper direction.  The latter is required behavior for cursors,
 * while the former case is generally undefined behavior in Postgres
 * so we don't care too much.
 * ----------------
 */
static void
heapgettup(HeapScanDesc scan,
           ScanDirection dir,
           int nkeys,
           ScanKey key)
{
    HeapTuple   tuple = &(scan->rs_ctup);
    Snapshot    snapshot = scan->rs_snapshot;
    bool        backward = ScanDirectionIsBackward(dir);
    BlockNumber page;
    bool        finished;
    Page        dp;
    int         lines;
    OffsetNumber lineoff;
    int         linesleft;
    ItemId      lpp;

    /*
     * calculate next starting lineoff, given scan direction
     */
    if (ScanDirectionIsForward(dir))
    {
        if (!scan->rs_inited)
        {
            /*
             * return null immediately if relation is empty
             */
            if (scan->rs_nblocks == 0)
            {
                Assert(!BufferIsValid(scan->rs_cbuf));
                tuple->t_data = NULL;
                return;
            }
            page = scan->rs_startblock; /* first page */
            heapgetpage(scan, page);
            lineoff = FirstOffsetNumber;        /* first offnum */
            scan->rs_inited = true;
        }
        else
        {
            /* continue from previously returned page/tuple */
            page = scan->rs_cblock;     /* current page */
            lineoff =           /* next offnum */
                OffsetNumberNext(ItemPointerGetOffsetNumber(&(tuple->t_self)));
        }

        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);

        dp = (Page) BufferGetPage(scan->rs_cbuf);
        lines = PageGetMaxOffsetNumber(dp);
        /* page and lineoff now reference the physically next tid */

        linesleft = lines - lineoff + 1;
    }
    else if (backward)
    {
        if (!scan->rs_inited)
        {
            /*
             * return null immediately if relation is empty
             */
            if (scan->rs_nblocks == 0)
            {
                Assert(!BufferIsValid(scan->rs_cbuf));
                tuple->t_data = NULL;
                return;
            }

            /*
             * Disable reporting to syncscan logic in a backwards scan; it's
             * not very likely anyone else is doing the same thing at the same
             * time, and much more likely that we'll just bollix things for
             * forward scanners.
             */
            scan->rs_syncscan = false;
            /* start from last page of the scan */
            if (scan->rs_startblock > 0)
                page = scan->rs_startblock - 1;
            else
                page = scan->rs_nblocks - 1;
            heapgetpage(scan, page);
        }
        else
        {
            /* continue from previously returned page/tuple */
            page = scan->rs_cblock;     /* current page */
        }

        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);

        dp = (Page) BufferGetPage(scan->rs_cbuf);
        lines = PageGetMaxOffsetNumber(dp);

        if (!scan->rs_inited)
        {
            lineoff = lines;    /* final offnum */
            scan->rs_inited = true;
        }
        else
        {
            lineoff =           /* previous offnum */
                OffsetNumberPrev(ItemPointerGetOffsetNumber(&(tuple->t_self)));
        }
        /* page and lineoff now reference the physically previous tid */

        linesleft = lineoff;
    }
    else
    {
        /*
         * ``no movement'' scan direction: refetch prior tuple
         */
        if (!scan->rs_inited)
        {
            Assert(!BufferIsValid(scan->rs_cbuf));
            tuple->t_data = NULL;
            return;
        }

        page = ItemPointerGetBlockNumber(&(tuple->t_self));
        if (page != scan->rs_cblock)
            heapgetpage(scan, page);

        /* Since the tuple was previously fetched, needn't lock page here */
        dp = (Page) BufferGetPage(scan->rs_cbuf);
        lineoff = ItemPointerGetOffsetNumber(&(tuple->t_self));
        lpp = PageGetItemId(dp, lineoff);
        Assert(ItemIdIsNormal(lpp));

        tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
        tuple->t_len = ItemIdGetLength(lpp);

        return;
    }

    /*
     * advance the scan until we find a qualifying tuple or run out of stuff
     * to scan
     */
    lpp = PageGetItemId(dp, lineoff);
    for (;;)
    {
        while (linesleft > 0)
        {
            if (ItemIdIsNormal(lpp))
            {
                bool        valid;

                tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
                tuple->t_len = ItemIdGetLength(lpp);
                ItemPointerSet(&(tuple->t_self), page, lineoff);

                /*
                 * if current tuple qualifies, return it.
                 */
                valid = HeapTupleSatisfiesVisibility(tuple,
                                                     snapshot,
                                                     scan->rs_cbuf);

                CheckForSerializableConflictOut(valid, scan->rs_rd, tuple,
                                                scan->rs_cbuf, snapshot);

                if (valid && key != NULL)
                    HeapKeyTest(tuple, RelationGetDescr(scan->rs_rd),
                                nkeys, key, valid);

                if (valid)
                {
                    LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);
                    return;
                }
            }

            /*
             * otherwise move to the next item on the page
             */
            --linesleft;
            if (backward)
            {
                --lpp;          /* move back in this page's ItemId array */
                --lineoff;
            }
            else
            {
                ++lpp;          /* move forward in this page's ItemId array */
                ++lineoff;
            }
        }

        /*
         * if we get here, it means we've exhausted the items on this page and
         * it's time to move to the next.
         */
        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_UNLOCK);

        /*
         * advance to next/prior page and detect end of scan
         */
        if (backward)
        {
            finished = (page == scan->rs_startblock);
            if (page == 0)
                page = scan->rs_nblocks;
            page--;
        }
        else
        {
            page++;
            if (page >= scan->rs_nblocks)
                page = 0;
            finished = (page == scan->rs_startblock);

            /*
             * Report our new scan position for synchronization purposes. We
             * don't do that when moving backwards, however. That would just
             * mess up any other forward-moving scanners.
             *
             * Note: we do this before checking for end of scan so that the
             * final state of the position hint is back at the start of the
             * rel.  That's not strictly necessary, but otherwise when you run
             * the same query multiple times the starting position would shift
             * a little bit backwards on every invocation, which is confusing.
             * We don't guarantee any specific ordering in general, though.
             */
            if (scan->rs_syncscan)
                ss_report_location(scan->rs_rd, page);
        }

        /*
         * return NULL if we've exhausted all the pages
         */
        if (finished)
        {
            if (BufferIsValid(scan->rs_cbuf))
                ReleaseBuffer(scan->rs_cbuf);
            scan->rs_cbuf = InvalidBuffer;
            scan->rs_cblock = InvalidBlockNumber;
            tuple->t_data = NULL;
            scan->rs_inited = false;
            return;
        }

        heapgetpage(scan, page);

        LockBuffer(scan->rs_cbuf, BUFFER_LOCK_SHARE);

        dp = (Page) BufferGetPage(scan->rs_cbuf);
        lines = PageGetMaxOffsetNumber((Page) dp);
        linesleft = lines;
        if (backward)
        {
            lineoff = lines;
            lpp = PageGetItemId(dp, lines);
        }
        else
        {
            lineoff = FirstOffsetNumber;
            lpp = PageGetItemId(dp, FirstOffsetNumber);
        }
    }
}

/* ----------------
 *      heapgettup_pagemode - fetch next heap tuple in page-at-a-time mode
 *
 *      Same API as heapgettup, but used in page-at-a-time mode
 *
 * The internal logic is much the same as heapgettup's too, but there are some
 * differences: we do not take the buffer content lock (that only needs to
 * happen inside heapgetpage), and we iterate through just the tuples listed
 * in rs_vistuples[] rather than all tuples on the page.  Notice that
 * lineindex is 0-based, where the corresponding loop variable lineoff in
 * heapgettup is 1-based.
 * ----------------
 */
static void
heapgettup_pagemode(HeapScanDesc scan,
                    ScanDirection dir,
                    int nkeys,
                    ScanKey key)
{
    HeapTuple   tuple = &(scan->rs_ctup);
    bool        backward = ScanDirectionIsBackward(dir);
    BlockNumber page;
    bool        finished;
    Page        dp;
    int         lines;
    int         lineindex;
    OffsetNumber lineoff;
    int         linesleft;
    ItemId      lpp;

    /*
     * calculate next starting lineindex, given scan direction
     */
    if (ScanDirectionIsForward(dir))
    {
        if (!scan->rs_inited)
        {
            /*
             * return null immediately if relation is empty
             */
            if (scan->rs_nblocks == 0)
            {
                Assert(!BufferIsValid(scan->rs_cbuf));
                tuple->t_data = NULL;
                return;
            }
            page = scan->rs_startblock; /* first page */
            heapgetpage(scan, page);
            lineindex = 0;
            scan->rs_inited = true;
        }
        else
        {
            /* continue from previously returned page/tuple */
            page = scan->rs_cblock;     /* current page */
            lineindex = scan->rs_cindex + 1;
        }

        dp = (Page) BufferGetPage(scan->rs_cbuf);
        lines = scan->rs_ntuples;
        /* page and lineindex now reference the next visible tid */

        linesleft = lines - lineindex;
    }
    else if (backward)
    {
        if (!scan->rs_inited)
        {
            /*
             * return null immediately if relation is empty
             */
            if (scan->rs_nblocks == 0)
            {
                Assert(!BufferIsValid(scan->rs_cbuf));
                tuple->t_data = NULL;
                return;
            }

            /*
             * Disable reporting to syncscan logic in a backwards scan; it's
             * not very likely anyone else is doing the same thing at the same
             * time, and much more likely that we'll just bollix things for
             * forward scanners.
             */
            scan->rs_syncscan = false;
            /* start from last page of the scan */
            if (scan->rs_startblock > 0)
                page = scan->rs_startblock - 1;
            else
                page = scan->rs_nblocks - 1;
            heapgetpage(scan, page);
        }
        else
        {
            /* continue from previously returned page/tuple */
            page = scan->rs_cblock;     /* current page */
        }

        dp = (Page) BufferGetPage(scan->rs_cbuf);
        lines = scan->rs_ntuples;

        if (!scan->rs_inited)
        {
            lineindex = lines - 1;
            scan->rs_inited = true;
        }
        else
        {
            lineindex = scan->rs_cindex - 1;
        }
        /* page and lineindex now reference the previous visible tid */

        linesleft = lineindex + 1;
    }
    else
    {
        /*
         * ``no movement'' scan direction: refetch prior tuple
         */
        if (!scan->rs_inited)
        {
            Assert(!BufferIsValid(scan->rs_cbuf));
            tuple->t_data = NULL;
            return;
        }

        page = ItemPointerGetBlockNumber(&(tuple->t_self));
        if (page != scan->rs_cblock)
            heapgetpage(scan, page);

        /* Since the tuple was previously fetched, needn't lock page here */
        dp = (Page) BufferGetPage(scan->rs_cbuf);
        lineoff = ItemPointerGetOffsetNumber(&(tuple->t_self));
        lpp = PageGetItemId(dp, lineoff);
        Assert(ItemIdIsNormal(lpp));

        tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
        tuple->t_len = ItemIdGetLength(lpp);

        /* check that rs_cindex is in sync */
        Assert(scan->rs_cindex < scan->rs_ntuples);
        Assert(lineoff == scan->rs_vistuples[scan->rs_cindex]);

        return;
    }

    /*
     * advance the scan until we find a qualifying tuple or run out of stuff
     * to scan
     */
    for (;;)
    {
        while (linesleft > 0)
        {
            lineoff = scan->rs_vistuples[lineindex];
            lpp = PageGetItemId(dp, lineoff);
            Assert(ItemIdIsNormal(lpp));

            tuple->t_data = (HeapTupleHeader) PageGetItem((Page) dp, lpp);
            tuple->t_len = ItemIdGetLength(lpp);
            ItemPointerSet(&(tuple->t_self), page, lineoff);

            /*
             * if current tuple qualifies, return it.
             */
            if (key != NULL)
            {
                bool        valid;

                HeapKeyTest(tuple, RelationGetDescr(scan->rs_rd),
                            nkeys, key, valid);
                if (valid)
                {
                    scan->rs_cindex = lineindex;
                    return;
                }
            }
            else
            {
                scan->rs_cindex = lineindex;
                return;
            }

            /*
             * otherwise move to the next item on the page
             */
            --linesleft;
            if (backward)
                --lineindex;
            else
                ++lineindex;
        }

        /*
         * if we get here, it means we've exhausted the items on this page and
         * it's time to move to the next.
         */
        if (backward)
        {
            finished = (page == scan->rs_startblock);
            if (page == 0)
                page = scan->rs_nblocks;
            page--;
        }
        else
        {
            page++;
            if (page >= scan->rs_nblocks)
                page = 0;
            finished = (page == scan->rs_startblock);

            /*
             * Report our new scan position for synchronization purposes. We
             * don't do that when moving backwards, however. That would just
             * mess up any other forward-moving scanners.
             *
             * Note: we do this before checking for end of scan so that the
             * final state of the position hint is back at the start of the
             * rel.  That's not strictly necessary, but otherwise when you run
             * the same query multiple times the starting position would shift
             * a little bit backwards on every invocation, which is confusing.
             * We don't guarantee any specific ordering in general, though.
             */
            if (scan->rs_syncscan)
                ss_report_location(scan->rs_rd, page);
        }

        /*
         * return NULL if we've exhausted all the pages
         */
        if (finished)
        {
            if (BufferIsValid(scan->rs_cbuf))
                ReleaseBuffer(scan->rs_cbuf);
            scan->rs_cbuf = InvalidBuffer;
            scan->rs_cblock = InvalidBlockNumber;
            tuple->t_data = NULL;
            scan->rs_inited = false;
            return;
        }

        heapgetpage(scan, page);

        dp = (Page) BufferGetPage(scan->rs_cbuf);
        lines = scan->rs_ntuples;
        linesleft = lines;
        if (backward)
            lineindex = lines - 1;
        else
            lineindex = 0;
    }
}


#if defined(DISABLE_COMPLEX_MACRO)
/*
 * This is formatted so oddly so that the correspondence to the macro
 * definition in access/htup.h is maintained.
 */
Datum
fastgetattr(HeapTuple tup, int attnum, TupleDesc tupleDesc,
            bool *isnull)
{
    return (
            (attnum) > 0 ?
            (
             (*(isnull) = false),
             HeapTupleNoNulls(tup) ?
             (
              (tupleDesc)->attrs[(attnum) - 1]->attcacheoff >= 0 ?
              (
               fetchatt((tupleDesc)->attrs[(attnum) - 1],
                        (char *) (tup)->t_data + (tup)->t_data->t_hoff +
                        (tupleDesc)->attrs[(attnum) - 1]->attcacheoff)
               )
              :
              nocachegetattr((tup), (attnum), (tupleDesc))
              )
             :
             (
              att_isnull((attnum) - 1, (tup)->t_data->t_bits) ?
              (
               (*(isnull) = true),
               (Datum) NULL
               )
              :
              (
               nocachegetattr((tup), (attnum), (tupleDesc))
               )
              )
             )
            :
            (
             (Datum) NULL
             )
        );
}
#endif   /* defined(DISABLE_COMPLEX_MACRO) */


/* ----------------------------------------------------------------
 *                   heap access method interface
 * ----------------------------------------------------------------
 */

/* ----------------
 *      relation_open - open any relation by relation OID
 *
 *      If lockmode is not "NoLock", the specified kind of lock is
 *      obtained on the relation.  (Generally, NoLock should only be
 *      used if the caller knows it has some appropriate lock on the
 *      relation already.)
 *
 *      An error is raised if the relation does not exist.
 *
 *      NB: a "relation" is anything with a pg_class entry.  The caller is
 *      expected to check whether the relkind is something it can handle.
 * ----------------
 */
Relation
relation_open(Oid relationId, LOCKMODE lockmode)
{
    Relation    r;

    Assert(lockmode >= NoLock && lockmode < MAX_LOCKMODES);

    /* Get the lock before trying to open the relcache entry */
    if (lockmode != NoLock)
        LockRelationOid(relationId, lockmode);

    /* The relcache does all the real work... */
    r = RelationIdGetRelation(relationId);

    if (!RelationIsValid(r))
        elog(ERROR, "could not open relation with OID %u", relationId);

    /* Make note that we've accessed a temporary relation */
    if (RelationUsesLocalBuffers(r))
        MyXactAccessedTempRel = true;

    pgstat_initstats(r);

    return r;
}

/* ----------------
 *      try_relation_open - open any relation by relation OID
 *
 *      Same as relation_open, except return NULL instead of failing
 *      if the relation does not exist.
 * ----------------
 */
Relation
try_relation_open(Oid relationId, LOCKMODE lockmode)
{
    Relation    r;

    Assert(lockmode >= NoLock && lockmode < MAX_LOCKMODES);

    /* Get the lock first */
    if (lockmode != NoLock)
        LockRelationOid(relationId, lockmode);

    /*
     * Now that we have the lock, probe to see if the relation really exists
     * or not.
     */
    if (!SearchSysCacheExists1(RELOID, ObjectIdGetDatum(relationId)))
    {
        /* Release useless lock */
        if (lockmode != NoLock)
            UnlockRelationOid(relationId, lockmode);

        return NULL;
    }

    /* Should be safe to do a relcache load */
    r = RelationIdGetRelation(relationId);

    if (!RelationIsValid(r))
        elog(ERROR, "could not open relation with OID %u", relationId);

    /* Make note that we've accessed a temporary relation */
    if (RelationUsesLocalBuffers(r))
        MyXactAccessedTempRel = true;

    pgstat_initstats(r);

    return r;
}

/* ----------------
 *      relation_openrv - open any relation specified by a RangeVar
 *
 *      Same as relation_open, but the relation is specified by a RangeVar.
 * ----------------
 */
Relation
relation_openrv(const RangeVar *relation, LOCKMODE lockmode)
{
    Oid         relOid;

    /*
     * Check for shared-cache-inval messages before trying to open the
     * relation.  This is needed even if we already hold a lock on the
     * relation, because GRANT/REVOKE are executed without taking any lock on
     * the target relation, and we want to be sure we see current ACL
     * information.  We can skip this if asked for NoLock, on the assumption
     * that such a call is not the first one in the current command, and so we
     * should be reasonably up-to-date already.  (XXX this all could stand to
     * be redesigned, but for the moment we'll keep doing this like it's been
     * done historically.)
     */
    if (lockmode != NoLock)
        AcceptInvalidationMessages();

    /* Look up and lock the appropriate relation using namespace search */
    relOid = RangeVarGetRelid(relation, lockmode, false);

    /* Let relation_open do the rest */
    return relation_open(relOid, NoLock);
}

/* ----------------
 *      relation_openrv_extended - open any relation specified by a RangeVar
 *
 *      Same as relation_openrv, but with an additional missing_ok argument
 *      allowing a NULL return rather than an error if the relation is not
 *      found.  (Note that some other causes, such as permissions problems,
 *      will still result in an ereport.)
 * ----------------
 */
Relation
relation_openrv_extended(const RangeVar *relation, LOCKMODE lockmode,
                         bool missing_ok)
{
    Oid         relOid;

    /*
     * Check for shared-cache-inval messages before trying to open the
     * relation.  See comments in relation_openrv().
     */
    if (lockmode != NoLock)
        AcceptInvalidationMessages();

    /* Look up and lock the appropriate relation using namespace search */
    relOid = RangeVarGetRelid(relation, lockmode, missing_ok);

    /* Return NULL on not-found */
    if (!OidIsValid(relOid))
        return NULL;

    /* Let relation_open do the rest */
    return relation_open(relOid, NoLock);
}

/* ----------------
 *      relation_close - close any relation
 *
 *      If lockmode is not "NoLock", we then release the specified lock.
 *
 *      Note that it is often sensible to hold a lock beyond relation_close;
 *      in that case, the lock is released automatically at xact end.
 * ----------------
 */
void
relation_close(Relation relation, LOCKMODE lockmode)
{
    LockRelId   relid = relation->rd_lockInfo.lockRelId;

    Assert(lockmode >= NoLock && lockmode < MAX_LOCKMODES);

    /* The relcache does the real work... */
    RelationClose(relation);

    if (lockmode != NoLock)
        UnlockRelationId(&relid, lockmode);
}


/* ----------------
 *      heap_open - open a heap relation by relation OID
 *
 *      This is essentially relation_open plus check that the relation
 *      is not an index nor a composite type.  (The caller should also
 *      check that it's not a view or foreign table before assuming it has
 *      storage.)
 * ----------------
 */
Relation
heap_open(Oid relationId, LOCKMODE lockmode)
{
    Relation    r;

    r = relation_open(relationId, lockmode);

    if (r->rd_rel->relkind == RELKIND_INDEX)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is an index",
                        RelationGetRelationName(r))));
    else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is a composite type",
                        RelationGetRelationName(r))));

    return r;
}

/* ----------------
 *      heap_openrv - open a heap relation specified
 *      by a RangeVar node
 *
 *      As above, but relation is specified by a RangeVar.
 * ----------------
 */
Relation
heap_openrv(const RangeVar *relation, LOCKMODE lockmode)
{
    Relation    r;

    r = relation_openrv(relation, lockmode);

    if (r->rd_rel->relkind == RELKIND_INDEX)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is an index",
                        RelationGetRelationName(r))));
    else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is a composite type",
                        RelationGetRelationName(r))));

    return r;
}

/* ----------------
 *      heap_openrv_extended - open a heap relation specified
 *      by a RangeVar node
 *
 *      As above, but optionally return NULL instead of failing for
 *      relation-not-found.
 * ----------------
 */
Relation
heap_openrv_extended(const RangeVar *relation, LOCKMODE lockmode,
                     bool missing_ok)
{
    Relation    r;

    r = relation_openrv_extended(relation, lockmode, missing_ok);

    if (r)
    {
        if (r->rd_rel->relkind == RELKIND_INDEX)
            ereport(ERROR,
                    (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                     errmsg("\"%s\" is an index",
                            RelationGetRelationName(r))));
        else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
            ereport(ERROR,
                    (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                     errmsg("\"%s\" is a composite type",
                            RelationGetRelationName(r))));
    }

    return r;
}


/* ----------------
 *      heap_beginscan  - begin relation scan
 *
 * heap_beginscan_strat offers an extended API that lets the caller control
 * whether a nondefault buffer access strategy can be used, and whether
 * syncscan can be chosen (possibly resulting in the scan not starting from
 * block zero).  Both of these default to TRUE with plain heap_beginscan.
 *
 * heap_beginscan_bm is an alternative entry point for setting up a
 * HeapScanDesc for a bitmap heap scan.  Although that scan technology is
 * really quite unlike a standard seqscan, there is just enough commonality
 * to make it worth using the same data structure.
 * ----------------
 */
HeapScanDesc
heap_beginscan(Relation relation, Snapshot snapshot,
               int nkeys, ScanKey key)
{
    return heap_beginscan_internal(relation, snapshot, nkeys, key,
                                   true, true, false);
}

HeapScanDesc
heap_beginscan_strat(Relation relation, Snapshot snapshot,
                     int nkeys, ScanKey key,
                     bool allow_strat, bool allow_sync)
{
    return heap_beginscan_internal(relation, snapshot, nkeys, key,
                                   allow_strat, allow_sync, false);
}

HeapScanDesc
heap_beginscan_bm(Relation relation, Snapshot snapshot,
                  int nkeys, ScanKey key)
{
    return heap_beginscan_internal(relation, snapshot, nkeys, key,
                                   false, false, true);
}

static HeapScanDesc
heap_beginscan_internal(Relation relation, Snapshot snapshot,
                        int nkeys, ScanKey key,
                        bool allow_strat, bool allow_sync,
                        bool is_bitmapscan)
{
    HeapScanDesc scan;

    /*
     * increment relation ref count while scanning relation
     *
     * This is just to make really sure the relcache entry won't go away while
     * the scan has a pointer to it.  Caller should be holding the rel open
     * anyway, so this is redundant in all normal scenarios...
     */
    RelationIncrementReferenceCount(relation);

    /*
     * allocate and initialize scan descriptor
     */
    scan = (HeapScanDesc) palloc(sizeof(HeapScanDescData));

    scan->rs_rd = relation;
    scan->rs_snapshot = snapshot;
    scan->rs_nkeys = nkeys;
    scan->rs_bitmapscan = is_bitmapscan;
    scan->rs_strategy = NULL;   /* set in initscan */
    scan->rs_allow_strat = allow_strat;
    scan->rs_allow_sync = allow_sync;

    /*
     * we can use page-at-a-time mode if it's an MVCC-safe snapshot
     */
    scan->rs_pageatatime = IsMVCCSnapshot(snapshot);

    /*
     * For a seqscan in a serializable transaction, acquire a predicate lock
     * on the entire relation. This is required not only to lock all the
     * matching tuples, but also to conflict with new insertions into the
     * table. In an indexscan, we take page locks on the index pages covering
     * the range specified in the scan qual, but in a heap scan there is
     * nothing more fine-grained to lock. A bitmap scan is a different story,
     * there we have already scanned the index and locked the index pages
     * covering the predicate. But in that case we still have to lock any
     * matching heap tuples.
     */
    if (!is_bitmapscan)
        PredicateLockRelation(relation, snapshot);

    /* we only need to set this up once */
    scan->rs_ctup.t_tableOid = RelationGetRelid(relation);

    /*
     * we do this here instead of in initscan() because heap_rescan also calls
     * initscan() and we don't want to allocate memory again
     */
    if (nkeys > 0)
        scan->rs_key = (ScanKey) palloc(sizeof(ScanKeyData) * nkeys);
    else
        scan->rs_key = NULL;

    initscan(scan, key, false);

    return scan;
}

/* ----------------
 *      heap_rescan     - restart a relation scan
 * ----------------
 */
void
heap_rescan(HeapScanDesc scan,
            ScanKey key)
{
    /*
     * unpin scan buffers
     */
    if (BufferIsValid(scan->rs_cbuf))
        ReleaseBuffer(scan->rs_cbuf);

    /*
     * reinitialize scan descriptor
     */
    initscan(scan, key, true);
}

/* ----------------
 *      heap_endscan    - end relation scan
 *
 *      See how to integrate with index scans.
 *      Check handling if reldesc caching.
 * ----------------
 */
void
heap_endscan(HeapScanDesc scan)
{
    /* Note: no locking manipulations needed */

    /*
     * unpin scan buffers
     */
    if (BufferIsValid(scan->rs_cbuf))
        ReleaseBuffer(scan->rs_cbuf);

    /*
     * decrement relation reference count and free scan descriptor storage
     */
    RelationDecrementReferenceCount(scan->rs_rd);

    if (scan->rs_key)
        pfree(scan->rs_key);

    if (scan->rs_strategy != NULL)
        FreeAccessStrategy(scan->rs_strategy);

    pfree(scan);
}

/* ----------------
 *      heap_getnext    - retrieve next tuple in scan
 *
 *      Fix to work with index relations.
 *      We don't return the buffer anymore, but you can get it from the
 *      returned HeapTuple.
 * ----------------
 */

#ifdef HEAPDEBUGALL
#define HEAPDEBUG_1 \
    elog(DEBUG2, "heap_getnext([%s,nkeys=%d],dir=%d) called", \
         RelationGetRelationName(scan->rs_rd), scan->rs_nkeys, (int) direction)
#define HEAPDEBUG_2 \
    elog(DEBUG2, "heap_getnext returning EOS")
#define HEAPDEBUG_3 \
    elog(DEBUG2, "heap_getnext returning tuple")
#else
#define HEAPDEBUG_1
#define HEAPDEBUG_2
#define HEAPDEBUG_3
#endif   /* !defined(HEAPDEBUGALL) */


HeapTuple
heap_getnext(HeapScanDesc scan, ScanDirection direction)
{
    /* Note: no locking manipulations needed */

    HEAPDEBUG_1;                /* heap_getnext( info ) */

    if (scan->rs_pageatatime)
        heapgettup_pagemode(scan, direction,
                            scan->rs_nkeys, scan->rs_key);
    else
        heapgettup(scan, direction, scan->rs_nkeys, scan->rs_key);

    if (scan->rs_ctup.t_data == NULL)
    {
        HEAPDEBUG_2;            /* heap_getnext returning EOS */
        return NULL;
    }

    /*
     * if we get here it means we have a new current scan tuple, so point to
     * the proper return buffer and return the tuple.
     */
    HEAPDEBUG_3;                /* heap_getnext returning tuple */

    pgstat_count_heap_getnext(scan->rs_rd);

    return &(scan->rs_ctup);
}

/*
 *  heap_fetch      - retrieve tuple with given tid
 *
 * On entry, tuple->t_self is the TID to fetch.  We pin the buffer holding
 * the tuple, fill in the remaining fields of *tuple, and check the tuple
 * against the specified snapshot.
 *
 * If successful (tuple found and passes snapshot time qual), then *userbuf
 * is set to the buffer holding the tuple and TRUE is returned.  The caller
 * must unpin the buffer when done with the tuple.
 *
 * If the tuple is not found (ie, item number references a deleted slot),
 * then tuple->t_data is set to NULL and FALSE is returned.
 *
 * If the tuple is found but fails the time qual check, then FALSE is returned
 * but tuple->t_data is left pointing to the tuple.
 *
 * keep_buf determines what is done with the buffer in the FALSE-result cases.
 * When the caller specifies keep_buf = true, we retain the pin on the buffer
 * and return it in *userbuf (so the caller must eventually unpin it); when
 * keep_buf = false, the pin is released and *userbuf is set to InvalidBuffer.
 *
 * stats_relation is the relation to charge the heap_fetch operation against
 * for statistical purposes.  (This could be the heap rel itself, an
 * associated index, or NULL to not count the fetch at all.)
 *
 * heap_fetch does not follow HOT chains: only the exact TID requested will
 * be fetched.
 *
 * It is somewhat inconsistent that we ereport() on invalid block number but
 * return false on invalid item number.  There are a couple of reasons though.
 * One is that the caller can relatively easily check the block number for
 * validity, but cannot check the item number without reading the page
 * himself.  Another is that when we are following a t_ctid link, we can be
 * reasonably confident that the page number is valid (since VACUUM shouldn't
 * truncate off the destination page without having killed the referencing
 * tuple first), but the item number might well not be good.
 */
bool
heap_fetch(Relation relation,
           Snapshot snapshot,
           HeapTuple tuple,
           Buffer *userbuf,
           bool keep_buf,
           Relation stats_relation)
{
    ItemPointer tid = &(tuple->t_self);
    ItemId      lp;
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    bool        valid;

    /*
     * Fetch and pin the appropriate page of the relation.
     */
    buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));

    /*
     * Need share lock on buffer to examine tuple commit status.
     */
    LockBuffer(buffer, BUFFER_LOCK_SHARE);
    page = BufferGetPage(buffer);

    /*
     * We'd better check for out-of-range offnum in case of VACUUM since the
     * TID was obtained.
     */
    offnum = ItemPointerGetOffsetNumber(tid);
    if (offnum < FirstOffsetNumber || offnum > PageGetMaxOffsetNumber(page))
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        if (keep_buf)
            *userbuf = buffer;
        else
        {
            ReleaseBuffer(buffer);
            *userbuf = InvalidBuffer;
        }
        tuple->t_data = NULL;
        return false;
    }

    /*
     * get the item line pointer corresponding to the requested tid
     */
    lp = PageGetItemId(page, offnum);

    /*
     * Must check for deleted tuple.
     */
    if (!ItemIdIsNormal(lp))
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        if (keep_buf)
            *userbuf = buffer;
        else
        {
            ReleaseBuffer(buffer);
            *userbuf = InvalidBuffer;
        }
        tuple->t_data = NULL;
        return false;
    }

    /*
     * fill in *tuple fields
     */
    tuple->t_data = (HeapTupleHeader) PageGetItem(page, lp);
    tuple->t_len = ItemIdGetLength(lp);
    tuple->t_tableOid = RelationGetRelid(relation);

    /*
     * check time qualification of tuple, then release lock
     */
    valid = HeapTupleSatisfiesVisibility(tuple, snapshot, buffer);

    if (valid)
        PredicateLockTuple(relation, tuple, snapshot);

    CheckForSerializableConflictOut(valid, relation, tuple, buffer, snapshot);

    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

    if (valid)
    {
        /*
         * All checks passed, so return the tuple as valid. Caller is now
         * responsible for releasing the buffer.
         */
        *userbuf = buffer;

        /* Count the successful fetch against appropriate rel, if any */
        if (stats_relation != NULL)
            pgstat_count_heap_fetch(stats_relation);

        return true;
    }

    /* Tuple failed time qual, but maybe caller wants to see it anyway. */
    if (keep_buf)
        *userbuf = buffer;
    else
    {
        ReleaseBuffer(buffer);
        *userbuf = InvalidBuffer;
    }

    return false;
}

/*
 *  heap_hot_search_buffer  - search HOT chain for tuple satisfying snapshot
 *
 * On entry, *tid is the TID of a tuple (either a simple tuple, or the root
 * of a HOT chain), and buffer is the buffer holding this tuple.  We search
 * for the first chain member satisfying the given snapshot.  If one is
 * found, we update *tid to reference that tuple's offset number, and
 * return TRUE.  If no match, return FALSE without modifying *tid.
 *
 * heapTuple is a caller-supplied buffer.  When a match is found, we return
 * the tuple here, in addition to updating *tid.  If no match is found, the
 * contents of this buffer on return are undefined.
 *
 * If all_dead is not NULL, we check non-visible tuples to see if they are
 * globally dead; *all_dead is set TRUE if all members of the HOT chain
 * are vacuumable, FALSE if not.
 *
 * Unlike heap_fetch, the caller must already have pin and (at least) share
 * lock on the buffer; it is still pinned/locked at exit.  Also unlike
 * heap_fetch, we do not report any pgstats count; caller may do so if wanted.
 */
bool
heap_hot_search_buffer(ItemPointer tid, Relation relation, Buffer buffer,
                       Snapshot snapshot, HeapTuple heapTuple,
                       bool *all_dead, bool first_call)
{
    Page        dp = (Page) BufferGetPage(buffer);
    TransactionId prev_xmax = InvalidTransactionId;
    OffsetNumber offnum;
    bool        at_chain_start;
    bool        valid;
    bool        skip;

    /* If this is not the first call, previous call returned a (live!) tuple */
    if (all_dead)
        *all_dead = first_call;

    Assert(TransactionIdIsValid(RecentGlobalXmin));

    Assert(ItemPointerGetBlockNumber(tid) == BufferGetBlockNumber(buffer));
    offnum = ItemPointerGetOffsetNumber(tid);
    at_chain_start = first_call;
    skip = !first_call;

    /* Scan through possible multiple members of HOT-chain */
    for (;;)
    {
        ItemId      lp;

        /* check for bogus TID */
        if (offnum < FirstOffsetNumber || offnum > PageGetMaxOffsetNumber(dp))
            break;

        lp = PageGetItemId(dp, offnum);

        /* check for unused, dead, or redirected items */
        if (!ItemIdIsNormal(lp))
        {
            /* We should only see a redirect at start of chain */
            if (ItemIdIsRedirected(lp) && at_chain_start)
            {
                /* Follow the redirect */
                offnum = ItemIdGetRedirect(lp);
                at_chain_start = false;
                continue;
            }
            /* else must be end of chain */
            break;
        }

        heapTuple->t_data = (HeapTupleHeader) PageGetItem(dp, lp);
        heapTuple->t_len = ItemIdGetLength(lp);
        heapTuple->t_tableOid = relation->rd_id;
        heapTuple->t_self = *tid;

        /*
         * Shouldn't see a HEAP_ONLY tuple at chain start.
         */
        if (at_chain_start && HeapTupleIsHeapOnly(heapTuple))
            break;

        /*
         * The xmin should match the previous xmax value, else chain is
         * broken.
         */
        if (TransactionIdIsValid(prev_xmax) &&
            !TransactionIdEquals(prev_xmax,
                                 HeapTupleHeaderGetXmin(heapTuple->t_data)))
            break;

        /*
         * When first_call is true (and thus, skip is initially false) we'll
         * return the first tuple we find.  But on later passes, heapTuple
         * will initially be pointing to the tuple we returned last time.
         * Returning it again would be incorrect (and would loop forever), so
         * we skip it and return the next match we find.
         */
        if (!skip)
        {
            /* If it's visible per the snapshot, we must return it */
            valid = HeapTupleSatisfiesVisibility(heapTuple, snapshot, buffer);
            CheckForSerializableConflictOut(valid, relation, heapTuple,
                                            buffer, snapshot);
            if (valid)
            {
                ItemPointerSetOffsetNumber(tid, offnum);
                PredicateLockTuple(relation, heapTuple, snapshot);
                if (all_dead)
                    *all_dead = false;
                return true;
            }
        }
        skip = false;

        /*
         * If we can't see it, maybe no one else can either.  At caller
         * request, check whether all chain members are dead to all
         * transactions.
         */
        if (all_dead && *all_dead &&
            !HeapTupleIsSurelyDead(heapTuple->t_data, RecentGlobalXmin))
            *all_dead = false;

        /*
         * Check to see if HOT chain continues past this tuple; if so fetch
         * the next offnum and loop around.
         */
        if (HeapTupleIsHotUpdated(heapTuple))
        {
            Assert(ItemPointerGetBlockNumber(&heapTuple->t_data->t_ctid) ==
                   ItemPointerGetBlockNumber(tid));
            offnum = ItemPointerGetOffsetNumber(&heapTuple->t_data->t_ctid);
            at_chain_start = false;
            prev_xmax = HeapTupleHeaderGetUpdateXid(heapTuple->t_data);
        }
        else
            break;              /* end of chain */
    }

    return false;
}

/*
 *  heap_hot_search     - search HOT chain for tuple satisfying snapshot
 *
 * This has the same API as heap_hot_search_buffer, except that the caller
 * does not provide the buffer containing the page, rather we access it
 * locally.
 */
bool
heap_hot_search(ItemPointer tid, Relation relation, Snapshot snapshot,
                bool *all_dead)
{
    bool        result;
    Buffer      buffer;
    HeapTupleData heapTuple;

    buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));
    LockBuffer(buffer, BUFFER_LOCK_SHARE);
    result = heap_hot_search_buffer(tid, relation, buffer, snapshot,
                                    &heapTuple, all_dead, true);
    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
    ReleaseBuffer(buffer);
    return result;
}

/*
 *  heap_get_latest_tid -  get the latest tid of a specified tuple
 *
 * Actually, this gets the latest version that is visible according to
 * the passed snapshot.  You can pass SnapshotDirty to get the very latest,
 * possibly uncommitted version.
 *
 * *tid is both an input and an output parameter: it is updated to
 * show the latest version of the row.  Note that it will not be changed
 * if no version of the row passes the snapshot test.
 */
void
heap_get_latest_tid(Relation relation,
                    Snapshot snapshot,
                    ItemPointer tid)
{
    BlockNumber blk;
    ItemPointerData ctid;
    TransactionId priorXmax;

    /* this is to avoid Assert failures on bad input */
    if (!ItemPointerIsValid(tid))
        return;

    /*
     * Since this can be called with user-supplied TID, don't trust the input
     * too much.  (RelationGetNumberOfBlocks is an expensive check, so we
     * don't check t_ctid links again this way.  Note that it would not do to
     * call it just once and save the result, either.)
     */
    blk = ItemPointerGetBlockNumber(tid);
    if (blk >= RelationGetNumberOfBlocks(relation))
        elog(ERROR, "block number %u is out of range for relation \"%s\"",
             blk, RelationGetRelationName(relation));

    /*
     * Loop to chase down t_ctid links.  At top of loop, ctid is the tuple we
     * need to examine, and *tid is the TID we will return if ctid turns out
     * to be bogus.
     *
     * Note that we will loop until we reach the end of the t_ctid chain.
     * Depending on the snapshot passed, there might be at most one visible
     * version of the row, but we don't try to optimize for that.
     */
    ctid = *tid;
    priorXmax = InvalidTransactionId;   /* cannot check first XMIN */
    for (;;)
    {
        Buffer      buffer;
        Page        page;
        OffsetNumber offnum;
        ItemId      lp;
        HeapTupleData tp;
        bool        valid;

        /*
         * Read, pin, and lock the page.
         */
        buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(&ctid));
        LockBuffer(buffer, BUFFER_LOCK_SHARE);
        page = BufferGetPage(buffer);

        /*
         * Check for bogus item number.  This is not treated as an error
         * condition because it can happen while following a t_ctid link. We
         * just assume that the prior tid is OK and return it unchanged.
         */
        offnum = ItemPointerGetOffsetNumber(&ctid);
        if (offnum < FirstOffsetNumber || offnum > PageGetMaxOffsetNumber(page))
        {
            UnlockReleaseBuffer(buffer);
            break;
        }
        lp = PageGetItemId(page, offnum);
        if (!ItemIdIsNormal(lp))
        {
            UnlockReleaseBuffer(buffer);
            break;
        }

        /* OK to access the tuple */
        tp.t_self = ctid;
        tp.t_data = (HeapTupleHeader) PageGetItem(page, lp);
        tp.t_len = ItemIdGetLength(lp);

        /*
         * After following a t_ctid link, we might arrive at an unrelated
         * tuple.  Check for XMIN match.
         */
        if (TransactionIdIsValid(priorXmax) &&
            !TransactionIdEquals(priorXmax, HeapTupleHeaderGetXmin(tp.t_data)))
        {
            UnlockReleaseBuffer(buffer);
            break;
        }

        /*
         * Check time qualification of tuple; if visible, set it as the new
         * result candidate.
         */
        valid = HeapTupleSatisfiesVisibility(&tp, snapshot, buffer);
        CheckForSerializableConflictOut(valid, relation, &tp, buffer, snapshot);
        if (valid)
            *tid = ctid;

        /*
         * If there's a valid t_ctid link, follow it, else we're done.
         */
        if ((tp.t_data->t_infomask & HEAP_XMAX_INVALID) ||
            HeapTupleHeaderIsOnlyLocked(tp.t_data) ||
            ItemPointerEquals(&tp.t_self, &tp.t_data->t_ctid))
        {
            UnlockReleaseBuffer(buffer);
            break;
        }

        ctid = tp.t_data->t_ctid;
        priorXmax = HeapTupleHeaderGetUpdateXid(tp.t_data);
        UnlockReleaseBuffer(buffer);
    }                           /* end of loop */
}


/*
 * UpdateXmaxHintBits - update tuple hint bits after xmax transaction ends
 *
 * This is called after we have waited for the XMAX transaction to terminate.
 * If the transaction aborted, we guarantee the XMAX_INVALID hint bit will
 * be set on exit.  If the transaction committed, we set the XMAX_COMMITTED
 * hint bit if possible --- but beware that that may not yet be possible,
 * if the transaction committed asynchronously.
 *
 * Note that if the transaction was a locker only, we set HEAP_XMAX_INVALID
 * even if it commits.
 *
 * Hence callers should look only at XMAX_INVALID.
 *
 * Note this is not allowed for tuples whose xmax is a multixact.
 */
static void
UpdateXmaxHintBits(HeapTupleHeader tuple, Buffer buffer, TransactionId xid)
{
    Assert(TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple), xid));
    Assert(!(tuple->t_infomask & HEAP_XMAX_IS_MULTI));

    if (!(tuple->t_infomask & (HEAP_XMAX_COMMITTED | HEAP_XMAX_INVALID)))
    {
        if (!HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask) &&
            TransactionIdDidCommit(xid))
            HeapTupleSetHintBits(tuple, buffer, HEAP_XMAX_COMMITTED,
                                 xid);
        else
            HeapTupleSetHintBits(tuple, buffer, HEAP_XMAX_INVALID,
                                 InvalidTransactionId);
    }
}


/*
 * GetBulkInsertState - prepare status object for a bulk insert
 */
BulkInsertState
GetBulkInsertState(void)
{
    BulkInsertState bistate;

    bistate = (BulkInsertState) palloc(sizeof(BulkInsertStateData));
    bistate->strategy = GetAccessStrategy(BAS_BULKWRITE);
    bistate->current_buf = InvalidBuffer;
    return bistate;
}

/*
 * FreeBulkInsertState - clean up after finishing a bulk insert
 */
void
FreeBulkInsertState(BulkInsertState bistate)
{
    if (bistate->current_buf != InvalidBuffer)
        ReleaseBuffer(bistate->current_buf);
    FreeAccessStrategy(bistate->strategy);
    pfree(bistate);
}


/*
 *  heap_insert     - insert tuple into a heap
 *
 * The new tuple is stamped with current transaction ID and the specified
 * command ID.
 *
 * If the HEAP_INSERT_SKIP_WAL option is specified, the new tuple is not
 * logged in WAL, even for a non-temp relation.  Safe usage of this behavior
 * requires that we arrange that all new tuples go into new pages not
 * containing any tuples from other transactions, and that the relation gets
 * fsync'd before commit.  (See also heap_sync() comments)
 *
 * The HEAP_INSERT_SKIP_FSM option is passed directly to
 * RelationGetBufferForTuple, which see for more info.
 *
 * HEAP_INSERT_FROZEN should only be specified for inserts into
 * relfilenodes created during the current subtransaction and when
 * there are no prior snapshots or pre-existing portals open.
 * This causes rows to be frozen, which is an MVCC violation and
 * requires explicit options chosen by user.
 *
 * Note that these options will be applied when inserting into the heap's
 * TOAST table, too, if the tuple requires any out-of-line data.
 *
 * The BulkInsertState object (if any; bistate can be NULL for default
 * behavior) is also just passed through to RelationGetBufferForTuple.
 *
 * The return value is the OID assigned to the tuple (either here or by the
 * caller), or InvalidOid if no OID.  The header fields of *tup are updated
 * to match the stored tuple; in particular tup->t_self receives the actual
 * TID where the tuple was stored.  But note that any toasting of fields
 * within the tuple data is NOT reflected into *tup.
 */
Oid
heap_insert(Relation relation, HeapTuple tup, CommandId cid,
            int options, BulkInsertState bistate)
{
    TransactionId xid = GetCurrentTransactionId();
    HeapTuple   heaptup;
    Buffer      buffer;
    Buffer      vmbuffer = InvalidBuffer;
    bool        all_visible_cleared = false;

    /*
     * Fill in tuple header fields, assign an OID, and toast the tuple if
     * necessary.
     *
     * Note: below this point, heaptup is the data we actually intend to store
     * into the relation; tup is the caller's original untoasted data.
     */
    heaptup = heap_prepare_insert(relation, tup, xid, cid, options);

    /*
     * We're about to do the actual insert -- but check for conflict first, to
     * avoid possibly having to roll back work we've just done.
     *
     * For a heap insert, we only need to check for table-level SSI locks. Our
     * new tuple can't possibly conflict with existing tuple locks, and heap
     * page locks are only consolidated versions of tuple locks; they do not
     * lock "gaps" as index page locks do.  So we don't need to identify a
     * buffer before making the call.
     */
    CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);

    /*
     * Find buffer to insert this tuple into.  If the page is all visible,
     * this will also pin the requisite visibility map page.
     */
    buffer = RelationGetBufferForTuple(relation, heaptup->t_len,
                                       InvalidBuffer, options, bistate,
                                       &vmbuffer, NULL);

    /* NO EREPORT(ERROR) from here till changes are logged */
    START_CRIT_SECTION();

    RelationPutHeapTuple(relation, buffer, heaptup);

    if (PageIsAllVisible(BufferGetPage(buffer)))
    {
        all_visible_cleared = true;
        PageClearAllVisible(BufferGetPage(buffer));
        visibilitymap_clear(relation,
                            ItemPointerGetBlockNumber(&(heaptup->t_self)),
                            vmbuffer);
    }

    /*
     * XXX Should we set PageSetPrunable on this page ?
     *
     * The inserting transaction may eventually abort thus making this tuple
     * DEAD and hence available for pruning. Though we don't want to optimize
     * for aborts, if no other tuple in this page is UPDATEd/DELETEd, the
     * aborted tuple will never be pruned until next vacuum is triggered.
     *
     * If you do add PageSetPrunable here, add it in heap_xlog_insert too.
     */

    MarkBufferDirty(buffer);

    /* XLOG stuff */
    if (!(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation))
    {
        xl_heap_insert xlrec;
        xl_heap_header xlhdr;
        XLogRecPtr  recptr;
        XLogRecData rdata[3];
        Page        page = BufferGetPage(buffer);
        uint8       info = XLOG_HEAP_INSERT;

        xlrec.all_visible_cleared = all_visible_cleared;
        xlrec.target.node = relation->rd_node;
        xlrec.target.tid = heaptup->t_self;
        rdata[0].data = (char *) &xlrec;
        rdata[0].len = SizeOfHeapInsert;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = &(rdata[1]);

        xlhdr.t_infomask2 = heaptup->t_data->t_infomask2;
        xlhdr.t_infomask = heaptup->t_data->t_infomask;
        xlhdr.t_hoff = heaptup->t_data->t_hoff;

        /*
         * note we mark rdata[1] as belonging to buffer; if XLogInsert decides
         * to write the whole page to the xlog, we don't need to store
         * xl_heap_header in the xlog.
         */
        rdata[1].data = (char *) &xlhdr;
        rdata[1].len = SizeOfHeapHeader;
        rdata[1].buffer = buffer;
        rdata[1].buffer_std = true;
        rdata[1].next = &(rdata[2]);

        /* PG73FORMAT: write bitmap [+ padding] [+ oid] + data */
        rdata[2].data = (char *) heaptup->t_data + offsetof(HeapTupleHeaderData, t_bits);
        rdata[2].len = heaptup->t_len - offsetof(HeapTupleHeaderData, t_bits);
        rdata[2].buffer = buffer;
        rdata[2].buffer_std = true;
        rdata[2].next = NULL;

        /*
         * If this is the single and first tuple on page, we can reinit the
         * page instead of restoring the whole thing.  Set flag, and hide
         * buffer references from XLogInsert.
         */
        if (ItemPointerGetOffsetNumber(&(heaptup->t_self)) == FirstOffsetNumber &&
            PageGetMaxOffsetNumber(page) == FirstOffsetNumber)
        {
            info |= XLOG_HEAP_INIT_PAGE;
            rdata[1].buffer = rdata[2].buffer = InvalidBuffer;
        }

        recptr = XLogInsert(RM_HEAP_ID, info, rdata);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();

    UnlockReleaseBuffer(buffer);
    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);

    /*
     * If tuple is cachable, mark it for invalidation from the caches in case
     * we abort.  Note it is OK to do this after releasing the buffer, because
     * the heaptup data structure is all in local memory, not in the shared
     * buffer.
     */
    CacheInvalidateHeapTuple(relation, heaptup, NULL);

    pgstat_count_heap_insert(relation, 1);

    /*
     * If heaptup is a private copy, release it.  Don't forget to copy t_self
     * back to the caller's image, too.
     */
    if (heaptup != tup)
    {
        tup->t_self = heaptup->t_self;
        heap_freetuple(heaptup);
    }

    return HeapTupleGetOid(tup);
}

/*
 * Subroutine for heap_insert(). Prepares a tuple for insertion. This sets the
 * tuple header fields, assigns an OID, and toasts the tuple if necessary.
 * Returns a toasted version of the tuple if it was toasted, or the original
 * tuple if not. Note that in any case, the header fields are also set in
 * the original tuple.
 */
static HeapTuple
heap_prepare_insert(Relation relation, HeapTuple tup, TransactionId xid,
                    CommandId cid, int options)
{
    if (relation->rd_rel->relhasoids)
    {
#ifdef NOT_USED
        /* this is redundant with an Assert in HeapTupleSetOid */
        Assert(tup->t_data->t_infomask & HEAP_HASOID);
#endif

        /*
         * If the object id of this tuple has already been assigned, trust the
         * caller.  There are a couple of ways this can happen.  At initial db
         * creation, the backend program sets oids for tuples. When we define
         * an index, we set the oid.  Finally, in the future, we may allow
         * users to set their own object ids in order to support a persistent
         * object store (objects need to contain pointers to one another).
         */
        if (!OidIsValid(HeapTupleGetOid(tup)))
            HeapTupleSetOid(tup, GetNewOid(relation));
    }
    else
    {
        /* check there is not space for an OID */
        Assert(!(tup->t_data->t_infomask & HEAP_HASOID));
    }

    tup->t_data->t_infomask &= ~(HEAP_XACT_MASK);
    tup->t_data->t_infomask2 &= ~(HEAP2_XACT_MASK);
    tup->t_data->t_infomask |= HEAP_XMAX_INVALID;
    if (options & HEAP_INSERT_FROZEN)
    {
        tup->t_data->t_infomask |= HEAP_XMIN_COMMITTED;
        HeapTupleHeaderSetXmin(tup->t_data, FrozenTransactionId);
    }
    else
        HeapTupleHeaderSetXmin(tup->t_data, xid);
    HeapTupleHeaderSetCmin(tup->t_data, cid);
    HeapTupleHeaderSetXmax(tup->t_data, 0);     /* for cleanliness */
    tup->t_tableOid = RelationGetRelid(relation);

    /*
     * If the new tuple is too big for storage or contains already toasted
     * out-of-line attributes from some other relation, invoke the toaster.
     */
    if (relation->rd_rel->relkind != RELKIND_RELATION &&
        relation->rd_rel->relkind != RELKIND_MATVIEW)
    {
        /* toast table entries should never be recursively toasted */
        Assert(!HeapTupleHasExternal(tup));
        return tup;
    }
    else if (HeapTupleHasExternal(tup) || tup->t_len > TOAST_TUPLE_THRESHOLD)
        return toast_insert_or_update(relation, tup, NULL, options);
    else
        return tup;
}

/*
 *  heap_multi_insert   - insert multiple tuple into a heap
 *
 * This is like heap_insert(), but inserts multiple tuples in one operation.
 * That's faster than calling heap_insert() in a loop, because when multiple
 * tuples can be inserted on a single page, we can write just a single WAL
 * record covering all of them, and only need to lock/unlock the page once.
 *
 * Note: this leaks memory into the current memory context. You can create a
 * temporary context before calling this, if that's a problem.
 */
void
heap_multi_insert(Relation relation, HeapTuple *tuples, int ntuples,
                  CommandId cid, int options, BulkInsertState bistate)
{
    TransactionId xid = GetCurrentTransactionId();
    HeapTuple  *heaptuples;
    int         i;
    int         ndone;
    char       *scratch = NULL;
    Page        page;
    bool        needwal;
    Size        saveFreeSpace;

    needwal = !(options & HEAP_INSERT_SKIP_WAL) && RelationNeedsWAL(relation);
    saveFreeSpace = RelationGetTargetPageFreeSpace(relation,
                                                   HEAP_DEFAULT_FILLFACTOR);

    /* Toast and set header data in all the tuples */
    heaptuples = palloc(ntuples * sizeof(HeapTuple));
    for (i = 0; i < ntuples; i++)
        heaptuples[i] = heap_prepare_insert(relation, tuples[i],
                                            xid, cid, options);

    /*
     * Allocate some memory to use for constructing the WAL record. Using
     * palloc() within a critical section is not safe, so we allocate this
     * beforehand.
     */
    if (needwal)
        scratch = palloc(BLCKSZ);

    /*
     * We're about to do the actual inserts -- but check for conflict first,
     * to avoid possibly having to roll back work we've just done.
     *
     * For a heap insert, we only need to check for table-level SSI locks. Our
     * new tuple can't possibly conflict with existing tuple locks, and heap
     * page locks are only consolidated versions of tuple locks; they do not
     * lock "gaps" as index page locks do.  So we don't need to identify a
     * buffer before making the call.
     */
    CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);

    ndone = 0;
    while (ndone < ntuples)
    {
        Buffer      buffer;
        Buffer      vmbuffer = InvalidBuffer;
        bool        all_visible_cleared = false;
        int         nthispage;

        /*
         * Find buffer where at least the next tuple will fit.  If the page is
         * all-visible, this will also pin the requisite visibility map page.
         */
        buffer = RelationGetBufferForTuple(relation, heaptuples[ndone]->t_len,
                                           InvalidBuffer, options, bistate,
                                           &vmbuffer, NULL);
        page = BufferGetPage(buffer);

        /* NO EREPORT(ERROR) from here till changes are logged */
        START_CRIT_SECTION();

        /*
         * RelationGetBufferForTuple has ensured that the first tuple fits.
         * Put that on the page, and then as many other tuples as fit.
         */
        RelationPutHeapTuple(relation, buffer, heaptuples[ndone]);
        for (nthispage = 1; ndone + nthispage < ntuples; nthispage++)
        {
            HeapTuple   heaptup = heaptuples[ndone + nthispage];

            if (PageGetHeapFreeSpace(page) < MAXALIGN(heaptup->t_len) + saveFreeSpace)
                break;

            RelationPutHeapTuple(relation, buffer, heaptup);
        }

        if (PageIsAllVisible(page))
        {
            all_visible_cleared = true;
            PageClearAllVisible(page);
            visibilitymap_clear(relation,
                                BufferGetBlockNumber(buffer),
                                vmbuffer);
        }

        /*
         * XXX Should we set PageSetPrunable on this page ? See heap_insert()
         */

        MarkBufferDirty(buffer);

        /* XLOG stuff */
        if (needwal)
        {
            XLogRecPtr  recptr;
            xl_heap_multi_insert *xlrec;
            XLogRecData rdata[2];
            uint8       info = XLOG_HEAP2_MULTI_INSERT;
            char       *tupledata;
            int         totaldatalen;
            char       *scratchptr = scratch;
            bool        init;

            /*
             * If the page was previously empty, we can reinit the page
             * instead of restoring the whole thing.
             */
            init = (ItemPointerGetOffsetNumber(&(heaptuples[ndone]->t_self)) == FirstOffsetNumber &&
                    PageGetMaxOffsetNumber(page) == FirstOffsetNumber + nthispage - 1);

            /* allocate xl_heap_multi_insert struct from the scratch area */
            xlrec = (xl_heap_multi_insert *) scratchptr;
            scratchptr += SizeOfHeapMultiInsert;

            /*
             * Allocate offsets array. Unless we're reinitializing the page,
             * in that case the tuples are stored in order starting at
             * FirstOffsetNumber and we don't need to store the offsets
             * explicitly.
             */
            if (!init)
                scratchptr += nthispage * sizeof(OffsetNumber);

            /* the rest of the scratch space is used for tuple data */
            tupledata = scratchptr;

            xlrec->all_visible_cleared = all_visible_cleared;
            xlrec->node = relation->rd_node;
            xlrec->blkno = BufferGetBlockNumber(buffer);
            xlrec->ntuples = nthispage;

            /*
             * Write out an xl_multi_insert_tuple and the tuple data itself
             * for each tuple.
             */
            for (i = 0; i < nthispage; i++)
            {
                HeapTuple   heaptup = heaptuples[ndone + i];
                xl_multi_insert_tuple *tuphdr;
                int         datalen;

                if (!init)
                    xlrec->offsets[i] = ItemPointerGetOffsetNumber(&heaptup->t_self);
                /* xl_multi_insert_tuple needs two-byte alignment. */
                tuphdr = (xl_multi_insert_tuple *) SHORTALIGN(scratchptr);
                scratchptr = ((char *) tuphdr) + SizeOfMultiInsertTuple;

                tuphdr->t_infomask2 = heaptup->t_data->t_infomask2;
                tuphdr->t_infomask = heaptup->t_data->t_infomask;
                tuphdr->t_hoff = heaptup->t_data->t_hoff;

                /* write bitmap [+ padding] [+ oid] + data */
                datalen = heaptup->t_len - offsetof(HeapTupleHeaderData, t_bits);
                memcpy(scratchptr,
                       (char *) heaptup->t_data + offsetof(HeapTupleHeaderData, t_bits),
                       datalen);
                tuphdr->datalen = datalen;
                scratchptr += datalen;
            }
            totaldatalen = scratchptr - tupledata;
            Assert((scratchptr - scratch) < BLCKSZ);

            rdata[0].data = (char *) xlrec;
            rdata[0].len = tupledata - scratch;
            rdata[0].buffer = InvalidBuffer;
            rdata[0].next = &rdata[1];

            rdata[1].data = tupledata;
            rdata[1].len = totaldatalen;
            rdata[1].buffer = buffer;
            rdata[1].buffer_std = true;
            rdata[1].next = NULL;

            /*
             * If we're going to reinitialize the whole page using the WAL
             * record, hide buffer reference from XLogInsert.
             */
            if (init)
            {
                rdata[1].buffer = InvalidBuffer;
                info |= XLOG_HEAP_INIT_PAGE;
            }

            recptr = XLogInsert(RM_HEAP2_ID, info, rdata);

            PageSetLSN(page, recptr);
        }

        END_CRIT_SECTION();

        UnlockReleaseBuffer(buffer);
        if (vmbuffer != InvalidBuffer)
            ReleaseBuffer(vmbuffer);

        ndone += nthispage;
    }

    /*
     * If tuples are cachable, mark them for invalidation from the caches in
     * case we abort.  Note it is OK to do this after releasing the buffer,
     * because the heaptuples data structure is all in local memory, not in
     * the shared buffer.
     */
    if (IsSystemRelation(relation))
    {
        for (i = 0; i < ntuples; i++)
            CacheInvalidateHeapTuple(relation, heaptuples[i], NULL);
    }

    /*
     * Copy t_self fields back to the caller's original tuples. This does
     * nothing for untoasted tuples (tuples[i] == heaptuples[i)], but it's
     * probably faster to always copy than check.
     */
    for (i = 0; i < ntuples; i++)
        tuples[i]->t_self = heaptuples[i]->t_self;

    pgstat_count_heap_insert(relation, ntuples);
}

/*
 *  simple_heap_insert - insert a tuple
 *
 * Currently, this routine differs from heap_insert only in supplying
 * a default command ID and not allowing access to the speedup options.
 *
 * This should be used rather than using heap_insert directly in most places
 * where we are modifying system catalogs.
 */
Oid
simple_heap_insert(Relation relation, HeapTuple tup)
{
    return heap_insert(relation, tup, GetCurrentCommandId(true), 0, NULL);
}

/*
 * Given infomask/infomask2, compute the bits that must be saved in the
 * "infobits" field of xl_heap_delete, xl_heap_update, xl_heap_lock,
 * xl_heap_lock_updated WAL records.
 *
 * See fix_infomask_from_infobits.
 */
static uint8
compute_infobits(uint16 infomask, uint16 infomask2)
{
    return
        ((infomask & HEAP_XMAX_IS_MULTI) != 0 ? XLHL_XMAX_IS_MULTI : 0) |
        ((infomask & HEAP_XMAX_LOCK_ONLY) != 0 ? XLHL_XMAX_LOCK_ONLY : 0) |
        ((infomask & HEAP_XMAX_EXCL_LOCK) != 0 ? XLHL_XMAX_EXCL_LOCK : 0) |
        /* note we ignore HEAP_XMAX_SHR_LOCK here */
        ((infomask & HEAP_XMAX_KEYSHR_LOCK) != 0 ? XLHL_XMAX_KEYSHR_LOCK : 0) |
        ((infomask2 & HEAP_KEYS_UPDATED) != 0 ?
         XLHL_KEYS_UPDATED : 0);
}

/*
 *  heap_delete - delete a tuple
 *
 * NB: do not call this directly unless you are prepared to deal with
 * concurrent-update conditions.  Use simple_heap_delete instead.
 *
 *  relation - table to be modified (caller must hold suitable lock)
 *  tid - TID of tuple to be deleted
 *  cid - delete command ID (used for visibility test, and stored into
 *      cmax if successful)
 *  crosscheck - if not InvalidSnapshot, also check tuple against this
 *  wait - true if should wait for any conflicting update to commit/abort
 *  hufd - output parameter, filled in failure cases (see below)
 *
 * Normal, successful return value is HeapTupleMayBeUpdated, which
 * actually means we did delete it.  Failure return codes are
 * HeapTupleSelfUpdated, HeapTupleUpdated, or HeapTupleBeingUpdated
 * (the last only possible if wait == false).
 *
 * In the failure cases, the routine fills *hufd with the tuple's t_ctid,
 * t_xmax (resolving a possible MultiXact, if necessary), and t_cmax
 * (the last only for HeapTupleSelfUpdated, since we
 * cannot obtain cmax from a combocid generated by another transaction).
 * See comments for struct HeapUpdateFailureData for additional info.
 */
HTSU_Result
heap_delete(Relation relation, ItemPointer tid,
            CommandId cid, Snapshot crosscheck, bool wait,
            HeapUpdateFailureData *hufd)
{
    HTSU_Result result;
    TransactionId xid = GetCurrentTransactionId();
    ItemId      lp;
    HeapTupleData tp;
    Page        page;
    BlockNumber block;
    Buffer      buffer;
    Buffer      vmbuffer = InvalidBuffer;
    TransactionId new_xmax;
    uint16      new_infomask,
                new_infomask2;
    bool        have_tuple_lock = false;
    bool        iscombo;
    bool        all_visible_cleared = false;

    Assert(ItemPointerIsValid(tid));

    block = ItemPointerGetBlockNumber(tid);
    buffer = ReadBuffer(relation, block);
    page = BufferGetPage(buffer);

    /*
     * Before locking the buffer, pin the visibility map page if it appears to
     * be necessary.  Since we haven't got the lock yet, someone else might be
     * in the middle of changing this, so we'll need to recheck after we have
     * the lock.
     */
    if (PageIsAllVisible(page))
        visibilitymap_pin(relation, block, &vmbuffer);

    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

    /*
     * If we didn't pin the visibility map page and the page has become all
     * visible while we were busy locking the buffer, we'll have to unlock and
     * re-lock, to avoid holding the buffer lock across an I/O.  That's a bit
     * unfortunate, but hopefully shouldn't happen often.
     */
    if (vmbuffer == InvalidBuffer && PageIsAllVisible(page))
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        visibilitymap_pin(relation, block, &vmbuffer);
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
    }

    lp = PageGetItemId(page, ItemPointerGetOffsetNumber(tid));
    Assert(ItemIdIsNormal(lp));

    tp.t_data = (HeapTupleHeader) PageGetItem(page, lp);
    tp.t_len = ItemIdGetLength(lp);
    tp.t_self = *tid;

l1:
    result = HeapTupleSatisfiesUpdate(tp.t_data, cid, buffer);

    if (result == HeapTupleInvisible)
    {
        UnlockReleaseBuffer(buffer);
        elog(ERROR, "attempted to delete invisible tuple");
    }
    else if (result == HeapTupleBeingUpdated && wait)
    {
        TransactionId xwait;
        uint16      infomask;

        /* must copy state data before unlocking buffer */
        xwait = HeapTupleHeaderGetRawXmax(tp.t_data);
        infomask = tp.t_data->t_infomask;

        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

        /*
         * Acquire tuple lock to establish our priority for the tuple (see
         * heap_lock_tuple).  LockTuple will release us when we are
         * next-in-line for the tuple.
         *
         * If we are forced to "start over" below, we keep the tuple lock;
         * this arranges that we stay at the head of the line while rechecking
         * tuple state.
         */
        if (!have_tuple_lock)
        {
            LockTupleTuplock(relation, &(tp.t_self), LockTupleExclusive);
            have_tuple_lock = true;
        }

        /*
         * Sleep until concurrent transaction ends.  Note that we don't care
         * which lock mode the locker has, because we need the strongest one.
         */

        if (infomask & HEAP_XMAX_IS_MULTI)
        {
            /* wait for multixact */
            MultiXactIdWait((MultiXactId) xwait, MultiXactStatusUpdate,
                            NULL, infomask);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

            /*
             * If xwait had just locked the tuple then some other xact could
             * update this tuple before we get to this point.  Check for xmax
             * change, and start over if so.
             */
            if (!(tp.t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tp.t_data),
                                     xwait))
                goto l1;

            /*
             * You might think the multixact is necessarily done here, but not
             * so: it could have surviving members, namely our own xact or
             * other subxacts of this backend.  It is legal for us to delete
             * the tuple in either case, however (the latter case is
             * essentially a situation of upgrading our former shared lock to
             * exclusive).  We don't bother changing the on-disk hint bits
             * since we are about to overwrite the xmax altogether.
             */
        }
        else
        {
            /* wait for regular transaction to end */
            XactLockTableWait(xwait);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

            /*
             * xwait is done, but if xwait had just locked the tuple then some
             * other xact could update this tuple before we get to this point.
             * Check for xmax change, and start over if so.
             */
            if ((tp.t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tp.t_data),
                                     xwait))
                goto l1;

            /* Otherwise check if it committed or aborted */
            UpdateXmaxHintBits(tp.t_data, buffer, xwait);
        }

        /*
         * We may overwrite if previous xmax aborted, or if it committed but
         * only locked the tuple without updating it.
         */
        if ((tp.t_data->t_infomask & HEAP_XMAX_INVALID) ||
            HEAP_XMAX_IS_LOCKED_ONLY(tp.t_data->t_infomask) ||
            HeapTupleHeaderIsOnlyLocked(tp.t_data))
            result = HeapTupleMayBeUpdated;
        else
            result = HeapTupleUpdated;
    }

    if (crosscheck != InvalidSnapshot && result == HeapTupleMayBeUpdated)
    {
        /* Perform additional check for transaction-snapshot mode RI updates */
        if (!HeapTupleSatisfiesVisibility(&tp, crosscheck, buffer))
            result = HeapTupleUpdated;
    }

    if (result != HeapTupleMayBeUpdated)
    {
        Assert(result == HeapTupleSelfUpdated ||
               result == HeapTupleUpdated ||
               result == HeapTupleBeingUpdated);
        Assert(!(tp.t_data->t_infomask & HEAP_XMAX_INVALID));
        hufd->ctid = tp.t_data->t_ctid;
        hufd->xmax = HeapTupleHeaderGetUpdateXid(tp.t_data);
        if (result == HeapTupleSelfUpdated)
            hufd->cmax = HeapTupleHeaderGetCmax(tp.t_data);
        else
            hufd->cmax = 0;     /* for lack of an InvalidCommandId value */
        UnlockReleaseBuffer(buffer);
        if (have_tuple_lock)
            UnlockTupleTuplock(relation, &(tp.t_self), LockTupleExclusive);
        if (vmbuffer != InvalidBuffer)
            ReleaseBuffer(vmbuffer);
        return result;
    }

    /*
     * We're about to do the actual delete -- check for conflict first, to
     * avoid possibly having to roll back work we've just done.
     */
    CheckForSerializableConflictIn(relation, &tp, buffer);

    /* replace cid with a combo cid if necessary */
    HeapTupleHeaderAdjustCmax(tp.t_data, &cid, &iscombo);

    START_CRIT_SECTION();

    /*
     * If this transaction commits, the tuple will become DEAD sooner or
     * later.  Set flag that this page is a candidate for pruning once our xid
     * falls below the OldestXmin horizon.  If the transaction finally aborts,
     * the subsequent page pruning will be a no-op and the hint will be
     * cleared.
     */
    PageSetPrunable(page, xid);

    if (PageIsAllVisible(page))
    {
        all_visible_cleared = true;
        PageClearAllVisible(page);
        visibilitymap_clear(relation, BufferGetBlockNumber(buffer),
                            vmbuffer);
    }

    /*
     * If this is the first possibly-multixact-able operation in the
     * current transaction, set my per-backend OldestMemberMXactId setting.
     * We can be certain that the transaction will never become a member of
     * any older MultiXactIds than that.  (We have to do this even if we
     * end up just using our own TransactionId below, since some other
     * backend could incorporate our XID into a MultiXact immediately
     * afterwards.)
     */
    MultiXactIdSetOldestMember();

    compute_new_xmax_infomask(HeapTupleHeaderGetRawXmax(tp.t_data),
                              tp.t_data->t_infomask, tp.t_data->t_infomask2,
                              xid, LockTupleExclusive, true,
                              &new_xmax, &new_infomask, &new_infomask2);

    /* store transaction information of xact deleting the tuple */
    tp.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
    tp.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
    tp.t_data->t_infomask |= new_infomask;
    tp.t_data->t_infomask2 |= new_infomask2;
    HeapTupleHeaderClearHotUpdated(tp.t_data);
    HeapTupleHeaderSetXmax(tp.t_data, new_xmax);
    HeapTupleHeaderSetCmax(tp.t_data, cid, iscombo);
    /* Make sure there is no forward chain link in t_ctid */
    tp.t_data->t_ctid = tp.t_self;

    MarkBufferDirty(buffer);

    /* XLOG stuff */
    if (RelationNeedsWAL(relation))
    {
        xl_heap_delete xlrec;
        XLogRecPtr  recptr;
        XLogRecData rdata[2];

        xlrec.all_visible_cleared = all_visible_cleared;
        xlrec.infobits_set = compute_infobits(tp.t_data->t_infomask,
                                              tp.t_data->t_infomask2);
        xlrec.target.node = relation->rd_node;
        xlrec.target.tid = tp.t_self;
        xlrec.xmax = new_xmax;
        rdata[0].data = (char *) &xlrec;
        rdata[0].len = SizeOfHeapDelete;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = &(rdata[1]);

        rdata[1].data = NULL;
        rdata[1].len = 0;
        rdata[1].buffer = buffer;
        rdata[1].buffer_std = true;
        rdata[1].next = NULL;

        recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_DELETE, rdata);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();

    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

    if (vmbuffer != InvalidBuffer)
        ReleaseBuffer(vmbuffer);

    /*
     * If the tuple has toasted out-of-line attributes, we need to delete
     * those items too.  We have to do this before releasing the buffer
     * because we need to look at the contents of the tuple, but it's OK to
     * release the content lock on the buffer first.
     */
    if (relation->rd_rel->relkind != RELKIND_RELATION &&
        relation->rd_rel->relkind != RELKIND_MATVIEW)
    {
        /* toast table entries should never be recursively toasted */
        Assert(!HeapTupleHasExternal(&tp));
    }
    else if (HeapTupleHasExternal(&tp))
        toast_delete(relation, &tp);

    /*
     * Mark tuple for invalidation from system caches at next command
     * boundary. We have to do this before releasing the buffer because we
     * need to look at the contents of the tuple.
     */
    CacheInvalidateHeapTuple(relation, &tp, NULL);

    /* Now we can release the buffer */
    ReleaseBuffer(buffer);

    /*
     * Release the lmgr tuple lock, if we had it.
     */
    if (have_tuple_lock)
        UnlockTupleTuplock(relation, &(tp.t_self), LockTupleExclusive);

    pgstat_count_heap_delete(relation);

    return HeapTupleMayBeUpdated;
}

/*
 *  simple_heap_delete - delete a tuple
 *
 * This routine may be used to delete a tuple when concurrent updates of
 * the target tuple are not expected (for example, because we have a lock
 * on the relation associated with the tuple).  Any failure is reported
 * via ereport().
 */
void
simple_heap_delete(Relation relation, ItemPointer tid)
{
    HTSU_Result result;
    HeapUpdateFailureData hufd;

    result = heap_delete(relation, tid,
                         GetCurrentCommandId(true), InvalidSnapshot,
                         true /* wait for commit */,
                         &hufd);
    switch (result)
    {
        case HeapTupleSelfUpdated:
            /* Tuple was already updated in current command? */
            elog(ERROR, "tuple already updated by self");
            break;

        case HeapTupleMayBeUpdated:
            /* done successfully */
            break;

        case HeapTupleUpdated:
            elog(ERROR, "tuple concurrently updated");
            break;

        default:
            elog(ERROR, "unrecognized heap_delete status: %u", result);
            break;
    }
}

/*
 *  heap_update - replace a tuple
 *
 * NB: do not call this directly unless you are prepared to deal with
 * concurrent-update conditions.  Use simple_heap_update instead.
 *
 *  relation - table to be modified (caller must hold suitable lock)
 *  otid - TID of old tuple to be replaced
 *  newtup - newly constructed tuple data to store
 *  cid - update command ID (used for visibility test, and stored into
 *      cmax/cmin if successful)
 *  crosscheck - if not InvalidSnapshot, also check old tuple against this
 *  wait - true if should wait for any conflicting update to commit/abort
 *  hufd - output parameter, filled in failure cases (see below)
 *  lockmode - output parameter, filled with lock mode acquired on tuple
 *
 * Normal, successful return value is HeapTupleMayBeUpdated, which
 * actually means we *did* update it.  Failure return codes are
 * HeapTupleSelfUpdated, HeapTupleUpdated, or HeapTupleBeingUpdated
 * (the last only possible if wait == false).
 *
 * On success, the header fields of *newtup are updated to match the new
 * stored tuple; in particular, newtup->t_self is set to the TID where the
 * new tuple was inserted, and its HEAP_ONLY_TUPLE flag is set iff a HOT
 * update was done.  However, any TOAST changes in the new tuple's
 * data are not reflected into *newtup.
 *
 * In the failure cases, the routine fills *hufd with the tuple's t_ctid,
 * t_xmax (resolving a possible MultiXact, if necessary), and t_cmax
 * (the last only for HeapTupleSelfUpdated, since we
 * cannot obtain cmax from a combocid generated by another transaction).
 * See comments for struct HeapUpdateFailureData for additional info.
 */
HTSU_Result
heap_update(Relation relation, ItemPointer otid, HeapTuple newtup,
            CommandId cid, Snapshot crosscheck, bool wait,
            HeapUpdateFailureData *hufd, LockTupleMode *lockmode)
{
    HTSU_Result result;
    TransactionId xid = GetCurrentTransactionId();
    Bitmapset  *hot_attrs;
    Bitmapset  *key_attrs;
    ItemId      lp;
    HeapTupleData oldtup;
    HeapTuple   heaptup;
    Page        page;
    BlockNumber block;
    MultiXactStatus mxact_status;
    Buffer      buffer,
                newbuf,
                vmbuffer = InvalidBuffer,
                vmbuffer_new = InvalidBuffer;
    bool        need_toast,
                already_marked;
    Size        newtupsize,
                pagefree;
    bool        have_tuple_lock = false;
    bool        iscombo;
    bool        satisfies_hot;
    bool        satisfies_key;
    bool        use_hot_update = false;
    bool        key_intact;
    bool        all_visible_cleared = false;
    bool        all_visible_cleared_new = false;
    bool        checked_lockers;
    bool        locker_remains;
    TransactionId xmax_new_tuple,
                  xmax_old_tuple;
    uint16      infomask_old_tuple,
                infomask2_old_tuple,
                infomask_new_tuple,
                infomask2_new_tuple;

    Assert(ItemPointerIsValid(otid));

    /*
     * Fetch the list of attributes to be checked for HOT update.  This is
     * wasted effort if we fail to update or have to put the new tuple on a
     * different page.  But we must compute the list before obtaining buffer
     * lock --- in the worst case, if we are doing an update on one of the
     * relevant system catalogs, we could deadlock if we try to fetch the list
     * later.  In any case, the relcache caches the data so this is usually
     * pretty cheap.
     *
     * Note that we get a copy here, so we need not worry about relcache flush
     * happening midway through.
     */
    hot_attrs = RelationGetIndexAttrBitmap(relation, false);
    key_attrs = RelationGetIndexAttrBitmap(relation, true);

    block = ItemPointerGetBlockNumber(otid);
    buffer = ReadBuffer(relation, block);
    page = BufferGetPage(buffer);

    /*
     * Before locking the buffer, pin the visibility map page if it appears to
     * be necessary.  Since we haven't got the lock yet, someone else might be
     * in the middle of changing this, so we'll need to recheck after we have
     * the lock.
     */
    if (PageIsAllVisible(page))
        visibilitymap_pin(relation, block, &vmbuffer);

    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

    lp = PageGetItemId(page, ItemPointerGetOffsetNumber(otid));
    Assert(ItemIdIsNormal(lp));

    /*
     * Fill in enough data in oldtup for HeapSatisfiesHOTandKeyUpdate to work
     * properly.
     */
    oldtup.t_tableOid = RelationGetRelid(relation);
    oldtup.t_data = (HeapTupleHeader) PageGetItem(page, lp);
    oldtup.t_len = ItemIdGetLength(lp);
    oldtup.t_self = *otid;

    /* the new tuple is ready, except for this: */
    newtup->t_tableOid = RelationGetRelid(relation);

    /* Fill in OID for newtup */
    if (relation->rd_rel->relhasoids)
    {
#ifdef NOT_USED
        /* this is redundant with an Assert in HeapTupleSetOid */
        Assert(newtup->t_data->t_infomask & HEAP_HASOID);
#endif
        HeapTupleSetOid(newtup, HeapTupleGetOid(&oldtup));
    }
    else
    {
        /* check there is not space for an OID */
        Assert(!(newtup->t_data->t_infomask & HEAP_HASOID));
    }

    /*
     * If we're not updating any "key" column, we can grab a weaker lock type.
     * This allows for more concurrency when we are running simultaneously with
     * foreign key checks.
     *
     * Note that if a column gets detoasted while executing the update, but the
     * value ends up being the same, this test will fail and we will use the
     * stronger lock.  This is acceptable; the important case to optimize is
     * updates that don't manipulate key columns, not those that
     * serendipitiously arrive at the same key values.
     */
    HeapSatisfiesHOTandKeyUpdate(relation, hot_attrs, key_attrs,
                                 &satisfies_hot, &satisfies_key,
                                 &oldtup, newtup);
    if (satisfies_key)
    {
        *lockmode = LockTupleNoKeyExclusive;
        mxact_status = MultiXactStatusNoKeyUpdate;
        key_intact = true;

        /*
         * If this is the first possibly-multixact-able operation in the
         * current transaction, set my per-backend OldestMemberMXactId setting.
         * We can be certain that the transaction will never become a member of
         * any older MultiXactIds than that.  (We have to do this even if we
         * end up just using our own TransactionId below, since some other
         * backend could incorporate our XID into a MultiXact immediately
         * afterwards.)
         */
        MultiXactIdSetOldestMember();
    }
    else
    {
        *lockmode = LockTupleExclusive;
        mxact_status = MultiXactStatusUpdate;
        key_intact = false;
    }

    /*
     * Note: beyond this point, use oldtup not otid to refer to old tuple.
     * otid may very well point at newtup->t_self, which we will overwrite
     * with the new tuple's location, so there's great risk of confusion if we
     * use otid anymore.
     */

l2:
    checked_lockers = false;
    locker_remains = false;
    result = HeapTupleSatisfiesUpdate(oldtup.t_data, cid, buffer);

    /* see below about the "no wait" case */
    Assert(result != HeapTupleBeingUpdated || wait);

    if (result == HeapTupleInvisible)
    {
        UnlockReleaseBuffer(buffer);
        elog(ERROR, "attempted to update invisible tuple");
    }
    else if (result == HeapTupleBeingUpdated && wait)
    {
        TransactionId   xwait;
        uint16      infomask;
        bool        can_continue = false;

        checked_lockers = true;

        /*
         * XXX note that we don't consider the "no wait" case here.  This
         * isn't a problem currently because no caller uses that case, but it
         * should be fixed if such a caller is introduced.  It wasn't a problem
         * previously because this code would always wait, but now that some
         * tuple locks do not conflict with one of the lock modes we use, it is
         * possible that this case is interesting to handle specially.
         *
         * This may cause failures with third-party code that calls heap_update
         * directly.
         */

        /* must copy state data before unlocking buffer */
        xwait = HeapTupleHeaderGetRawXmax(oldtup.t_data);
        infomask = oldtup.t_data->t_infomask;

        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

        /*
         * Acquire tuple lock to establish our priority for the tuple (see
         * heap_lock_tuple).  LockTuple will release us when we are
         * next-in-line for the tuple.
         *
         * If we are forced to "start over" below, we keep the tuple lock;
         * this arranges that we stay at the head of the line while rechecking
         * tuple state.
         */
        if (!have_tuple_lock)
        {
            LockTupleTuplock(relation, &(oldtup.t_self), *lockmode);
            have_tuple_lock = true;
        }

        /*
         * Now we have to do something about the existing locker.  If it's a
         * multi, sleep on it; we might be awakened before it is completely
         * gone (or even not sleep at all in some cases); we need to preserve
         * it as locker, unless it is gone completely.
         *
         * If it's not a multi, we need to check for sleeping conditions before
         * actually going to sleep.  If the update doesn't conflict with the
         * locks, we just continue without sleeping (but making sure it is
         * preserved).
         */
        if (infomask & HEAP_XMAX_IS_MULTI)
        {
            TransactionId   update_xact;
            int             remain;

            /* wait for multixact */
            MultiXactIdWait((MultiXactId) xwait, mxact_status, &remain,
                            infomask);
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

            /*
             * If xwait had just locked the tuple then some other xact could
             * update this tuple before we get to this point.  Check for xmax
             * change, and start over if so.
             */
            if (!(oldtup.t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                !TransactionIdEquals(HeapTupleHeaderGetRawXmax(oldtup.t_data),
                                     xwait))
                goto l2;

            /*
             * Note that the multixact may not be done by now.  It could have
             * surviving members; our own xact or other subxacts of this
             * backend, and also any other concurrent transaction that locked
             * the tuple with KeyShare if we only got TupleLockUpdate.  If this
             * is the case, we have to be careful to mark the updated tuple
             * with the surviving members in Xmax.
             *
             * Note that there could have been another update in the MultiXact.
             * In that case, we need to check whether it committed or aborted.
             * If it aborted we are safe to update it again; otherwise there is
             * an update conflict, and we have to return HeapTupleUpdated
             * below.
             *
             * In the LockTupleExclusive case, we still need to preserve the
             * surviving members: those would include the tuple locks we had
             * before this one, which are important to keep in case this
             * subxact aborts.
             */
            update_xact = InvalidTransactionId;
            if (!HEAP_XMAX_IS_LOCKED_ONLY(oldtup.t_data->t_infomask))
                update_xact = HeapTupleGetUpdateXid(oldtup.t_data);

            /* there was no UPDATE in the MultiXact; or it aborted. */
            if (!TransactionIdIsValid(update_xact) ||
                TransactionIdDidAbort(update_xact))
                can_continue = true;

            locker_remains = remain != 0;
        }
        else
        {
            /*
             * If it's just a key-share locker, and we're not changing the
             * key columns, we don't need to wait for it to end; but we
             * need to preserve it as locker.
             */
            if (HEAP_XMAX_IS_KEYSHR_LOCKED(infomask) && key_intact)
            {
                LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * recheck the locker; if someone else changed the tuple while we
                 * weren't looking, start over.
                 */
                if ((oldtup.t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                    !TransactionIdEquals(HeapTupleHeaderGetRawXmax(oldtup.t_data),
                                         xwait))
                    goto l2;

                can_continue = true;
                locker_remains = true;
            }
            else
            {
                /* wait for regular transaction to end */
                XactLockTableWait(xwait);
                LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * xwait is done, but if xwait had just locked the tuple then some
                 * other xact could update this tuple before we get to this point.
                 * Check for xmax change, and start over if so.
                 */
                if ((oldtup.t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                    !TransactionIdEquals(HeapTupleHeaderGetRawXmax(oldtup.t_data),
                                         xwait))
                    goto l2;

                /* Otherwise check if it committed or aborted */
                UpdateXmaxHintBits(oldtup.t_data, buffer, xwait);
                if (oldtup.t_data->t_infomask & HEAP_XMAX_INVALID)
                    can_continue = true;
            }
        }

        result = can_continue ? HeapTupleMayBeUpdated : HeapTupleUpdated;
    }

    if (crosscheck != InvalidSnapshot && result == HeapTupleMayBeUpdated)
    {
        /* Perform additional check for transaction-snapshot mode RI updates */
        if (!HeapTupleSatisfiesVisibility(&oldtup, crosscheck, buffer))
            result = HeapTupleUpdated;
    }

    if (result != HeapTupleMayBeUpdated)
    {
        Assert(result == HeapTupleSelfUpdated ||
               result == HeapTupleUpdated ||
               result == HeapTupleBeingUpdated);
        Assert(!(oldtup.t_data->t_infomask & HEAP_XMAX_INVALID));
        hufd->ctid = oldtup.t_data->t_ctid;
        hufd->xmax = HeapTupleHeaderGetUpdateXid(oldtup.t_data);
        if (result == HeapTupleSelfUpdated)
            hufd->cmax = HeapTupleHeaderGetCmax(oldtup.t_data);
        else
            hufd->cmax = 0;     /* for lack of an InvalidCommandId value */
        UnlockReleaseBuffer(buffer);
        if (have_tuple_lock)
            UnlockTupleTuplock(relation, &(oldtup.t_self), *lockmode);
        if (vmbuffer != InvalidBuffer)
            ReleaseBuffer(vmbuffer);
        bms_free(hot_attrs);
        bms_free(key_attrs);
        return result;
    }

    /*
     * If we didn't pin the visibility map page and the page has become all
     * visible while we were busy locking the buffer, or during some
     * subsequent window during which we had it unlocked, we'll have to unlock
     * and re-lock, to avoid holding the buffer lock across an I/O.  That's a
     * bit unfortunate, especially since we'll now have to recheck whether
     * the tuple has been locked or updated under us, but hopefully it won't
     * happen very often.
     */
    if (vmbuffer == InvalidBuffer && PageIsAllVisible(page))
    {
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
        visibilitymap_pin(relation, block, &vmbuffer);
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        goto l2;
    }

    /*
     * We're about to do the actual update -- check for conflict first, to
     * avoid possibly having to roll back work we've just done.
     */
    CheckForSerializableConflictIn(relation, &oldtup, buffer);

    /* Fill in transaction status data */

    /*
     * If the tuple we're updating is locked, we need to preserve the locking
     * info in the old tuple's Xmax.  Prepare a new Xmax value for this.
     */
    compute_new_xmax_infomask(HeapTupleHeaderGetRawXmax(oldtup.t_data),
                              oldtup.t_data->t_infomask,
                              oldtup.t_data->t_infomask2,
                              xid, *lockmode, true,
                              &xmax_old_tuple, &infomask_old_tuple,
                              &infomask2_old_tuple);

    /*
     * And also prepare an Xmax value for the new copy of the tuple.  If there
     * was no xmax previously, or there was one but all lockers are now gone,
     * then use InvalidXid; otherwise, get the xmax from the old tuple.  (In
     * rare cases that might also be InvalidXid and yet not have the
     * HEAP_XMAX_INVALID bit set; that's fine.)
     */
    if ((oldtup.t_data->t_infomask & HEAP_XMAX_INVALID) ||
        (checked_lockers && !locker_remains))
        xmax_new_tuple = InvalidTransactionId;
    else
        xmax_new_tuple = HeapTupleHeaderGetRawXmax(oldtup.t_data);

    if (!TransactionIdIsValid(xmax_new_tuple))
    {
        infomask_new_tuple = HEAP_XMAX_INVALID;
        infomask2_new_tuple = 0;
    }
    else
    {
        /*
         * If we found a valid Xmax for the new tuple, then the infomask bits
         * to use on the new tuple depend on what was there on the old one.
         * Note that since we're doing an update, the only possibility is that
         * the lockers had FOR KEY SHARE lock.
         */
        if (oldtup.t_data->t_infomask & HEAP_XMAX_IS_MULTI)
        {
            GetMultiXactIdHintBits(xmax_new_tuple, &infomask_new_tuple,
                                   &infomask2_new_tuple);
        }
        else
        {
            infomask_new_tuple = HEAP_XMAX_KEYSHR_LOCK | HEAP_XMAX_LOCK_ONLY;
            infomask2_new_tuple = 0;
        }
    }

    /*
     * Prepare the new tuple with the appropriate initial values of Xmin and
     * Xmax, as well as initial infomask bits as computed above.
     */
    newtup->t_data->t_infomask &= ~(HEAP_XACT_MASK);
    newtup->t_data->t_infomask2 &= ~(HEAP2_XACT_MASK);
    HeapTupleHeaderSetXmin(newtup->t_data, xid);
    HeapTupleHeaderSetCmin(newtup->t_data, cid);
    newtup->t_data->t_infomask |= HEAP_UPDATED | infomask_new_tuple;
    newtup->t_data->t_infomask2 |= infomask2_new_tuple;
    HeapTupleHeaderSetXmax(newtup->t_data, xmax_new_tuple);

    /*
     * Replace cid with a combo cid if necessary.  Note that we already put
     * the plain cid into the new tuple.
     */
    HeapTupleHeaderAdjustCmax(oldtup.t_data, &cid, &iscombo);

    /*
     * If the toaster needs to be activated, OR if the new tuple will not fit
     * on the same page as the old, then we need to release the content lock
     * (but not the pin!) on the old tuple's buffer while we are off doing
     * TOAST and/or table-file-extension work.  We must mark the old tuple to
     * show that it's already being updated, else other processes may try to
     * update it themselves.
     *
     * We need to invoke the toaster if there are already any out-of-line
     * toasted values present, or if the new tuple is over-threshold.
     */
    if (relation->rd_rel->relkind != RELKIND_RELATION &&
        relation->rd_rel->relkind != RELKIND_MATVIEW)
    {
        /* toast table entries should never be recursively toasted */
        Assert(!HeapTupleHasExternal(&oldtup));
        Assert(!HeapTupleHasExternal(newtup));
        need_toast = false;
    }
    else
        need_toast = (HeapTupleHasExternal(&oldtup) ||
                      HeapTupleHasExternal(newtup) ||
                      newtup->t_len > TOAST_TUPLE_THRESHOLD);

    pagefree = PageGetHeapFreeSpace(page);

    newtupsize = MAXALIGN(newtup->t_len);

    if (need_toast || newtupsize > pagefree)
    {
        /* Clear obsolete visibility flags ... */
        oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
        oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
        HeapTupleClearHotUpdated(&oldtup);
        /* ... and store info about transaction updating this tuple */
        Assert(TransactionIdIsValid(xmax_old_tuple));
        HeapTupleHeaderSetXmax(oldtup.t_data, xmax_old_tuple);
        oldtup.t_data->t_infomask |= infomask_old_tuple;
        oldtup.t_data->t_infomask2 |= infomask2_old_tuple;
        HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);
        /* temporarily make it look not-updated */
        oldtup.t_data->t_ctid = oldtup.t_self;
        already_marked = true;
        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

        /*
         * Let the toaster do its thing, if needed.
         *
         * Note: below this point, heaptup is the data we actually intend to
         * store into the relation; newtup is the caller's original untoasted
         * data.
         */
        if (need_toast)
        {
            /* Note we always use WAL and FSM during updates */
            heaptup = toast_insert_or_update(relation, newtup, &oldtup, 0);
            newtupsize = MAXALIGN(heaptup->t_len);
        }
        else
            heaptup = newtup;

        /*
         * Now, do we need a new page for the tuple, or not?  This is a bit
         * tricky since someone else could have added tuples to the page while
         * we weren't looking.  We have to recheck the available space after
         * reacquiring the buffer lock.  But don't bother to do that if the
         * former amount of free space is still not enough; it's unlikely
         * there's more free now than before.
         *
         * What's more, if we need to get a new page, we will need to acquire
         * buffer locks on both old and new pages.  To avoid deadlock against
         * some other backend trying to get the same two locks in the other
         * order, we must be consistent about the order we get the locks in.
         * We use the rule "lock the lower-numbered page of the relation
         * first".  To implement this, we must do RelationGetBufferForTuple
         * while not holding the lock on the old page, and we must rely on it
         * to get the locks on both pages in the correct order.
         */
        if (newtupsize > pagefree)
        {
            /* Assume there's no chance to put heaptup on same page. */
            newbuf = RelationGetBufferForTuple(relation, heaptup->t_len,
                                               buffer, 0, NULL,
                                               &vmbuffer_new, &vmbuffer);
        }
        else
        {
            /* Re-acquire the lock on the old tuple's page. */
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
            /* Re-check using the up-to-date free space */
            pagefree = PageGetHeapFreeSpace(page);
            if (newtupsize > pagefree)
            {
                /*
                 * Rats, it doesn't fit anymore.  We must now unlock and
                 * relock to avoid deadlock.  Fortunately, this path should
                 * seldom be taken.
                 */
                LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
                newbuf = RelationGetBufferForTuple(relation, heaptup->t_len,
                                                   buffer, 0, NULL,
                                                   &vmbuffer_new, &vmbuffer);
            }
            else
            {
                /* OK, it fits here, so we're done. */
                newbuf = buffer;
            }
        }
    }
    else
    {
        /* No TOAST work needed, and it'll fit on same page */
        already_marked = false;
        newbuf = buffer;
        heaptup = newtup;
    }

    /*
     * We're about to create the new tuple -- check for conflict first, to
     * avoid possibly having to roll back work we've just done.
     *
     * NOTE: For a tuple insert, we only need to check for table locks, since
     * predicate locking at the index level will cover ranges for anything
     * except a table scan.  Therefore, only provide the relation.
     */
    CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);

    /*
     * At this point newbuf and buffer are both pinned and locked, and newbuf
     * has enough space for the new tuple.  If they are the same buffer, only
     * one pin is held.
     */

    if (newbuf == buffer)
    {
        /*
         * Since the new tuple is going into the same page, we might be able
         * to do a HOT update.  Check if any of the index columns have been
         * changed.  If not, then HOT update is possible.
         */
        if (satisfies_hot)
            use_hot_update = true;
    }
    else
    {
        /* Set a hint that the old page could use prune/defrag */
        PageSetFull(page);
    }

    /* NO EREPORT(ERROR) from here till changes are logged */
    START_CRIT_SECTION();

    /*
     * If this transaction commits, the old tuple will become DEAD sooner or
     * later.  Set flag that this page is a candidate for pruning once our xid
     * falls below the OldestXmin horizon.  If the transaction finally aborts,
     * the subsequent page pruning will be a no-op and the hint will be
     * cleared.
     *
     * XXX Should we set hint on newbuf as well?  If the transaction aborts,
     * there would be a prunable tuple in the newbuf; but for now we choose
     * not to optimize for aborts.  Note that heap_xlog_update must be kept in
     * sync if this decision changes.
     */
    PageSetPrunable(page, xid);

    if (use_hot_update)
    {
        /* Mark the old tuple as HOT-updated */
        HeapTupleSetHotUpdated(&oldtup);
        /* And mark the new tuple as heap-only */
        HeapTupleSetHeapOnly(heaptup);
        /* Mark the caller's copy too, in case different from heaptup */
        HeapTupleSetHeapOnly(newtup);
    }
    else
    {
        /* Make sure tuples are correctly marked as not-HOT */
        HeapTupleClearHotUpdated(&oldtup);
        HeapTupleClearHeapOnly(heaptup);
        HeapTupleClearHeapOnly(newtup);
    }

    RelationPutHeapTuple(relation, newbuf, heaptup);    /* insert new tuple */

    if (!already_marked)
    {
        /* Clear obsolete visibility flags ... */
        oldtup.t_data->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
        oldtup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
        /* ... and store info about transaction updating this tuple */
        Assert(TransactionIdIsValid(xmax_old_tuple));
        HeapTupleHeaderSetXmax(oldtup.t_data, xmax_old_tuple);
        oldtup.t_data->t_infomask |= infomask_old_tuple;
        oldtup.t_data->t_infomask2 |= infomask2_old_tuple;
        HeapTupleHeaderSetCmax(oldtup.t_data, cid, iscombo);
    }

    /* record address of new tuple in t_ctid of old one */
    oldtup.t_data->t_ctid = heaptup->t_self;

    /* clear PD_ALL_VISIBLE flags */
    if (PageIsAllVisible(BufferGetPage(buffer)))
    {
        all_visible_cleared = true;
        PageClearAllVisible(BufferGetPage(buffer));
        visibilitymap_clear(relation, BufferGetBlockNumber(buffer),
                            vmbuffer);
    }
    if (newbuf != buffer && PageIsAllVisible(BufferGetPage(newbuf)))
    {
        all_visible_cleared_new = true;
        PageClearAllVisible(BufferGetPage(newbuf));
        visibilitymap_clear(relation, BufferGetBlockNumber(newbuf),
                            vmbuffer_new);
    }

    if (newbuf != buffer)
        MarkBufferDirty(newbuf);
    MarkBufferDirty(buffer);

    /* XLOG stuff */
    if (RelationNeedsWAL(relation))
    {
        XLogRecPtr  recptr = log_heap_update(relation, buffer,
                                             newbuf, &oldtup, heaptup,
                                             all_visible_cleared,
                                             all_visible_cleared_new);

        if (newbuf != buffer)
        {
            PageSetLSN(BufferGetPage(newbuf), recptr);
        }
        PageSetLSN(BufferGetPage(buffer), recptr);
    }

    END_CRIT_SECTION();

    if (newbuf != buffer)
        LockBuffer(newbuf, BUFFER_LOCK_UNLOCK);
    LockBuffer(buffer, BUFFER_LOCK_UNLOCK);

    /*
     * Mark old tuple for invalidation from system caches at next command
     * boundary, and mark the new tuple for invalidation in case we abort. We
     * have to do this before releasing the buffer because oldtup is in the
     * buffer.  (heaptup is all in local memory, but it's necessary to process
     * both tuple versions in one call to inval.c so we can avoid redundant
     * sinval messages.)
     */
    CacheInvalidateHeapTuple(relation, &oldtup, heaptup);

    /* Now we can release the buffer(s) */
    if (newbuf != buffer)
        ReleaseBuffer(newbuf);
    ReleaseBuffer(buffer);
    if (BufferIsValid(vmbuffer_new))
        ReleaseBuffer(vmbuffer_new);
    if (BufferIsValid(vmbuffer))
        ReleaseBuffer(vmbuffer);

    /*
     * Release the lmgr tuple lock, if we had it.
     */
    if (have_tuple_lock)
        UnlockTupleTuplock(relation, &(oldtup.t_self), *lockmode);

    pgstat_count_heap_update(relation, use_hot_update);

    /*
     * If heaptup is a private copy, release it.  Don't forget to copy t_self
     * back to the caller's image, too.
     */
    if (heaptup != newtup)
    {
        newtup->t_self = heaptup->t_self;
        heap_freetuple(heaptup);
    }

    bms_free(hot_attrs);
    bms_free(key_attrs);

    return HeapTupleMayBeUpdated;
}

/*
 * Check if the specified attribute's value is same in both given tuples.
 * Subroutine for HeapSatisfiesHOTandKeyUpdate.
 */
static bool
heap_tuple_attr_equals(TupleDesc tupdesc, int attrnum,
                       HeapTuple tup1, HeapTuple tup2)
{
    Datum       value1,
                value2;
    bool        isnull1,
                isnull2;
    Form_pg_attribute att;

    /*
     * If it's a whole-tuple reference, say "not equal".  It's not really
     * worth supporting this case, since it could only succeed after a no-op
     * update, which is hardly a case worth optimizing for.
     */
    if (attrnum == 0)
        return false;

    /*
     * Likewise, automatically say "not equal" for any system attribute other
     * than OID and tableOID; we cannot expect these to be consistent in a HOT
     * chain, or even to be set correctly yet in the new tuple.
     */
    if (attrnum < 0)
    {
        if (attrnum != ObjectIdAttributeNumber &&
            attrnum != TableOidAttributeNumber)
            return false;
    }

    /*
     * Extract the corresponding values.  XXX this is pretty inefficient if
     * there are many indexed columns.  Should HeapSatisfiesHOTandKeyUpdate do a
     * single heap_deform_tuple call on each tuple, instead?  But that doesn't
     * work for system columns ...
     */
    value1 = heap_getattr(tup1, attrnum, tupdesc, &isnull1);
    value2 = heap_getattr(tup2, attrnum, tupdesc, &isnull2);

    /*
     * If one value is NULL and other is not, then they are certainly not
     * equal
     */
    if (isnull1 != isnull2)
        return false;

    /*
     * If both are NULL, they can be considered equal.
     */
    if (isnull1)
        return true;

    /*
     * We do simple binary comparison of the two datums.  This may be overly
     * strict because there can be multiple binary representations for the
     * same logical value.  But we should be OK as long as there are no false
     * positives.  Using a type-specific equality operator is messy because
     * there could be multiple notions of equality in different operator
     * classes; furthermore, we cannot safely invoke user-defined functions
     * while holding exclusive buffer lock.
     */
    if (attrnum <= 0)
    {
        /* The only allowed system columns are OIDs, so do this */
        return (DatumGetObjectId(value1) == DatumGetObjectId(value2));
    }
    else
    {
        Assert(attrnum <= tupdesc->natts);
        att = tupdesc->attrs[attrnum - 1];
        return datumIsEqual(value1, value2, att->attbyval, att->attlen);
    }
}

/*
 * Check which columns are being updated.
 *
 * This simultaneously checks conditions for HOT updates and for FOR KEY
 * SHARE updates.  Since much of the time they will be checking very similar
 * sets of columns, and doing the same tests on them, it makes sense to
 * optimize and do them together.
 *
 * We receive two bitmapsets comprising the two sets of columns we're
 * interested in.  Note these are destructively modified; that is OK since
 * this is invoked at most once in heap_update.
 *
 * hot_result is set to TRUE if it's okay to do a HOT update (i.e. it does not
 * modified indexed columns); key_result is set to TRUE if the update does not
 * modify columns used in the key.
 */
static void
HeapSatisfiesHOTandKeyUpdate(Relation relation,
                             Bitmapset *hot_attrs, Bitmapset *key_attrs,
                             bool *satisfies_hot, bool *satisfies_key,
                             HeapTuple oldtup, HeapTuple newtup)
{
    int     next_hot_attnum;
    int     next_key_attnum;
    bool    hot_result = true;
    bool    key_result = true;
    bool    key_done = false;
    bool    hot_done = false;

    next_hot_attnum = bms_first_member(hot_attrs);
    if (next_hot_attnum == -1)
        hot_done = true;
    else
        /* Adjust for system attributes */
        next_hot_attnum += FirstLowInvalidHeapAttributeNumber;

    next_key_attnum = bms_first_member(key_attrs);
    if (next_key_attnum == -1)
        key_done = true;
    else
        /* Adjust for system attributes */
        next_key_attnum += FirstLowInvalidHeapAttributeNumber;

    for (;;)
    {
        int     check_now;
        bool    changed;

        /* both bitmapsets are now empty */
        if (key_done && hot_done)
            break;

        /* XXX there's probably an easier way ... */
        if (hot_done)
            check_now = next_key_attnum;
        if (key_done)
            check_now = next_hot_attnum;
        else
            check_now = Min(next_hot_attnum, next_key_attnum);

        changed = !heap_tuple_attr_equals(RelationGetDescr(relation),
                                          check_now, oldtup, newtup);
        if (changed)
        {
            if (check_now == next_hot_attnum)
                hot_result = false;
            if (check_now == next_key_attnum)
                key_result = false;
        }

        /* if both are false now, we can stop checking */
        if (!hot_result && !key_result)
            break;

        if (check_now == next_hot_attnum)
        {
            next_hot_attnum = bms_first_member(hot_attrs);
            if (next_hot_attnum == -1)
                hot_done = true;
            else
                /* Adjust for system attributes */
                next_hot_attnum += FirstLowInvalidHeapAttributeNumber;
        }
        if (check_now == next_key_attnum)
        {
            next_key_attnum = bms_first_member(key_attrs);
            if (next_key_attnum == -1)
                key_done = true;
            else
                /* Adjust for system attributes */
                next_key_attnum += FirstLowInvalidHeapAttributeNumber;
        }
    }

    *satisfies_hot = hot_result;
    *satisfies_key = key_result;
}

/*
 *  simple_heap_update - replace a tuple
 *
 * This routine may be used to update a tuple when concurrent updates of
 * the target tuple are not expected (for example, because we have a lock
 * on the relation associated with the tuple).  Any failure is reported
 * via ereport().
 */
void
simple_heap_update(Relation relation, ItemPointer otid, HeapTuple tup)
{
    HTSU_Result result;
    HeapUpdateFailureData hufd;
    LockTupleMode lockmode;

    result = heap_update(relation, otid, tup,
                         GetCurrentCommandId(true), InvalidSnapshot,
                         true /* wait for commit */,
                         &hufd, &lockmode);
    switch (result)
    {
        case HeapTupleSelfUpdated:
            /* Tuple was already updated in current command? */
            elog(ERROR, "tuple already updated by self");
            break;

        case HeapTupleMayBeUpdated:
            /* done successfully */
            break;

        case HeapTupleUpdated:
            elog(ERROR, "tuple concurrently updated");
            break;

        default:
            elog(ERROR, "unrecognized heap_update status: %u", result);
            break;
    }
}


/*
 * Return the MultiXactStatus corresponding to the given tuple lock mode.
 */
static MultiXactStatus
get_mxact_status_for_lock(LockTupleMode mode, bool is_update)
{
    MultiXactStatus     retval;

    if (is_update)
        retval = tupleLockExtraInfo[mode].updstatus;
    else
        retval = tupleLockExtraInfo[mode].lockstatus;

    if (retval == -1)
        elog(ERROR, "invalid lock tuple mode %d/%s", mode,
             is_update ? "true" : "false");

    return retval;
}


/*
 *  heap_lock_tuple - lock a tuple in shared or exclusive mode
 *
 * Note that this acquires a buffer pin, which the caller must release.
 *
 * Input parameters:
 *  relation: relation containing tuple (caller must hold suitable lock)
 *  tuple->t_self: TID of tuple to lock (rest of struct need not be valid)
 *  cid: current command ID (used for visibility test, and stored into
 *      tuple's cmax if lock is successful)
 *  mode: indicates if shared or exclusive tuple lock is desired
 *  nowait: if true, ereport rather than blocking if lock not available
 *  follow_updates: if true, follow the update chain to also lock descendant
 *      tuples.
 *
 * Output parameters:
 *  *tuple: all fields filled in
 *  *buffer: set to buffer holding tuple (pinned but not locked at exit)
 *  *hufd: filled in failure cases (see below)
 *
 * Function result may be:
 *  HeapTupleMayBeUpdated: lock was successfully acquired
 *  HeapTupleSelfUpdated: lock failed because tuple updated by self
 *  HeapTupleUpdated: lock failed because tuple updated by other xact
 *
 * In the failure cases, the routine fills *hufd with the tuple's t_ctid,
 * t_xmax (resolving a possible MultiXact, if necessary), and t_cmax
 * (the last only for HeapTupleSelfUpdated, since we
 * cannot obtain cmax from a combocid generated by another transaction).
 * See comments for struct HeapUpdateFailureData for additional info.
 *
 * See README.tuplock for a thorough explanation of this mechanism.
 */
HTSU_Result
heap_lock_tuple(Relation relation, HeapTuple tuple,
                CommandId cid, LockTupleMode mode, bool nowait,
                bool follow_updates,
                Buffer *buffer, HeapUpdateFailureData *hufd)
{
    HTSU_Result result;
    ItemPointer tid = &(tuple->t_self);
    ItemId      lp;
    Page        page;
    TransactionId xid,
                xmax;
    uint16      old_infomask,
                new_infomask,
                new_infomask2;
    bool        have_tuple_lock = false;

    *buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(tid));
    LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

    page = BufferGetPage(*buffer);
    lp = PageGetItemId(page, ItemPointerGetOffsetNumber(tid));
    Assert(ItemIdIsNormal(lp));

    tuple->t_data = (HeapTupleHeader) PageGetItem(page, lp);
    tuple->t_len = ItemIdGetLength(lp);
    tuple->t_tableOid = RelationGetRelid(relation);

l3:
    result = HeapTupleSatisfiesUpdate(tuple->t_data, cid, *buffer);

    if (result == HeapTupleInvisible)
    {
        UnlockReleaseBuffer(*buffer);
        elog(ERROR, "attempted to lock invisible tuple");
    }
    else if (result == HeapTupleBeingUpdated)
    {
        TransactionId xwait;
        uint16      infomask;
        uint16      infomask2;
        bool        require_sleep;
        ItemPointerData t_ctid;

        /* must copy state data before unlocking buffer */
        xwait = HeapTupleHeaderGetRawXmax(tuple->t_data);
        infomask = tuple->t_data->t_infomask;
        infomask2 = tuple->t_data->t_infomask2;
        ItemPointerCopy(&tuple->t_data->t_ctid, &t_ctid);

        LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);

        /*
         * If any subtransaction of the current top transaction already holds a
         * lock as strong or stronger than what we're requesting, we
         * effectively hold the desired lock already.  We *must* succeed
         * without trying to take the tuple lock, else we will deadlock against
         * anyone wanting to acquire a stronger lock.
         */
        if (infomask & HEAP_XMAX_IS_MULTI)
        {
            int     i;
            int     nmembers;
            MultiXactMember *members;

            /*
             * We don't need to allow old multixacts here; if that had been the
             * case, HeapTupleSatisfiesUpdate would have returned MayBeUpdated
             * and we wouldn't be here.
             */
            nmembers = GetMultiXactIdMembers(xwait, &members, false);

            for (i = 0; i < nmembers; i++)
            {
                if (TransactionIdIsCurrentTransactionId(members[i].xid))
                {
                    LockTupleMode   membermode;

                    membermode = TUPLOCK_from_mxstatus(members[i].status);

                    if (membermode >= mode)
                    {
                        if (have_tuple_lock)
                            UnlockTupleTuplock(relation, tid, mode);

                        pfree(members);
                        return HeapTupleMayBeUpdated;
                    }
                }
            }

            pfree(members);
        }

        /*
         * Acquire tuple lock to establish our priority for the tuple.
         * LockTuple will release us when we are next-in-line for the tuple.
         * We must do this even if we are share-locking.
         *
         * If we are forced to "start over" below, we keep the tuple lock;
         * this arranges that we stay at the head of the line while rechecking
         * tuple state.
         */
        if (!have_tuple_lock)
        {
            if (nowait)
            {
                if (!ConditionalLockTupleTuplock(relation, tid, mode))
                    ereport(ERROR,
                            (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                             errmsg("could not obtain lock on row in relation \"%s\"",
                                    RelationGetRelationName(relation))));
            }
            else
                LockTupleTuplock(relation, tid, mode);
            have_tuple_lock = true;
        }

        /*
         * Initially assume that we will have to wait for the locking
         * transaction(s) to finish.  We check various cases below in which
         * this can be turned off.
         */
        require_sleep = true;
        if (mode == LockTupleKeyShare)
        {
            /*
             * If we're requesting KeyShare, and there's no update present, we
             * don't need to wait.  Even if there is an update, we can still
             * continue if the key hasn't been modified.
             *
             * However, if there are updates, we need to walk the update chain
             * to mark future versions of the row as locked, too.  That way, if
             * somebody deletes that future version, we're protected against
             * the key going away.  This locking of future versions could block
             * momentarily, if a concurrent transaction is deleting a key; or
             * it could return a value to the effect that the transaction
             * deleting the key has already committed.  So we do this before
             * re-locking the buffer; otherwise this would be prone to
             * deadlocks.
             *
             * Note that the TID we're locking was grabbed before we unlocked
             * the buffer.  For it to change while we're not looking, the other
             * properties we're testing for below after re-locking the buffer
             * would also change, in which case we would restart this loop
             * above.
             */
            if (!(infomask2 & HEAP_KEYS_UPDATED))
            {
                bool    updated;

                updated = !HEAP_XMAX_IS_LOCKED_ONLY(infomask);

                /*
                 * If there are updates, follow the update chain; bail out
                 * if that cannot be done.
                 */
                if (follow_updates && updated)
                {
                    HTSU_Result     res;

                    res = heap_lock_updated_tuple(relation, tuple, &t_ctid,
                                                  GetCurrentTransactionId(),
                                                  mode);
                    if (res != HeapTupleMayBeUpdated)
                    {
                        result = res;
                        /* recovery code expects to have buffer lock held */
                        LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                        goto failed;
                    }
                }

                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * Make sure it's still an appropriate lock, else start over.
                 * Also, if it wasn't updated before we released the lock, but
                 * is updated now, we start over too; the reason is that we now
                 * need to follow the update chain to lock the new versions.
                 */
                if (!HeapTupleHeaderIsOnlyLocked(tuple->t_data) &&
                    ((tuple->t_data->t_infomask2 & HEAP_KEYS_UPDATED) ||
                     !updated))
                    goto l3;

                /* Things look okay, so we can skip sleeping */
                require_sleep = false;

                /*
                 * Note we allow Xmax to change here; other updaters/lockers
                 * could have modified it before we grabbed the buffer lock.
                 * However, this is not a problem, because with the recheck we
                 * just did we ensure that they still don't conflict with the
                 * lock we want.
                 */
            }
        }
        else if (mode == LockTupleShare)
        {
            /*
             * If we're requesting Share, we can similarly avoid sleeping if
             * there's no update and no exclusive lock present.
             */
            if (HEAP_XMAX_IS_LOCKED_ONLY(infomask) &&
                !HEAP_XMAX_IS_EXCL_LOCKED(infomask))
            {
                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * Make sure it's still an appropriate lock, else start over.
                 * See above about allowing xmax to change.
                 */
                if (!HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_data->t_infomask) ||
                    HEAP_XMAX_IS_EXCL_LOCKED(tuple->t_data->t_infomask))
                    goto l3;
                require_sleep = false;
            }
        }
        else if (mode == LockTupleNoKeyExclusive)
        {
            /*
             * If we're requesting NoKeyExclusive, we might also be able to
             * avoid sleeping; just ensure that there's no other lock type than
             * KeyShare.  Note that this is a bit more involved than just
             * checking hint bits -- we need to expand the multixact to figure
             * out lock modes for each one (unless there was only one such
             * locker).
             */
            if (infomask & HEAP_XMAX_IS_MULTI)
            {
                int     nmembers;
                MultiXactMember *members;

                /*
                 * We don't need to allow old multixacts here; if that had been
                 * the case, HeapTupleSatisfiesUpdate would have returned
                 * MayBeUpdated and we wouldn't be here.
                 */
                nmembers = GetMultiXactIdMembers(xwait, &members, false);

                if (nmembers <= 0)
                {
                    /*
                     * No need to keep the previous xmax here. This is unlikely
                     * to happen.
                     */
                    require_sleep = false;
                }
                else
                {
                    int     i;
                    bool    allowed = true;

                    for (i = 0; i < nmembers; i++)
                    {
                        if (members[i].status != MultiXactStatusForKeyShare)
                        {
                            allowed = false;
                            break;
                        }
                    }
                    if (allowed)
                    {
                        /*
                         * if the xmax changed under us in the meantime, start
                         * over.
                         */
                        LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                        if (!(tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                            !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple->t_data),
                                                 xwait))
                        {
                            pfree(members);
                            goto l3;
                        }
                        /* otherwise, we're good */
                        require_sleep = false;
                    }

                    pfree(members);
                }
            }
            else if (HEAP_XMAX_IS_KEYSHR_LOCKED(infomask))
            {
                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

                /* if the xmax changed in the meantime, start over */
                if ((tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                    !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple->t_data),
                                         xwait))
                    goto l3;
                /* otherwise, we're good */
                require_sleep = false;
            }
        }

        /*
         * By here, we either have already acquired the buffer exclusive lock,
         * or we must wait for the locking transaction or multixact; so below
         * we ensure that we grab buffer lock after the sleep.
         */

        if (require_sleep)
        {
            if (infomask & HEAP_XMAX_IS_MULTI)
            {
                MultiXactStatus status = get_mxact_status_for_lock(mode, false);

                /* We only ever lock tuples, never update them */
                if (status >= MultiXactStatusNoKeyUpdate)
                    elog(ERROR, "invalid lock mode in heap_lock_tuple");

                /* wait for multixact to end */
                if (nowait)
                {
                    if (!ConditionalMultiXactIdWait((MultiXactId) xwait,
                                                    status, NULL, infomask))
                        ereport(ERROR,
                                (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                 errmsg("could not obtain lock on row in relation \"%s\"",
                                        RelationGetRelationName(relation))));
                }
                else
                    MultiXactIdWait((MultiXactId) xwait, status, NULL, infomask);

                /* if there are updates, follow the update chain */
                if (follow_updates &&
                    !HEAP_XMAX_IS_LOCKED_ONLY(infomask))
                {
                    HTSU_Result     res;

                    res = heap_lock_updated_tuple(relation, tuple, &t_ctid,
                                                  GetCurrentTransactionId(),
                                                  mode);
                    if (res != HeapTupleMayBeUpdated)
                    {
                        result = res;
                        /* recovery code expects to have buffer lock held */
                        LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                        goto failed;
                    }
                }

                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * If xwait had just locked the tuple then some other xact
                 * could update this tuple before we get to this point. Check
                 * for xmax change, and start over if so.
                 */
                if (!(tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                    !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple->t_data),
                                         xwait))
                    goto l3;

                /*
                 * Of course, the multixact might not be done here: if we're
                 * requesting a light lock mode, other transactions with light
                 * locks could still be alive, as well as locks owned by our
                 * own xact or other subxacts of this backend.  We need to
                 * preserve the surviving MultiXact members.  Note that it
                 * isn't absolutely necessary in the latter case, but doing so
                 * is simpler.
                 */
            }
            else
            {
                /* wait for regular transaction to end */
                if (nowait)
                {
                    if (!ConditionalXactLockTableWait(xwait))
                        ereport(ERROR,
                                (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                                 errmsg("could not obtain lock on row in relation \"%s\"",
                                        RelationGetRelationName(relation))));
                }
                else
                    XactLockTableWait(xwait);

                /* if there are updates, follow the update chain */
                if (follow_updates &&
                    !HEAP_XMAX_IS_LOCKED_ONLY(infomask))
                {
                    HTSU_Result     res;

                    res = heap_lock_updated_tuple(relation, tuple, &t_ctid,
                                                  GetCurrentTransactionId(),
                                                  mode);
                    if (res != HeapTupleMayBeUpdated)
                    {
                        result = res;
                        /* recovery code expects to have buffer lock held */
                        LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);
                        goto failed;
                    }
                }

                LockBuffer(*buffer, BUFFER_LOCK_EXCLUSIVE);

                /*
                 * xwait is done, but if xwait had just locked the tuple then
                 * some other xact could update this tuple before we get to
                 * this point.  Check for xmax change, and start over if so.
                 */
                if ((tuple->t_data->t_infomask & HEAP_XMAX_IS_MULTI) ||
                    !TransactionIdEquals(HeapTupleHeaderGetRawXmax(tuple->t_data),
                                         xwait))
                    goto l3;

                /*
                 * Otherwise check if it committed or aborted.  Note we cannot
                 * be here if the tuple was only locked by somebody who didn't
                 * conflict with us; that should have been handled above.  So
                 * that transaction must necessarily be gone by now.
                 */
                UpdateXmaxHintBits(tuple->t_data, *buffer, xwait);
            }
        }

        /* By here, we're certain that we hold buffer exclusive lock again */

        /*
         * We may lock if previous xmax aborted, or if it committed but only
         * locked the tuple without updating it; or if we didn't have to wait
         * at all for whatever reason.
         */
        if (!require_sleep ||
            (tuple->t_data->t_infomask & HEAP_XMAX_INVALID) ||
            HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_data->t_infomask) ||
            HeapTupleHeaderIsOnlyLocked(tuple->t_data))
            result = HeapTupleMayBeUpdated;
        else
            result = HeapTupleUpdated;
    }

failed:
    if (result != HeapTupleMayBeUpdated)
    {
        Assert(result == HeapTupleSelfUpdated || result == HeapTupleUpdated);
        Assert(!(tuple->t_data->t_infomask & HEAP_XMAX_INVALID));
        hufd->ctid = tuple->t_data->t_ctid;
        hufd->xmax = HeapTupleHeaderGetUpdateXid(tuple->t_data);
        if (result == HeapTupleSelfUpdated)
            hufd->cmax = HeapTupleHeaderGetCmax(tuple->t_data);
        else
            hufd->cmax = 0;     /* for lack of an InvalidCommandId value */
        LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
        if (have_tuple_lock)
            UnlockTupleTuplock(relation, tid, mode);
        return result;
    }

    xmax = HeapTupleHeaderGetRawXmax(tuple->t_data);
    old_infomask = tuple->t_data->t_infomask;

    /*
     * We might already hold the desired lock (or stronger), possibly under a
     * different subtransaction of the current top transaction.  If so, there
     * is no need to change state or issue a WAL record.  We already handled
     * the case where this is true for xmax being a MultiXactId, so now check
     * for cases where it is a plain TransactionId.
     *
     * Note in particular that this covers the case where we already hold
     * exclusive lock on the tuple and the caller only wants key share or share
     * lock. It would certainly not do to give up the exclusive lock.
     */
    if (!(old_infomask & (HEAP_XMAX_INVALID |
                          HEAP_XMAX_COMMITTED |
                          HEAP_XMAX_IS_MULTI)) &&
        (mode == LockTupleKeyShare ?
         (HEAP_XMAX_IS_KEYSHR_LOCKED(old_infomask) ||
          HEAP_XMAX_IS_SHR_LOCKED(old_infomask) ||
          HEAP_XMAX_IS_EXCL_LOCKED(old_infomask)) :
         mode == LockTupleShare ?
         (HEAP_XMAX_IS_SHR_LOCKED(old_infomask) ||
          HEAP_XMAX_IS_EXCL_LOCKED(old_infomask)) :
         (HEAP_XMAX_IS_EXCL_LOCKED(old_infomask))) &&
        TransactionIdIsCurrentTransactionId(xmax))
    {
        LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
        /* Probably can't hold tuple lock here, but may as well check */
        if (have_tuple_lock)
            UnlockTupleTuplock(relation, tid, mode);
        return HeapTupleMayBeUpdated;
    }

    /*
     * If this is the first possibly-multixact-able operation in the
     * current transaction, set my per-backend OldestMemberMXactId setting.
     * We can be certain that the transaction will never become a member of
     * any older MultiXactIds than that.  (We have to do this even if we
     * end up just using our own TransactionId below, since some other
     * backend could incorporate our XID into a MultiXact immediately
     * afterwards.)
     */
    MultiXactIdSetOldestMember();

    /*
     * Compute the new xmax and infomask to store into the tuple.  Note we do
     * not modify the tuple just yet, because that would leave it in the wrong
     * state if multixact.c elogs.
     */
    compute_new_xmax_infomask(xmax, old_infomask, tuple->t_data->t_infomask2,
                              GetCurrentTransactionId(), mode, false,
                              &xid, &new_infomask, &new_infomask2);

    START_CRIT_SECTION();

    /*
     * Store transaction information of xact locking the tuple.
     *
     * Note: Cmax is meaningless in this context, so don't set it; this avoids
     * possibly generating a useless combo CID.  Moreover, if we're locking a
     * previously updated tuple, it's important to preserve the Cmax.
     *
     * Also reset the HOT UPDATE bit, but only if there's no update; otherwise
     * we would break the HOT chain.
     */
    tuple->t_data->t_infomask &= ~HEAP_XMAX_BITS;
    tuple->t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
    tuple->t_data->t_infomask |= new_infomask;
    tuple->t_data->t_infomask2 |= new_infomask2;
    if (HEAP_XMAX_IS_LOCKED_ONLY(new_infomask))
        HeapTupleHeaderClearHotUpdated(tuple->t_data);
    HeapTupleHeaderSetXmax(tuple->t_data, xid);

    /*
     * Make sure there is no forward chain link in t_ctid.  Note that in the
     * cases where the tuple has been updated, we must not overwrite t_ctid,
     * because it was set by the updater.  Moreover, if the tuple has been
     * updated, we need to follow the update chain to lock the new versions
     * of the tuple as well.
     */
    if (HEAP_XMAX_IS_LOCKED_ONLY(new_infomask))
        tuple->t_data->t_ctid = *tid;

    MarkBufferDirty(*buffer);

    /*
     * XLOG stuff.  You might think that we don't need an XLOG record because
     * there is no state change worth restoring after a crash.  You would be
     * wrong however: we have just written either a TransactionId or a
     * MultiXactId that may never have been seen on disk before, and we need
     * to make sure that there are XLOG entries covering those ID numbers.
     * Else the same IDs might be re-used after a crash, which would be
     * disastrous if this page made it to disk before the crash.  Essentially
     * we have to enforce the WAL log-before-data rule even in this case.
     * (Also, in a PITR log-shipping or 2PC environment, we have to have XLOG
     * entries for everything anyway.)
     */
    if (RelationNeedsWAL(relation))
    {
        xl_heap_lock xlrec;
        XLogRecPtr  recptr;
        XLogRecData rdata[2];

        xlrec.target.node = relation->rd_node;
        xlrec.target.tid = tuple->t_self;
        xlrec.locking_xid = xid;
        xlrec.infobits_set = compute_infobits(new_infomask,
                                              tuple->t_data->t_infomask2);
        rdata[0].data = (char *) &xlrec;
        rdata[0].len = SizeOfHeapLock;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = &(rdata[1]);

        rdata[1].data = NULL;
        rdata[1].len = 0;
        rdata[1].buffer = *buffer;
        rdata[1].buffer_std = true;
        rdata[1].next = NULL;

        recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_LOCK, rdata);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();

    LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);

    /*
     * Don't update the visibility map here. Locking a tuple doesn't change
     * visibility info.
     */

    /*
     * Now that we have successfully marked the tuple as locked, we can
     * release the lmgr tuple lock, if we had it.
     */
    if (have_tuple_lock)
        UnlockTupleTuplock(relation, tid, mode);

    return HeapTupleMayBeUpdated;
}


/*
 * Given an original set of Xmax and infomask, and a transaction (identified by
 * add_to_xmax) acquiring a new lock of some mode, compute the new Xmax and
 * corresponding infomasks to use on the tuple.
 *
 * Note that this might have side effects such as creating a new MultiXactId.
 *
 * Most callers will have called HeapTupleSatisfiesUpdate before this function;
 * that will have set the HEAP_XMAX_INVALID bit if the xmax was a MultiXactId
 * but it was not running anymore. There is a race condition, which is that the
 * MultiXactId may have finished since then, but that uncommon case is handled
 * either here, or within MultiXactIdExpand.
 *
 * There is a similar race condition possible when the old xmax was a regular
 * TransactionId.  We test TransactionIdIsInProgress again just to narrow the
 * window, but it's still possible to end up creating an unnecessary
 * MultiXactId.  Fortunately this is harmless.
 */
static void
compute_new_xmax_infomask(TransactionId xmax, uint16 old_infomask,
                          uint16 old_infomask2, TransactionId add_to_xmax,
                          LockTupleMode mode, bool is_update,
                          TransactionId *result_xmax, uint16 *result_infomask,
                          uint16 *result_infomask2)
{
    TransactionId   new_xmax;
    uint16          new_infomask,
                    new_infomask2;

l5:
    new_infomask = 0;
    new_infomask2 = 0;
    if (old_infomask & HEAP_XMAX_INVALID)
    {
        /*
         * No previous locker; we just insert our own TransactionId.
         */
        if (is_update)
        {
            new_xmax = add_to_xmax;
            if (mode == LockTupleExclusive)
                new_infomask2 |= HEAP_KEYS_UPDATED;
        }
        else
        {
            new_infomask |= HEAP_XMAX_LOCK_ONLY;
            switch (mode)
            {
                case LockTupleKeyShare:
                    new_xmax = add_to_xmax;
                    new_infomask |= HEAP_XMAX_KEYSHR_LOCK;
                    break;
                case LockTupleShare:
                    new_xmax = add_to_xmax;
                    new_infomask |= HEAP_XMAX_SHR_LOCK;
                    break;
                case LockTupleNoKeyExclusive:
                    new_xmax = add_to_xmax;
                    new_infomask |= HEAP_XMAX_EXCL_LOCK;
                    break;
                case LockTupleExclusive:
                    new_xmax = add_to_xmax;
                    new_infomask |= HEAP_XMAX_EXCL_LOCK;
                    new_infomask2 |= HEAP_KEYS_UPDATED;
                    break;
                default:
                    new_xmax = InvalidTransactionId;    /* silence compiler */
                    elog(ERROR, "invalid lock mode");
            }
        }
    }
    else if (old_infomask & HEAP_XMAX_IS_MULTI)
    {
        MultiXactStatus     new_status;

        /*
         * Currently we don't allow XMAX_COMMITTED to be set for multis,
         * so cross-check.
         */
        Assert(!(old_infomask & HEAP_XMAX_COMMITTED));

        /*
         * A multixact together with LOCK_ONLY set but neither lock bit set
         * (i.e. a pg_upgraded share locked tuple) cannot possibly be running
         * anymore.  This check is critical for databases upgraded by
         * pg_upgrade; both MultiXactIdIsRunning and MultiXactIdExpand assume
         * that such multis are never passed.
         */
        if (!(old_infomask & HEAP_LOCK_MASK) &&
            HEAP_XMAX_IS_LOCKED_ONLY(old_infomask))
        {
            old_infomask &= ~HEAP_XMAX_IS_MULTI;
            old_infomask |= HEAP_XMAX_INVALID;
            goto l5;
        }

        /*
         * If the XMAX is already a MultiXactId, then we need to expand it to
         * include add_to_xmax; but if all the members were lockers and are all
         * gone, we can do away with the IS_MULTI bit and just set add_to_xmax
         * as the only locker/updater.  If all lockers are gone and we have an
         * updater that aborted, we can also do without a multi.
         *
         * The cost of doing GetMultiXactIdMembers would be paid by
         * MultiXactIdExpand if we weren't to do this, so this check is not
         * incurring extra work anyhow.
         */
        if (!MultiXactIdIsRunning(xmax))
        {
            if (HEAP_XMAX_IS_LOCKED_ONLY(old_infomask) ||
                TransactionIdDidAbort(MultiXactIdGetUpdateXid(xmax,
                                                              old_infomask)))
            {
                /*
                 * Reset these bits and restart; otherwise fall through to
                 * create a new multi below.
                 */
                old_infomask &= ~HEAP_XMAX_IS_MULTI;
                old_infomask |= HEAP_XMAX_INVALID;
                goto l5;
            }
        }

        new_status = get_mxact_status_for_lock(mode, is_update);

        new_xmax = MultiXactIdExpand((MultiXactId) xmax, add_to_xmax,
                                     new_status);
        GetMultiXactIdHintBits(new_xmax, &new_infomask, &new_infomask2);
    }
    else if (old_infomask & HEAP_XMAX_COMMITTED)
    {
        /*
         * It's a committed update, so we need to preserve him as updater of
         * the tuple.
         */
        MultiXactStatus     status;
        MultiXactStatus     new_status;

        if (old_infomask2 & HEAP_KEYS_UPDATED)
            status = MultiXactStatusUpdate;
        else
            status = MultiXactStatusNoKeyUpdate;

        new_status = get_mxact_status_for_lock(mode, is_update);
        /*
         * since it's not running, it's obviously impossible for the old
         * updater to be identical to the current one, so we need not check
         * for that case as we do in the block above.
         */
        new_xmax = MultiXactIdCreate(xmax, status, add_to_xmax, new_status);
        GetMultiXactIdHintBits(new_xmax, &new_infomask, &new_infomask2);
    }
    else if (TransactionIdIsInProgress(xmax))
    {
        /*
         * If the XMAX is a valid, in-progress TransactionId, then we need to
         * create a new MultiXactId that includes both the old locker or
         * updater and our own TransactionId.
         */
        MultiXactStatus     status;
        MultiXactStatus     new_status;

        if (HEAP_XMAX_IS_LOCKED_ONLY(old_infomask))
        {
            if (HEAP_XMAX_IS_KEYSHR_LOCKED(old_infomask))
                status = MultiXactStatusForKeyShare;
            else if (HEAP_XMAX_IS_SHR_LOCKED(old_infomask))
                status = MultiXactStatusForShare;
            else if (HEAP_XMAX_IS_EXCL_LOCKED(old_infomask))
            {
                if (old_infomask2 & HEAP_KEYS_UPDATED)
                    status = MultiXactStatusForUpdate;
                else
                    status = MultiXactStatusForNoKeyUpdate;
            }
            else
            {
                /*
                 * LOCK_ONLY can be present alone only when a page has been
                 * upgraded by pg_upgrade.  But in that case,
                 * TransactionIdIsInProgress() should have returned false.  We
                 * assume it's no longer locked in this case.
                 */
                elog(WARNING, "LOCK_ONLY found for Xid in progress %u", xmax);
                old_infomask |= HEAP_XMAX_INVALID;
                old_infomask &= ~HEAP_XMAX_LOCK_ONLY;
                goto l5;
            }
        }
        else
        {
            /* it's an update, but which kind? */
            if (old_infomask2 & HEAP_KEYS_UPDATED)
                status = MultiXactStatusUpdate;
            else
                status = MultiXactStatusNoKeyUpdate;
        }

        new_status = get_mxact_status_for_lock(mode, is_update);

        /*
         * If the existing lock mode is identical to or weaker than the new
         * one, we can act as though there is no existing lock, so set
         * XMAX_INVALID and restart.
         */
        if (xmax == add_to_xmax)
        {
            LockTupleMode   old_mode = TUPLOCK_from_mxstatus(status);
            bool            old_isupd = ISUPDATE_from_mxstatus(status);

            /*
             * We can do this if the new LockTupleMode is higher or equal than
             * the old one; and if there was previously an update, we need an
             * update, but if there wasn't, then we can accept there not being
             * one.
             */
            if ((mode >= old_mode) && (is_update || !old_isupd))
            {
                /*
                 * Note that the infomask might contain some other dirty bits.
                 * However, since the new infomask is reset to zero, we only
                 * set what's minimally necessary, and that the case that
                 * checks HEAP_XMAX_INVALID is the very first above, there is
                 * no need for extra cleanup of the infomask here.
                 */
                old_infomask |= HEAP_XMAX_INVALID;
                goto l5;
            }
        }
        new_xmax = MultiXactIdCreate(xmax, status, add_to_xmax, new_status);
        GetMultiXactIdHintBits(new_xmax, &new_infomask, &new_infomask2);
    }
    else if (!HEAP_XMAX_IS_LOCKED_ONLY(old_infomask) &&
             TransactionIdDidCommit(xmax))
    {
        /*
         * It's a committed update, so we gotta preserve him as updater of the
         * tuple.
         */
        MultiXactStatus     status;
        MultiXactStatus     new_status;

        if (old_infomask2 & HEAP_KEYS_UPDATED)
            status = MultiXactStatusUpdate;
        else
            status = MultiXactStatusNoKeyUpdate;

        new_status = get_mxact_status_for_lock(mode, is_update);
        /*
         * since it's not running, it's obviously impossible for the old
         * updater to be identical to the current one, so we need not check
         * for that case as we do in the block above.
         */
        new_xmax = MultiXactIdCreate(xmax, status, add_to_xmax, new_status);
        GetMultiXactIdHintBits(new_xmax, &new_infomask, &new_infomask2);
    }
    else
    {
        /*
         * Can get here iff the locking/updating transaction was running when
         * the infomask was extracted from the tuple, but finished before
         * TransactionIdIsInProgress got to run.  Deal with it as if there was
         * no locker at all in the first place.
         */
        old_infomask |= HEAP_XMAX_INVALID;
        goto l5;
    }

    *result_infomask = new_infomask;
    *result_infomask2 = new_infomask2;
    *result_xmax = new_xmax;
}


/*
 * Recursive part of heap_lock_updated_tuple
 *
 * Fetch the tuple pointed to by tid in rel, and mark it as locked by the given
 * xid with the given mode; if this tuple is updated, recurse to lock the new
 * version as well.
 */
static HTSU_Result
heap_lock_updated_tuple_rec(Relation rel, ItemPointer tid, TransactionId xid,
                            LockTupleMode mode)
{
    ItemPointerData tupid;
    HeapTupleData   mytup;
    Buffer          buf;
    uint16          new_infomask,
                    new_infomask2,
                    old_infomask;
    TransactionId   xmax,
                    new_xmax;

    ItemPointerCopy(tid, &tupid);

    for (;;)
    {
        new_infomask = 0;
        new_xmax = InvalidTransactionId;
        ItemPointerCopy(&tupid, &(mytup.t_self));

        if (!heap_fetch(rel, SnapshotAny, &mytup, &buf, false, NULL))
            elog(ERROR, "unable to fetch updated version of tuple");

l4:
        CHECK_FOR_INTERRUPTS();
        LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

        old_infomask = mytup.t_data->t_infomask;
        xmax = HeapTupleHeaderGetRawXmax(mytup.t_data);

        /*
         * If this tuple is updated and the key has been modified (or deleted),
         * what we do depends on the status of the updating transaction: if
         * it's live, we sleep until it finishes; if it has committed, we have
         * to fail (i.e. return HeapTupleUpdated); if it aborted, we ignore it.
         * For updates that didn't touch the key, we can just plough ahead.
         */
        if (!(old_infomask & HEAP_XMAX_INVALID) &&
            (mytup.t_data->t_infomask2 & HEAP_KEYS_UPDATED))
        {
            TransactionId   update_xid;

            /*
             * Note: we *must* check TransactionIdIsInProgress before
             * TransactionIdDidAbort/Commit; see comment at top of tqual.c for
             * an explanation.
             */
            update_xid = HeapTupleHeaderGetUpdateXid(mytup.t_data);
            if (TransactionIdIsCurrentTransactionId(update_xid))
            {
                UnlockReleaseBuffer(buf);
                return HeapTupleSelfUpdated;
            }
            else if (TransactionIdIsInProgress(update_xid))
            {
                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
                /* No LockTupleTuplock here -- see heap_lock_updated_tuple */
                XactLockTableWait(update_xid);
                goto l4;
            }
            else if (TransactionIdDidAbort(update_xid))
                ;   /* okay to proceed */
            else if (TransactionIdDidCommit(update_xid))
            {
                UnlockReleaseBuffer(buf);
                return HeapTupleUpdated;
            }
        }

        /* compute the new Xmax and infomask values for the tuple ... */
        compute_new_xmax_infomask(xmax, old_infomask, mytup.t_data->t_infomask2,
                                  xid, mode, false,
                                  &new_xmax, &new_infomask, &new_infomask2);

        START_CRIT_SECTION();

        /* ... and set them */
        HeapTupleHeaderSetXmax(mytup.t_data, new_xmax);
        mytup.t_data->t_infomask &= ~HEAP_XMAX_BITS;
        mytup.t_data->t_infomask2 &= ~HEAP_KEYS_UPDATED;
        mytup.t_data->t_infomask |= new_infomask;
        mytup.t_data->t_infomask2 |= new_infomask2;

        MarkBufferDirty(buf);

        /* XLOG stuff */
        if (RelationNeedsWAL(rel))
        {
            xl_heap_lock_updated xlrec;
            XLogRecPtr  recptr;
            XLogRecData rdata[2];
            Page        page = BufferGetPage(buf);

            xlrec.target.node = rel->rd_node;
            xlrec.target.tid = mytup.t_self;
            xlrec.xmax = new_xmax;
            xlrec.infobits_set = compute_infobits(new_infomask, new_infomask2);

            rdata[0].data = (char *) &xlrec;
            rdata[0].len = SizeOfHeapLockUpdated;
            rdata[0].buffer = InvalidBuffer;
            rdata[0].next = &(rdata[1]);

            rdata[1].data = NULL;
            rdata[1].len = 0;
            rdata[1].buffer = buf;
            rdata[1].buffer_std = true;
            rdata[1].next = NULL;

            recptr = XLogInsert(RM_HEAP2_ID, XLOG_HEAP2_LOCK_UPDATED, rdata);

            PageSetLSN(page, recptr);
        }

        END_CRIT_SECTION();

        /* if we find the end of update chain, we're done. */
        if (mytup.t_data->t_infomask & HEAP_XMAX_INVALID ||
            ItemPointerEquals(&mytup.t_self, &mytup.t_data->t_ctid)  ||
            HeapTupleHeaderIsOnlyLocked(mytup.t_data))
        {
            UnlockReleaseBuffer(buf);
            return HeapTupleMayBeUpdated;
        }

        /* tail recursion */
        ItemPointerCopy(&(mytup.t_data->t_ctid), &tupid);
        UnlockReleaseBuffer(buf);
    }
}

/*
 * heap_lock_updated_tuple
 *      Follow update chain when locking an updated tuple, acquiring locks (row
 *      marks) on the updated versions.
 *
 * The initial tuple is assumed to be already locked.
 *
 * This function doesn't check visibility, it just inconditionally marks the
 * tuple(s) as locked.  If any tuple in the updated chain is being deleted
 * concurrently (or updated with the key being modified), sleep until the
 * transaction doing it is finished.
 *
 * Note that we don't acquire heavyweight tuple locks on the tuples we walk
 * when we have to wait for other transactions to release them, as opposed to
 * what heap_lock_tuple does.  The reason is that having more than one
 * transaction walking the chain is probably uncommon enough that risk of
 * starvation is not likely: one of the preconditions for being here is that
 * the snapshot in use predates the update that created this tuple (because we
 * started at an earlier version of the tuple), but at the same time such a
 * transaction cannot be using repeatable read or serializable isolation
 * levels, because that would lead to a serializability failure.
 */
static HTSU_Result
heap_lock_updated_tuple(Relation rel, HeapTuple tuple, ItemPointer ctid,
                        TransactionId xid, LockTupleMode mode)
{
    if (!ItemPointerEquals(&tuple->t_self, ctid))
    {
        /*
         * If this is the first possibly-multixact-able operation in the
         * current transaction, set my per-backend OldestMemberMXactId setting.
         * We can be certain that the transaction will never become a member of
         * any older MultiXactIds than that.  (We have to do this even if we
         * end up just using our own TransactionId below, since some other
         * backend could incorporate our XID into a MultiXact immediately
         * afterwards.)
         */
        MultiXactIdSetOldestMember();

        return heap_lock_updated_tuple_rec(rel, ctid, xid, mode);
    }

    /* nothing to lock */
    return HeapTupleMayBeUpdated;
}


/*
 * heap_inplace_update - update a tuple "in place" (ie, overwrite it)
 *
 * Overwriting violates both MVCC and transactional safety, so the uses
 * of this function in Postgres are extremely limited.  Nonetheless we
 * find some places to use it.
 *
 * The tuple cannot change size, and therefore it's reasonable to assume
 * that its null bitmap (if any) doesn't change either.  So we just
 * overwrite the data portion of the tuple without touching the null
 * bitmap or any of the header fields.
 *
 * tuple is an in-memory tuple structure containing the data to be written
 * over the target tuple.  Also, tuple->t_self identifies the target tuple.
 */
void
heap_inplace_update(Relation relation, HeapTuple tuple)
{
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    ItemId      lp = NULL;
    HeapTupleHeader htup;
    uint32      oldlen;
    uint32      newlen;

    buffer = ReadBuffer(relation, ItemPointerGetBlockNumber(&(tuple->t_self)));
    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
    page = (Page) BufferGetPage(buffer);

    offnum = ItemPointerGetOffsetNumber(&(tuple->t_self));
    if (PageGetMaxOffsetNumber(page) >= offnum)
        lp = PageGetItemId(page, offnum);

    if (PageGetMaxOffsetNumber(page) < offnum || !ItemIdIsNormal(lp))
        elog(ERROR, "heap_inplace_update: invalid lp");

    htup = (HeapTupleHeader) PageGetItem(page, lp);

    oldlen = ItemIdGetLength(lp) - htup->t_hoff;
    newlen = tuple->t_len - tuple->t_data->t_hoff;
    if (oldlen != newlen || htup->t_hoff != tuple->t_data->t_hoff)
        elog(ERROR, "heap_inplace_update: wrong tuple length");

    /* NO EREPORT(ERROR) from here till changes are logged */
    START_CRIT_SECTION();

    memcpy((char *) htup + htup->t_hoff,
           (char *) tuple->t_data + tuple->t_data->t_hoff,
           newlen);

    MarkBufferDirty(buffer);

    /* XLOG stuff */
    if (RelationNeedsWAL(relation))
    {
        xl_heap_inplace xlrec;
        XLogRecPtr  recptr;
        XLogRecData rdata[2];

        xlrec.target.node = relation->rd_node;
        xlrec.target.tid = tuple->t_self;

        rdata[0].data = (char *) &xlrec;
        rdata[0].len = SizeOfHeapInplace;
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = &(rdata[1]);

        rdata[1].data = (char *) htup + htup->t_hoff;
        rdata[1].len = newlen;
        rdata[1].buffer = buffer;
        rdata[1].buffer_std = true;
        rdata[1].next = NULL;

        recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_INPLACE, rdata);

        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();

    UnlockReleaseBuffer(buffer);

    /*
     * Send out shared cache inval if necessary.  Note that because we only
     * pass the new version of the tuple, this mustn't be used for any
     * operations that could change catcache lookup keys.  But we aren't
     * bothering with index updates either, so that's true a fortiori.
     */
    if (!IsBootstrapProcessingMode())
        CacheInvalidateHeapTuple(relation, tuple, NULL);
}


/*
 * heap_freeze_tuple
 *
 * Check to see whether any of the XID fields of a tuple (xmin, xmax, xvac)
 * are older than the specified cutoff XID.  If so, replace them with
 * FrozenTransactionId or InvalidTransactionId as appropriate, and return
 * TRUE.  Return FALSE if nothing was changed.
 *
 * It is assumed that the caller has checked the tuple with
 * HeapTupleSatisfiesVacuum() and determined that it is not HEAPTUPLE_DEAD
 * (else we should be removing the tuple, not freezing it).
 *
 * NB: cutoff_xid *must* be <= the current global xmin, to ensure that any
 * XID older than it could neither be running nor seen as running by any
 * open transaction.  This ensures that the replacement will not change
 * anyone's idea of the tuple state.  Also, since we assume the tuple is
 * not HEAPTUPLE_DEAD, the fact that an XID is not still running allows us
 * to assume that it is either committed good or aborted, as appropriate;
 * so we need no external state checks to decide what to do.  (This is good
 * because this function is applied during WAL recovery, when we don't have
 * access to any such state, and can't depend on the hint bits to be set.)
 *
 * Similarly, cutoff_multi must be less than or equal to the smallest
 * MultiXactId used by any transaction currently open.
 *
 * If the tuple is in a shared buffer, caller must hold an exclusive lock on
 * that buffer.
 *
 * Note: it might seem we could make the changes without exclusive lock, since
 * TransactionId read/write is assumed atomic anyway.  However there is a race
 * condition: someone who just fetched an old XID that we overwrite here could
 * conceivably not finish checking the XID against pg_clog before we finish
 * the VACUUM and perhaps truncate off the part of pg_clog he needs.  Getting
 * exclusive lock ensures no other backend is in process of checking the
 * tuple status.  Also, getting exclusive lock makes it safe to adjust the
 * infomask bits.
 */
bool
heap_freeze_tuple(HeapTupleHeader tuple, TransactionId cutoff_xid,
                  MultiXactId cutoff_multi)
{
    bool        changed = false;
    TransactionId xid;

    xid = HeapTupleHeaderGetXmin(tuple);
    if (TransactionIdIsNormal(xid) &&
        TransactionIdPrecedes(xid, cutoff_xid))
    {
        HeapTupleHeaderSetXmin(tuple, FrozenTransactionId);

        /*
         * Might as well fix the hint bits too; usually XMIN_COMMITTED will
         * already be set here, but there's a small chance not.
         */
        Assert(!(tuple->t_infomask & HEAP_XMIN_INVALID));
        tuple->t_infomask |= HEAP_XMIN_COMMITTED;
        changed = true;
    }

    /*
     * Note that this code handles IS_MULTI Xmax values, too, but only to mark
     * the tuple frozen if the updating Xid in the mxact is below the freeze
     * cutoff; it doesn't remove dead members of a very old multixact.
     */
    xid = HeapTupleHeaderGetRawXmax(tuple);
    if ((tuple->t_infomask & HEAP_XMAX_IS_MULTI) ?
        (MultiXactIdIsValid(xid) &&
         MultiXactIdPrecedes(xid, cutoff_multi)) :
        (TransactionIdIsNormal(xid) &&
         TransactionIdPrecedes(xid, cutoff_xid)))
    {
        HeapTupleHeaderSetXmax(tuple, InvalidTransactionId);

        /*
         * The tuple might be marked either XMAX_INVALID or XMAX_COMMITTED
         * + LOCKED.  Normalize to INVALID just to be sure no one gets
         * confused.  Also get rid of the HEAP_KEYS_UPDATED bit.
         */
        tuple->t_infomask &= ~HEAP_XMAX_BITS;
        tuple->t_infomask |= HEAP_XMAX_INVALID;
        HeapTupleHeaderClearHotUpdated(tuple);
        tuple->t_infomask2 &= ~HEAP_KEYS_UPDATED;
        changed = true;
    }

    /*
     * Old-style VACUUM FULL is gone, but we have to keep this code as long as
     * we support having MOVED_OFF/MOVED_IN tuples in the database.
     */
    if (tuple->t_infomask & HEAP_MOVED)
    {
        xid = HeapTupleHeaderGetXvac(tuple);
        if (TransactionIdIsNormal(xid) &&
            TransactionIdPrecedes(xid, cutoff_xid))
        {
            /*
             * If a MOVED_OFF tuple is not dead, the xvac transaction must
             * have failed; whereas a non-dead MOVED_IN tuple must mean the
             * xvac transaction succeeded.
             */
            if (tuple->t_infomask & HEAP_MOVED_OFF)
                HeapTupleHeaderSetXvac(tuple, InvalidTransactionId);
            else
                HeapTupleHeaderSetXvac(tuple, FrozenTransactionId);

            /*
             * Might as well fix the hint bits too; usually XMIN_COMMITTED
             * will already be set here, but there's a small chance not.
             */
            Assert(!(tuple->t_infomask & HEAP_XMIN_INVALID));
            tuple->t_infomask |= HEAP_XMIN_COMMITTED;
            changed = true;
        }
    }

    return changed;
}

/*
 * For a given MultiXactId, return the hint bits that should be set in the
 * tuple's infomask.
 *
 * Normally this should be called for a multixact that was just created, and
 * so is on our local cache, so the GetMembers call is fast.
 */
static void
GetMultiXactIdHintBits(MultiXactId multi, uint16 *new_infomask,
                       uint16 *new_infomask2)
{
    int     nmembers;
    MultiXactMember *members;
    int     i;
    uint16  bits = HEAP_XMAX_IS_MULTI;
    uint16  bits2 = 0;
    bool    has_update = false;
    LockTupleMode   strongest = LockTupleKeyShare;

    /*
     * We only use this in multis we just created, so they cannot be values
     * pre-pg_upgrade.
     */
    nmembers = GetMultiXactIdMembers(multi, &members, false);

    for (i = 0; i < nmembers; i++)
    {
        LockTupleMode   mode;

        /*
         * Remember the strongest lock mode held by any member of the
         * multixact.
         */
        mode = TUPLOCK_from_mxstatus(members[i].status);
        if (mode > strongest)
            strongest = mode;

        /* See what other bits we need */
        switch (members[i].status)
        {
            case MultiXactStatusForKeyShare:
            case MultiXactStatusForShare:
            case MultiXactStatusForNoKeyUpdate:
                break;

            case MultiXactStatusForUpdate:
                bits2 |= HEAP_KEYS_UPDATED;
                break;

            case MultiXactStatusNoKeyUpdate:
                has_update = true;
                break;

            case MultiXactStatusUpdate:
                bits2 |= HEAP_KEYS_UPDATED;
                has_update = true;
                break;
        }
    }

    if (strongest == LockTupleExclusive ||
        strongest == LockTupleNoKeyExclusive)
        bits |= HEAP_XMAX_EXCL_LOCK;
    else if (strongest == LockTupleShare)
        bits |= HEAP_XMAX_SHR_LOCK;
    else if (strongest == LockTupleKeyShare)
        bits |= HEAP_XMAX_KEYSHR_LOCK;

    if (!has_update)
        bits |= HEAP_XMAX_LOCK_ONLY;

    if (nmembers > 0)
        pfree(members);

    *new_infomask = bits;
    *new_infomask2 = bits2;
}

/*
 * MultiXactIdGetUpdateXid
 *
 * Given a multixact Xmax and corresponding infomask, which does not have the
 * HEAP_XMAX_LOCK_ONLY bit set, obtain and return the Xid of the updating
 * transaction.
 */
static TransactionId
MultiXactIdGetUpdateXid(TransactionId xmax, uint16 t_infomask)
{
    TransactionId   update_xact = InvalidTransactionId;
    MultiXactMember *members;
    int             nmembers;

    Assert(!(t_infomask & HEAP_XMAX_LOCK_ONLY));
    Assert(t_infomask & HEAP_XMAX_IS_MULTI);

    /*
     * Since we know the LOCK_ONLY bit is not set, this cannot be a
     * multi from pre-pg_upgrade.
     */
    nmembers = GetMultiXactIdMembers(xmax, &members, false);

    if (nmembers > 0)
    {
        int     i;

        for (i = 0; i < nmembers; i++)
        {
            /* Ignore lockers */
            if (members[i].status == MultiXactStatusForKeyShare ||
                members[i].status == MultiXactStatusForShare ||
                members[i].status == MultiXactStatusForNoKeyUpdate ||
                members[i].status == MultiXactStatusForUpdate)
                continue;

            /* ignore aborted transactions */
            if (TransactionIdDidAbort(members[i].xid))
                continue;
            /* there should be at most one non-aborted updater */
            Assert(update_xact == InvalidTransactionId);
            Assert(members[i].status == MultiXactStatusNoKeyUpdate ||
                   members[i].status == MultiXactStatusUpdate);
            update_xact = members[i].xid;
#ifndef USE_ASSERT_CHECKING
            /*
             * in an assert-enabled build, walk the whole array to ensure
             * there's no other updater.
             */
            break;
#endif
        }

        pfree(members);
    }

    return update_xact;
}

/*
 * HeapTupleGetUpdateXid
 *      As above, but use a HeapTupleHeader
 *
 * See also HeapTupleHeaderGetUpdateXid, which can be used without previously
 * checking the hint bits.
 */
TransactionId
HeapTupleGetUpdateXid(HeapTupleHeader tuple)
{
    return MultiXactIdGetUpdateXid(HeapTupleHeaderGetRawXmax(tuple),
                                   tuple->t_infomask);
}

/*
 * Do_MultiXactIdWait
 *      Actual implementation for the two functions below.
 *
 * We do this by sleeping on each member using XactLockTableWait.  Any
 * members that belong to the current backend are *not* waited for, however;
 * this would not merely be useless but would lead to Assert failure inside
 * XactLockTableWait.  By the time this returns, it is certain that all
 * transactions *of other backends* that were members of the MultiXactId
 * that conflict with the requested status are dead (and no new ones can have
 * been added, since it is not legal to add members to an existing
 * MultiXactId).
 *
 * But by the time we finish sleeping, someone else may have changed the Xmax
 * of the containing tuple, so the caller needs to iterate on us somehow.
 *
 * Note that in case we return false, the number of remaining members is
 * not to be trusted.
 */
static bool
Do_MultiXactIdWait(MultiXactId multi, MultiXactStatus status,
                   int *remaining, uint16 infomask, bool nowait)
{
    bool        allow_old;
    bool        result = true;
    MultiXactMember *members;
    int         nmembers;
    int         remain = 0;

    allow_old = !(infomask & HEAP_LOCK_MASK) && HEAP_XMAX_IS_LOCKED_ONLY(infomask);
    nmembers = GetMultiXactIdMembers(multi, &members, allow_old);

    if (nmembers >= 0)
    {
        int         i;

        for (i = 0; i < nmembers; i++)
        {
            TransactionId memxid = members[i].xid;
            MultiXactStatus memstatus = members[i].status;

            if (TransactionIdIsCurrentTransactionId(memxid))
            {
                remain++;
                continue;
            }

            if (!DoLockModesConflict(LOCKMODE_from_mxstatus(memstatus),
                                     LOCKMODE_from_mxstatus(status)))
            {
                if (remaining && TransactionIdIsInProgress(memxid))
                    remain++;
                continue;
            }

            /*
             * This member conflicts with our multi, so we have to sleep (or
             * return failure, if asked to avoid waiting.)
             */
            if (nowait)
            {
                result = ConditionalXactLockTableWait(memxid);
                if (!result)
                    break;
            }
            else
                XactLockTableWait(memxid);
        }

        pfree(members);
    }

    if (remaining)
        *remaining = remain;

    return result;
}

/*
 * MultiXactIdWait
 *      Sleep on a MultiXactId.
 *
 * By the time we finish sleeping, someone else may have changed the Xmax
 * of the containing tuple, so the caller needs to iterate on us somehow.
 *
 * We return (in *remaining, if not NULL) the number of members that are still
 * running, including any (non-aborted) subtransactions of our own transaction.
 *
 */
static void
MultiXactIdWait(MultiXactId multi, MultiXactStatus status,
                int *remaining, uint16 infomask)
{
    Do_MultiXactIdWait(multi, status, remaining, infomask, false);
}

/*
 * ConditionalMultiXactIdWait
 *      As above, but only lock if we can get the lock without blocking.
 *
 * By the time we finish sleeping, someone else may have changed the Xmax
 * of the containing tuple, so the caller needs to iterate on us somehow.
 *
 * If the multixact is now all gone, return true.  Returns false if some
 * transactions might still be running.
 *
 * We return (in *remaining, if not NULL) the number of members that are still
 * running, including any (non-aborted) subtransactions of our own transaction.
 */
static bool
ConditionalMultiXactIdWait(MultiXactId multi, MultiXactStatus status,
                           int *remaining, uint16 infomask)
{
    return Do_MultiXactIdWait(multi, status, remaining, infomask, true);
}

/*
 * heap_tuple_needs_freeze
 *
 * Check to see whether any of the XID fields of a tuple (xmin, xmax, xvac)
 * are older than the specified cutoff XID or MultiXactId.  If so, return TRUE.
 *
 * It doesn't matter whether the tuple is alive or dead, we are checking
 * to see if a tuple needs to be removed or frozen to avoid wraparound.
 */
bool
heap_tuple_needs_freeze(HeapTupleHeader tuple, TransactionId cutoff_xid,
                        MultiXactId cutoff_multi, Buffer buf)
{
    TransactionId xid;

    xid = HeapTupleHeaderGetXmin(tuple);
    if (TransactionIdIsNormal(xid) &&
        TransactionIdPrecedes(xid, cutoff_xid))
        return true;

    if (!(tuple->t_infomask & HEAP_XMAX_INVALID))
    {
        if (!(tuple->t_infomask & HEAP_XMAX_IS_MULTI))
        {
            xid = HeapTupleHeaderGetRawXmax(tuple);
            if (TransactionIdIsNormal(xid) &&
                TransactionIdPrecedes(xid, cutoff_xid))
                return true;
        }
        else
        {
            MultiXactId multi;

            multi = HeapTupleHeaderGetRawXmax(tuple);
            if (MultiXactIdPrecedes(multi, cutoff_multi))
                return true;
        }
    }

    if (tuple->t_infomask & HEAP_MOVED)
    {
        xid = HeapTupleHeaderGetXvac(tuple);
        if (TransactionIdIsNormal(xid) &&
            TransactionIdPrecedes(xid, cutoff_xid))
            return true;
    }

    return false;
}

/* ----------------
 *      heap_markpos    - mark scan position
 * ----------------
 */
void
heap_markpos(HeapScanDesc scan)
{
    /* Note: no locking manipulations needed */

    if (scan->rs_ctup.t_data != NULL)
    {
        scan->rs_mctid = scan->rs_ctup.t_self;
        if (scan->rs_pageatatime)
            scan->rs_mindex = scan->rs_cindex;
    }
    else
        ItemPointerSetInvalid(&scan->rs_mctid);
}

/* ----------------
 *      heap_restrpos   - restore position to marked location
 * ----------------
 */
void
heap_restrpos(HeapScanDesc scan)
{
    /* XXX no amrestrpos checking that ammarkpos called */

    if (!ItemPointerIsValid(&scan->rs_mctid))
    {
        scan->rs_ctup.t_data = NULL;

        /*
         * unpin scan buffers
         */
        if (BufferIsValid(scan->rs_cbuf))
            ReleaseBuffer(scan->rs_cbuf);
        scan->rs_cbuf = InvalidBuffer;
        scan->rs_cblock = InvalidBlockNumber;
        scan->rs_inited = false;
    }
    else
    {
        /*
         * If we reached end of scan, rs_inited will now be false.  We must
         * reset it to true to keep heapgettup from doing the wrong thing.
         */
        scan->rs_inited = true;
        scan->rs_ctup.t_self = scan->rs_mctid;
        if (scan->rs_pageatatime)
        {
            scan->rs_cindex = scan->rs_mindex;
            heapgettup_pagemode(scan,
                                NoMovementScanDirection,
                                0,      /* needn't recheck scan keys */
                                NULL);
        }
        else
            heapgettup(scan,
                       NoMovementScanDirection,
                       0,       /* needn't recheck scan keys */
                       NULL);
    }
}

/*
 * If 'tuple' contains any visible XID greater than latestRemovedXid,
 * ratchet forwards latestRemovedXid to the greatest one found.
 * This is used as the basis for generating Hot Standby conflicts, so
 * if a tuple was never visible then removing it should not conflict
 * with queries.
 */
void
HeapTupleHeaderAdvanceLatestRemovedXid(HeapTupleHeader tuple,
                                       TransactionId *latestRemovedXid)
{
    TransactionId xmin = HeapTupleHeaderGetXmin(tuple);
    TransactionId xmax = HeapTupleHeaderGetUpdateXid(tuple);
    TransactionId xvac = HeapTupleHeaderGetXvac(tuple);

    if (tuple->t_infomask & HEAP_MOVED)
    {
        if (TransactionIdPrecedes(*latestRemovedXid, xvac))
            *latestRemovedXid = xvac;
    }

    /*
     * Ignore tuples inserted by an aborted transaction or if the tuple was
     * updated/deleted by the inserting transaction.
     *
     * Look for a committed hint bit, or if no xmin bit is set, check clog.
     * This needs to work on both master and standby, where it is used to
     * assess btree delete records.
     */
    if ((tuple->t_infomask & HEAP_XMIN_COMMITTED) ||
        (!(tuple->t_infomask & HEAP_XMIN_COMMITTED) &&
         !(tuple->t_infomask & HEAP_XMIN_INVALID) &&
         TransactionIdDidCommit(xmin)))
    {
        if (xmax != xmin &&
            TransactionIdFollows(xmax, *latestRemovedXid))
            *latestRemovedXid = xmax;
    }

    /* *latestRemovedXid may still be invalid at end */
}

/*
 * Perform XLogInsert to register a heap cleanup info message. These
 * messages are sent once per VACUUM and are required because
 * of the phasing of removal operations during a lazy VACUUM.
 * see comments for vacuum_log_cleanup_info().
 */
XLogRecPtr
log_heap_cleanup_info(RelFileNode rnode, TransactionId latestRemovedXid)
{
    xl_heap_cleanup_info xlrec;
    XLogRecPtr  recptr;
    XLogRecData rdata;

    xlrec.node = rnode;
    xlrec.latestRemovedXid = latestRemovedXid;

    rdata.data = (char *) &xlrec;
    rdata.len = SizeOfHeapCleanupInfo;
    rdata.buffer = InvalidBuffer;
    rdata.next = NULL;

    recptr = XLogInsert(RM_HEAP2_ID, XLOG_HEAP2_CLEANUP_INFO, &rdata);

    return recptr;
}

/*
 * Perform XLogInsert for a heap-clean operation.  Caller must already
 * have modified the buffer and marked it dirty.
 *
 * Note: prior to Postgres 8.3, the entries in the nowunused[] array were
 * zero-based tuple indexes.  Now they are one-based like other uses
 * of OffsetNumber.
 *
 * We also include latestRemovedXid, which is the greatest XID present in
 * the removed tuples. That allows recovery processing to cancel or wait
 * for long standby queries that can still see these tuples.
 */
XLogRecPtr
log_heap_clean(Relation reln, Buffer buffer,
               OffsetNumber *redirected, int nredirected,
               OffsetNumber *nowdead, int ndead,
               OffsetNumber *nowunused, int nunused,
               TransactionId latestRemovedXid)
{
    xl_heap_clean xlrec;
    uint8       info;
    XLogRecPtr  recptr;
    XLogRecData rdata[4];

    /* Caller should not call me on a non-WAL-logged relation */
    Assert(RelationNeedsWAL(reln));

    xlrec.node = reln->rd_node;
    xlrec.block = BufferGetBlockNumber(buffer);
    xlrec.latestRemovedXid = latestRemovedXid;
    xlrec.nredirected = nredirected;
    xlrec.ndead = ndead;

    rdata[0].data = (char *) &xlrec;
    rdata[0].len = SizeOfHeapClean;
    rdata[0].buffer = InvalidBuffer;
    rdata[0].next = &(rdata[1]);

    /*
     * The OffsetNumber arrays are not actually in the buffer, but we pretend
     * that they are.  When XLogInsert stores the whole buffer, the offset
     * arrays need not be stored too.  Note that even if all three arrays are
     * empty, we want to expose the buffer as a candidate for whole-page
     * storage, since this record type implies a defragmentation operation
     * even if no item pointers changed state.
     */
    if (nredirected > 0)
    {
        rdata[1].data = (char *) redirected;
        rdata[1].len = nredirected * sizeof(OffsetNumber) * 2;
    }
    else
    {
        rdata[1].data = NULL;
        rdata[1].len = 0;
    }
    rdata[1].buffer = buffer;
    rdata[1].buffer_std = true;
    rdata[1].next = &(rdata[2]);

    if (ndead > 0)
    {
        rdata[2].data = (char *) nowdead;
        rdata[2].len = ndead * sizeof(OffsetNumber);
    }
    else
    {
        rdata[2].data = NULL;
        rdata[2].len = 0;
    }
    rdata[2].buffer = buffer;
    rdata[2].buffer_std = true;
    rdata[2].next = &(rdata[3]);

    if (nunused > 0)
    {
        rdata[3].data = (char *) nowunused;
        rdata[3].len = nunused * sizeof(OffsetNumber);
    }
    else
    {
        rdata[3].data = NULL;
        rdata[3].len = 0;
    }
    rdata[3].buffer = buffer;
    rdata[3].buffer_std = true;
    rdata[3].next = NULL;

    info = XLOG_HEAP2_CLEAN;
    recptr = XLogInsert(RM_HEAP2_ID, info, rdata);

    return recptr;
}

/*
 * Perform XLogInsert for a heap-freeze operation.  Caller must already
 * have modified the buffer and marked it dirty.
 */
XLogRecPtr
log_heap_freeze(Relation reln, Buffer buffer,
                TransactionId cutoff_xid, MultiXactId cutoff_multi,
                OffsetNumber *offsets, int offcnt)
{
    xl_heap_freeze xlrec;
    XLogRecPtr  recptr;
    XLogRecData rdata[2];

    /* Caller should not call me on a non-WAL-logged relation */
    Assert(RelationNeedsWAL(reln));
    /* nor when there are no tuples to freeze */
    Assert(offcnt > 0);

    xlrec.node = reln->rd_node;
    xlrec.block = BufferGetBlockNumber(buffer);
    xlrec.cutoff_xid = cutoff_xid;
    xlrec.cutoff_multi = cutoff_multi;

    rdata[0].data = (char *) &xlrec;
    rdata[0].len = SizeOfHeapFreeze;
    rdata[0].buffer = InvalidBuffer;
    rdata[0].next = &(rdata[1]);

    /*
     * The tuple-offsets array is not actually in the buffer, but pretend that
     * it is.  When XLogInsert stores the whole buffer, the offsets array need
     * not be stored too.
     */
    rdata[1].data = (char *) offsets;
    rdata[1].len = offcnt * sizeof(OffsetNumber);
    rdata[1].buffer = buffer;
    rdata[1].buffer_std = true;
    rdata[1].next = NULL;

    recptr = XLogInsert(RM_HEAP2_ID, XLOG_HEAP2_FREEZE, rdata);

    return recptr;
}

/*
 * Perform XLogInsert for a heap-visible operation.  'block' is the block
 * being marked all-visible, and vm_buffer is the buffer containing the
 * corresponding visibility map block.  Both should have already been modified
 * and dirtied.
 *
 * If checksums are enabled, we also add the heap_buffer to the chain to
 * protect it from being torn.
 */
XLogRecPtr
log_heap_visible(RelFileNode rnode, Buffer heap_buffer, Buffer vm_buffer,
                 TransactionId cutoff_xid)
{
    xl_heap_visible xlrec;
    XLogRecPtr  recptr;
    XLogRecData rdata[3];

    Assert(BufferIsValid(heap_buffer));
    Assert(BufferIsValid(vm_buffer));

    xlrec.node = rnode;
    xlrec.block = BufferGetBlockNumber(heap_buffer);
    xlrec.cutoff_xid = cutoff_xid;

    rdata[0].data = (char *) &xlrec;
    rdata[0].len = SizeOfHeapVisible;
    rdata[0].buffer = InvalidBuffer;
    rdata[0].next = &(rdata[1]);

    rdata[1].data = NULL;
    rdata[1].len = 0;
    rdata[1].buffer = vm_buffer;
    rdata[1].buffer_std = false;
    rdata[1].next = NULL;

    if (DataChecksumsEnabled())
    {
        rdata[1].next = &(rdata[2]);

        rdata[2].data = NULL;
        rdata[2].len = 0;
        rdata[2].buffer = heap_buffer;
        rdata[2].buffer_std = true;
        rdata[2].next = NULL;
    }

    recptr = XLogInsert(RM_HEAP2_ID, XLOG_HEAP2_VISIBLE, rdata);

    return recptr;
}

/*
 * Perform XLogInsert for a heap-update operation.  Caller must already
 * have modified the buffer(s) and marked them dirty.
 */
static XLogRecPtr
log_heap_update(Relation reln, Buffer oldbuf,
                Buffer newbuf, HeapTuple oldtup, HeapTuple newtup,
                bool all_visible_cleared, bool new_all_visible_cleared)
{
    xl_heap_update xlrec;
    xl_heap_header xlhdr;
    uint8       info;
    XLogRecPtr  recptr;
    XLogRecData rdata[4];
    Page        page = BufferGetPage(newbuf);

    /* Caller should not call me on a non-WAL-logged relation */
    Assert(RelationNeedsWAL(reln));

    if (HeapTupleIsHeapOnly(newtup))
        info = XLOG_HEAP_HOT_UPDATE;
    else
        info = XLOG_HEAP_UPDATE;

    xlrec.target.node = reln->rd_node;
    xlrec.target.tid = oldtup->t_self;
    xlrec.old_xmax = HeapTupleHeaderGetRawXmax(oldtup->t_data);
    xlrec.old_infobits_set = compute_infobits(oldtup->t_data->t_infomask,
                                              oldtup->t_data->t_infomask2);
    xlrec.new_xmax = HeapTupleHeaderGetRawXmax(newtup->t_data);
    xlrec.all_visible_cleared = all_visible_cleared;
    xlrec.newtid = newtup->t_self;
    xlrec.new_all_visible_cleared = new_all_visible_cleared;

    rdata[0].data = (char *) &xlrec;
    rdata[0].len = SizeOfHeapUpdate;
    rdata[0].buffer = InvalidBuffer;
    rdata[0].next = &(rdata[1]);

    rdata[1].data = NULL;
    rdata[1].len = 0;
    rdata[1].buffer = oldbuf;
    rdata[1].buffer_std = true;
    rdata[1].next = &(rdata[2]);

    xlhdr.t_infomask2 = newtup->t_data->t_infomask2;
    xlhdr.t_infomask = newtup->t_data->t_infomask;
    xlhdr.t_hoff = newtup->t_data->t_hoff;

    /*
     * As with insert records, we need not store the rdata[2] segment if we
     * decide to store the whole buffer instead.
     */
    rdata[2].data = (char *) &xlhdr;
    rdata[2].len = SizeOfHeapHeader;
    rdata[2].buffer = newbuf;
    rdata[2].buffer_std = true;
    rdata[2].next = &(rdata[3]);

    /* PG73FORMAT: write bitmap [+ padding] [+ oid] + data */
    rdata[3].data = (char *) newtup->t_data + offsetof(HeapTupleHeaderData, t_bits);
    rdata[3].len = newtup->t_len - offsetof(HeapTupleHeaderData, t_bits);
    rdata[3].buffer = newbuf;
    rdata[3].buffer_std = true;
    rdata[3].next = NULL;

    /* If new tuple is the single and first tuple on page... */
    if (ItemPointerGetOffsetNumber(&(newtup->t_self)) == FirstOffsetNumber &&
        PageGetMaxOffsetNumber(page) == FirstOffsetNumber)
    {
        info |= XLOG_HEAP_INIT_PAGE;
        rdata[2].buffer = rdata[3].buffer = InvalidBuffer;
    }

    recptr = XLogInsert(RM_HEAP_ID, info, rdata);

    return recptr;
}

/*
 * Perform XLogInsert of a HEAP_NEWPAGE record to WAL. Caller is responsible
 * for writing the page to disk after calling this routine.
 *
 * Note: If you're using this function, you should be building pages in private
 * memory and writing them directly to smgr.  If you're using buffers, call
 * log_newpage_buffer instead.
 *
 * Note: the NEWPAGE log record is used for both heaps and indexes, so do
 * not do anything that assumes we are touching a heap.
 */
XLogRecPtr
log_newpage(RelFileNode *rnode, ForkNumber forkNum, BlockNumber blkno,
            Page page)
{
    xl_heap_newpage xlrec;
    XLogRecPtr  recptr;
    XLogRecData rdata[2];

    /* NO ELOG(ERROR) from here till newpage op is logged */
    START_CRIT_SECTION();

    xlrec.node = *rnode;
    xlrec.forknum = forkNum;
    xlrec.blkno = blkno;

    rdata[0].data = (char *) &xlrec;
    rdata[0].len = SizeOfHeapNewpage;
    rdata[0].buffer = InvalidBuffer;
    rdata[0].next = &(rdata[1]);

    rdata[1].data = (char *) page;
    rdata[1].len = BLCKSZ;
    rdata[1].buffer = InvalidBuffer;
    rdata[1].next = NULL;

    recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_NEWPAGE, rdata);

    /*
     * The page may be uninitialized. If so, we can't set the LSN and TLI
     * because that would corrupt the page.
     */
    if (!PageIsNew(page))
    {
        PageSetLSN(page, recptr);
    }

    END_CRIT_SECTION();

    return recptr;
}

/*
 * Perform XLogInsert of a HEAP_NEWPAGE record to WAL.
 *
 * Caller should initialize the buffer and mark it dirty before calling this
 * function.  This function will set the page LSN and TLI.
 *
 * Note: the NEWPAGE log record is used for both heaps and indexes, so do
 * not do anything that assumes we are touching a heap.
 */
XLogRecPtr
log_newpage_buffer(Buffer buffer)
{
    xl_heap_newpage xlrec;
    XLogRecPtr  recptr;
    XLogRecData rdata[2];
    Page        page = BufferGetPage(buffer);

    /* We should be in a critical section. */
    Assert(CritSectionCount > 0);

    BufferGetTag(buffer, &xlrec.node, &xlrec.forknum, &xlrec.blkno);

    rdata[0].data = (char *) &xlrec;
    rdata[0].len = SizeOfHeapNewpage;
    rdata[0].buffer = InvalidBuffer;
    rdata[0].next = &(rdata[1]);

    rdata[1].data = page;
    rdata[1].len = BLCKSZ;
    rdata[1].buffer = InvalidBuffer;
    rdata[1].next = NULL;

    recptr = XLogInsert(RM_HEAP_ID, XLOG_HEAP_NEWPAGE, rdata);

    /*
     * The page may be uninitialized. If so, we can't set the LSN and TLI
     * because that would corrupt the page.
     */
    if (!PageIsNew(page))
    {
        PageSetLSN(page, recptr);
    }

    return recptr;
}

/*
 * Handles CLEANUP_INFO
 */
static void
heap_xlog_cleanup_info(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_cleanup_info *xlrec = (xl_heap_cleanup_info *) XLogRecGetData(record);

    if (InHotStandby)
        ResolveRecoveryConflictWithSnapshot(xlrec->latestRemovedXid, xlrec->node);

    /*
     * Actual operation is a no-op. Record type exists to provide a means for
     * conflict processing to occur before we begin index vacuum actions. see
     * vacuumlazy.c and also comments in btvacuumpage()
     */

    /* Backup blocks are not used in cleanup_info records */
    Assert(!(record->xl_info & XLR_BKP_BLOCK_MASK));
}

/*
 * Handles HEAP2_CLEAN record type
 */
static void
heap_xlog_clean(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_clean *xlrec = (xl_heap_clean *) XLogRecGetData(record);
    Buffer      buffer;
    Page        page;
    OffsetNumber *end;
    OffsetNumber *redirected;
    OffsetNumber *nowdead;
    OffsetNumber *nowunused;
    int         nredirected;
    int         ndead;
    int         nunused;
    Size        freespace;

    /*
     * We're about to remove tuples. In Hot Standby mode, ensure that there's
     * no queries running for which the removed tuples are still visible.
     *
     * Not all HEAP2_CLEAN records remove tuples with xids, so we only want to
     * conflict on the records that cause MVCC failures for user queries. If
     * latestRemovedXid is invalid, skip conflict processing.
     */
    if (InHotStandby && TransactionIdIsValid(xlrec->latestRemovedXid))
        ResolveRecoveryConflictWithSnapshot(xlrec->latestRemovedXid,
                                            xlrec->node);

    /*
     * If we have a full-page image, restore it (using a cleanup lock) and
     * we're done.
     */
    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        (void) RestoreBackupBlock(lsn, record, 0, true, false);
        return;
    }

    buffer = XLogReadBufferExtended(xlrec->node, MAIN_FORKNUM, xlrec->block, RBM_NORMAL);
    if (!BufferIsValid(buffer))
        return;
    LockBufferForCleanup(buffer);
    page = (Page) BufferGetPage(buffer);

    if (lsn <= PageGetLSN(page))
    {
        UnlockReleaseBuffer(buffer);
        return;
    }

    nredirected = xlrec->nredirected;
    ndead = xlrec->ndead;
    end = (OffsetNumber *) ((char *) xlrec + record->xl_len);
    redirected = (OffsetNumber *) ((char *) xlrec + SizeOfHeapClean);
    nowdead = redirected + (nredirected * 2);
    nowunused = nowdead + ndead;
    nunused = (end - nowunused);
    Assert(nunused >= 0);

    /* Update all item pointers per the record, and repair fragmentation */
    heap_page_prune_execute(buffer,
                            redirected, nredirected,
                            nowdead, ndead,
                            nowunused, nunused);

    freespace = PageGetHeapFreeSpace(page);     /* needed to update FSM below */

    /*
     * Note: we don't worry about updating the page's prunability hints. At
     * worst this will cause an extra prune cycle to occur soon.
     */

    PageSetLSN(page, lsn);
    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);

    /*
     * Update the FSM as well.
     *
     * XXX: We don't get here if the page was restored from full page image.
     * We don't bother to update the FSM in that case, it doesn't need to be
     * totally accurate anyway.
     */
    XLogRecordPageWithFreeSpace(xlrec->node, xlrec->block, freespace);
}

static void
heap_xlog_freeze(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_freeze *xlrec = (xl_heap_freeze *) XLogRecGetData(record);
    TransactionId cutoff_xid = xlrec->cutoff_xid;
    MultiXactId cutoff_multi = xlrec->cutoff_multi;
    Buffer      buffer;
    Page        page;

    /*
     * In Hot Standby mode, ensure that there's no queries running which still
     * consider the frozen xids as running.
     */
    if (InHotStandby)
        ResolveRecoveryConflictWithSnapshot(cutoff_xid, xlrec->node);

    /* If we have a full-page image, restore it and we're done */
    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        (void) RestoreBackupBlock(lsn, record, 0, false, false);
        return;
    }

    buffer = XLogReadBuffer(xlrec->node, xlrec->block, false);
    if (!BufferIsValid(buffer))
        return;
    page = (Page) BufferGetPage(buffer);

    if (lsn <= PageGetLSN(page))
    {
        UnlockReleaseBuffer(buffer);
        return;
    }

    if (record->xl_len > SizeOfHeapFreeze)
    {
        OffsetNumber *offsets;
        OffsetNumber *offsets_end;

        offsets = (OffsetNumber *) ((char *) xlrec + SizeOfHeapFreeze);
        offsets_end = (OffsetNumber *) ((char *) xlrec + record->xl_len);

        while (offsets < offsets_end)
        {
            /* offsets[] entries are one-based */
            ItemId      lp = PageGetItemId(page, *offsets);
            HeapTupleHeader tuple = (HeapTupleHeader) PageGetItem(page, lp);

            (void) heap_freeze_tuple(tuple, cutoff_xid, cutoff_multi);
            offsets++;
        }
    }

    PageSetLSN(page, lsn);
    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);
}

/*
 * Replay XLOG_HEAP2_VISIBLE record.
 *
 * The critical integrity requirement here is that we must never end up with
 * a situation where the visibility map bit is set, and the page-level
 * PD_ALL_VISIBLE bit is clear.  If that were to occur, then a subsequent
 * page modification would fail to clear the visibility map bit.
 */
static void
heap_xlog_visible(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_visible *xlrec = (xl_heap_visible *) XLogRecGetData(record);

    /*
     * If there are any Hot Standby transactions running that have an xmin
     * horizon old enough that this page isn't all-visible for them, they
     * might incorrectly decide that an index-only scan can skip a heap fetch.
     *
     * NB: It might be better to throw some kind of "soft" conflict here that
     * forces any index-only scan that is in flight to perform heap fetches,
     * rather than killing the transaction outright.
     */
    if (InHotStandby)
        ResolveRecoveryConflictWithSnapshot(xlrec->cutoff_xid, xlrec->node);

    /*
     * If heap block was backed up, restore it. This can only happen with
     * checksums enabled.
     */
    if (record->xl_info & XLR_BKP_BLOCK(1))
    {
        Assert(DataChecksumsEnabled());
        (void) RestoreBackupBlock(lsn, record, 1, false, false);
    }
    else
    {
        Buffer      buffer;
        Page        page;

        /*
         * Read the heap page, if it still exists. If the heap file has been
         * dropped or truncated later in recovery, we don't need to update the
         * page, but we'd better still update the visibility map.
         */
        buffer = XLogReadBufferExtended(xlrec->node, MAIN_FORKNUM,
                                        xlrec->block, RBM_NORMAL);
        if (BufferIsValid(buffer))
        {
            LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

            page = (Page) BufferGetPage(buffer);

            /*
             * We don't bump the LSN of the heap page when setting the
             * visibility map bit (unless checksums are enabled, in which case
             * we must), because that would generate an unworkable volume of
             * full-page writes.  This exposes us to torn page hazards, but
             * since we're not inspecting the existing page contents in any
             * way, we don't care.
             *
             * However, all operations that clear the visibility map bit *do*
             * bump the LSN, and those operations will only be replayed if the
             * XLOG LSN follows the page LSN.  Thus, if the page LSN has
             * advanced past our XLOG record's LSN, we mustn't mark the page
             * all-visible, because the subsequent update won't be replayed to
             * clear the flag.
             */
            if (lsn > PageGetLSN(page))
            {
                PageSetAllVisible(page);
                MarkBufferDirty(buffer);
            }

            /* Done with heap page. */
            UnlockReleaseBuffer(buffer);
        }
    }

    /*
     * Even if we skipped the heap page update due to the LSN interlock, it's
     * still safe to update the visibility map.  Any WAL record that clears
     * the visibility map bit does so before checking the page LSN, so any
     * bits that need to be cleared will still be cleared.
     */
    if (record->xl_info & XLR_BKP_BLOCK(0))
        (void) RestoreBackupBlock(lsn, record, 0, false, false);
    else
    {
        Relation    reln;
        Buffer      vmbuffer = InvalidBuffer;

        reln = CreateFakeRelcacheEntry(xlrec->node);
        visibilitymap_pin(reln, xlrec->block, &vmbuffer);

        /*
         * Don't set the bit if replay has already passed this point.
         *
         * It might be safe to do this unconditionally; if replay has passed
         * this point, we'll replay at least as far this time as we did
         * before, and if this bit needs to be cleared, the record responsible
         * for doing so should be again replayed, and clear it.  For right
         * now, out of an abundance of conservatism, we use the same test here
         * we did for the heap page.  If this results in a dropped bit, no
         * real harm is done; and the next VACUUM will fix it.
         */
        if (lsn > PageGetLSN(BufferGetPage(vmbuffer)))
            visibilitymap_set(reln, xlrec->block, InvalidBuffer, lsn, vmbuffer,
                              xlrec->cutoff_xid);

        ReleaseBuffer(vmbuffer);
        FreeFakeRelcacheEntry(reln);
    }
}

static void
heap_xlog_newpage(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_newpage *xlrec = (xl_heap_newpage *) XLogRecGetData(record);
    Buffer      buffer;
    Page        page;

    /* Backup blocks are not used in newpage records */
    Assert(!(record->xl_info & XLR_BKP_BLOCK_MASK));

    /*
     * Note: the NEWPAGE log record is used for both heaps and indexes, so do
     * not do anything that assumes we are touching a heap.
     */
    buffer = XLogReadBufferExtended(xlrec->node, xlrec->forknum, xlrec->blkno,
                                    RBM_ZERO);
    Assert(BufferIsValid(buffer));
    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
    page = (Page) BufferGetPage(buffer);

    Assert(record->xl_len == SizeOfHeapNewpage + BLCKSZ);
    memcpy(page, (char *) xlrec + SizeOfHeapNewpage, BLCKSZ);

    /*
     * The page may be uninitialized. If so, we can't set the LSN because that
     * would corrupt the page.
     */
    if (!PageIsNew(page))
    {
        PageSetLSN(page, lsn);
    }

    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);
}

/*
 * Given an "infobits" field from an XLog record, set the correct bits in the
 * given infomask and infomask2 for the tuple touched by the record.
 *
 * (This is the reverse of compute_infobits).
 */
static void
fix_infomask_from_infobits(uint8 infobits, uint16 *infomask, uint16 *infomask2)
{
    *infomask &= ~(HEAP_XMAX_IS_MULTI | HEAP_XMAX_LOCK_ONLY |
                   HEAP_XMAX_KEYSHR_LOCK | HEAP_XMAX_EXCL_LOCK);
    *infomask2 &= ~HEAP_KEYS_UPDATED;

    if (infobits & XLHL_XMAX_IS_MULTI)
        *infomask |= HEAP_XMAX_IS_MULTI;
    if (infobits & XLHL_XMAX_LOCK_ONLY)
        *infomask |= HEAP_XMAX_LOCK_ONLY;
    if (infobits & XLHL_XMAX_EXCL_LOCK)
        *infomask |= HEAP_XMAX_EXCL_LOCK;
    /* note HEAP_XMAX_SHR_LOCK isn't considered here */
    if (infobits & XLHL_XMAX_KEYSHR_LOCK)
        *infomask |= HEAP_XMAX_KEYSHR_LOCK;

    if (infobits & XLHL_KEYS_UPDATED)
        *infomask2 |= HEAP_KEYS_UPDATED;
}

static void
heap_xlog_delete(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_delete *xlrec = (xl_heap_delete *) XLogRecGetData(record);
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    ItemId      lp = NULL;
    HeapTupleHeader htup;
    BlockNumber blkno;

    blkno = ItemPointerGetBlockNumber(&(xlrec->target.tid));

    /*
     * The visibility map may need to be fixed even if the heap page is
     * already up-to-date.
     */
    if (xlrec->all_visible_cleared)
    {
        Relation    reln = CreateFakeRelcacheEntry(xlrec->target.node);
        Buffer      vmbuffer = InvalidBuffer;

        visibilitymap_pin(reln, blkno, &vmbuffer);
        visibilitymap_clear(reln, blkno, vmbuffer);
        ReleaseBuffer(vmbuffer);
        FreeFakeRelcacheEntry(reln);
    }

    /* If we have a full-page image, restore it and we're done */
    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        (void) RestoreBackupBlock(lsn, record, 0, false, false);
        return;
    }

    buffer = XLogReadBuffer(xlrec->target.node, blkno, false);
    if (!BufferIsValid(buffer))
        return;
    page = (Page) BufferGetPage(buffer);

    if (lsn <= PageGetLSN(page))        /* changes are applied */
    {
        UnlockReleaseBuffer(buffer);
        return;
    }

    offnum = ItemPointerGetOffsetNumber(&(xlrec->target.tid));
    if (PageGetMaxOffsetNumber(page) >= offnum)
        lp = PageGetItemId(page, offnum);

    if (PageGetMaxOffsetNumber(page) < offnum || !ItemIdIsNormal(lp))
        elog(PANIC, "heap_delete_redo: invalid lp");

    htup = (HeapTupleHeader) PageGetItem(page, lp);

    htup->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
    htup->t_infomask2 &= ~HEAP_KEYS_UPDATED;
    HeapTupleHeaderClearHotUpdated(htup);
    fix_infomask_from_infobits(xlrec->infobits_set,
                               &htup->t_infomask, &htup->t_infomask2);
    HeapTupleHeaderSetXmax(htup, xlrec->xmax);
    HeapTupleHeaderSetCmax(htup, FirstCommandId, false);

    /* Mark the page as a candidate for pruning */
    PageSetPrunable(page, record->xl_xid);

    if (xlrec->all_visible_cleared)
        PageClearAllVisible(page);

    /* Make sure there is no forward chain link in t_ctid */
    htup->t_ctid = xlrec->target.tid;
    PageSetLSN(page, lsn);
    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);
}

static void
heap_xlog_insert(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_insert *xlrec = (xl_heap_insert *) XLogRecGetData(record);
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    struct
    {
        HeapTupleHeaderData hdr;
        char        data[MaxHeapTupleSize];
    }           tbuf;
    HeapTupleHeader htup;
    xl_heap_header xlhdr;
    uint32      newlen;
    Size        freespace;
    BlockNumber blkno;

    blkno = ItemPointerGetBlockNumber(&(xlrec->target.tid));

    /*
     * The visibility map may need to be fixed even if the heap page is
     * already up-to-date.
     */
    if (xlrec->all_visible_cleared)
    {
        Relation    reln = CreateFakeRelcacheEntry(xlrec->target.node);
        Buffer      vmbuffer = InvalidBuffer;

        visibilitymap_pin(reln, blkno, &vmbuffer);
        visibilitymap_clear(reln, blkno, vmbuffer);
        ReleaseBuffer(vmbuffer);
        FreeFakeRelcacheEntry(reln);
    }

    /* If we have a full-page image, restore it and we're done */
    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        (void) RestoreBackupBlock(lsn, record, 0, false, false);
        return;
    }

    if (record->xl_info & XLOG_HEAP_INIT_PAGE)
    {
        buffer = XLogReadBuffer(xlrec->target.node, blkno, true);
        Assert(BufferIsValid(buffer));
        page = (Page) BufferGetPage(buffer);

        PageInit(page, BufferGetPageSize(buffer), 0);
    }
    else
    {
        buffer = XLogReadBuffer(xlrec->target.node, blkno, false);
        if (!BufferIsValid(buffer))
            return;
        page = (Page) BufferGetPage(buffer);

        if (lsn <= PageGetLSN(page))    /* changes are applied */
        {
            UnlockReleaseBuffer(buffer);
            return;
        }
    }

    offnum = ItemPointerGetOffsetNumber(&(xlrec->target.tid));
    if (PageGetMaxOffsetNumber(page) + 1 < offnum)
        elog(PANIC, "heap_insert_redo: invalid max offset number");

    newlen = record->xl_len - SizeOfHeapInsert - SizeOfHeapHeader;
    Assert(newlen <= MaxHeapTupleSize);
    memcpy((char *) &xlhdr,
           (char *) xlrec + SizeOfHeapInsert,
           SizeOfHeapHeader);
    htup = &tbuf.hdr;
    MemSet((char *) htup, 0, sizeof(HeapTupleHeaderData));
    /* PG73FORMAT: get bitmap [+ padding] [+ oid] + data */
    memcpy((char *) htup + offsetof(HeapTupleHeaderData, t_bits),
           (char *) xlrec + SizeOfHeapInsert + SizeOfHeapHeader,
           newlen);
    newlen += offsetof(HeapTupleHeaderData, t_bits);
    htup->t_infomask2 = xlhdr.t_infomask2;
    htup->t_infomask = xlhdr.t_infomask;
    htup->t_hoff = xlhdr.t_hoff;
    HeapTupleHeaderSetXmin(htup, record->xl_xid);
    HeapTupleHeaderSetCmin(htup, FirstCommandId);
    htup->t_ctid = xlrec->target.tid;

    offnum = PageAddItem(page, (Item) htup, newlen, offnum, true, true);
    if (offnum == InvalidOffsetNumber)
        elog(PANIC, "heap_insert_redo: failed to add tuple");

    freespace = PageGetHeapFreeSpace(page);     /* needed to update FSM below */

    PageSetLSN(page, lsn);

    if (xlrec->all_visible_cleared)
        PageClearAllVisible(page);

    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);

    /*
     * If the page is running low on free space, update the FSM as well.
     * Arbitrarily, our definition of "low" is less than 20%. We can't do much
     * better than that without knowing the fill-factor for the table.
     *
     * XXX: We don't get here if the page was restored from full page image.
     * We don't bother to update the FSM in that case, it doesn't need to be
     * totally accurate anyway.
     */
    if (freespace < BLCKSZ / 5)
        XLogRecordPageWithFreeSpace(xlrec->target.node, blkno, freespace);
}

/*
 * Handles MULTI_INSERT record type.
 */
static void
heap_xlog_multi_insert(XLogRecPtr lsn, XLogRecord *record)
{
    char       *recdata = XLogRecGetData(record);
    xl_heap_multi_insert *xlrec;
    Buffer      buffer;
    Page        page;
    struct
    {
        HeapTupleHeaderData hdr;
        char        data[MaxHeapTupleSize];
    }           tbuf;
    HeapTupleHeader htup;
    uint32      newlen;
    Size        freespace;
    BlockNumber blkno;
    int         i;
    bool        isinit = (record->xl_info & XLOG_HEAP_INIT_PAGE) != 0;

    /*
     * Insertion doesn't overwrite MVCC data, so no conflict processing is
     * required.
     */

    xlrec = (xl_heap_multi_insert *) recdata;
    recdata += SizeOfHeapMultiInsert;

    /*
     * If we're reinitializing the page, the tuples are stored in order from
     * FirstOffsetNumber. Otherwise there's an array of offsets in the WAL
     * record.
     */
    if (!isinit)
        recdata += sizeof(OffsetNumber) * xlrec->ntuples;

    blkno = xlrec->blkno;

    /*
     * The visibility map may need to be fixed even if the heap page is
     * already up-to-date.
     */
    if (xlrec->all_visible_cleared)
    {
        Relation    reln = CreateFakeRelcacheEntry(xlrec->node);
        Buffer      vmbuffer = InvalidBuffer;

        visibilitymap_pin(reln, blkno, &vmbuffer);
        visibilitymap_clear(reln, blkno, vmbuffer);
        ReleaseBuffer(vmbuffer);
        FreeFakeRelcacheEntry(reln);
    }

    /* If we have a full-page image, restore it and we're done */
    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        (void) RestoreBackupBlock(lsn, record, 0, false, false);
        return;
    }

    if (isinit)
    {
        buffer = XLogReadBuffer(xlrec->node, blkno, true);
        Assert(BufferIsValid(buffer));
        page = (Page) BufferGetPage(buffer);

        PageInit(page, BufferGetPageSize(buffer), 0);
    }
    else
    {
        buffer = XLogReadBuffer(xlrec->node, blkno, false);
        if (!BufferIsValid(buffer))
            return;
        page = (Page) BufferGetPage(buffer);

        if (lsn <= PageGetLSN(page))    /* changes are applied */
        {
            UnlockReleaseBuffer(buffer);
            return;
        }
    }

    for (i = 0; i < xlrec->ntuples; i++)
    {
        OffsetNumber offnum;
        xl_multi_insert_tuple *xlhdr;

        if (isinit)
            offnum = FirstOffsetNumber + i;
        else
            offnum = xlrec->offsets[i];
        if (PageGetMaxOffsetNumber(page) + 1 < offnum)
            elog(PANIC, "heap_multi_insert_redo: invalid max offset number");

        xlhdr = (xl_multi_insert_tuple *) SHORTALIGN(recdata);
        recdata = ((char *) xlhdr) + SizeOfMultiInsertTuple;

        newlen = xlhdr->datalen;
        Assert(newlen <= MaxHeapTupleSize);
        htup = &tbuf.hdr;
        MemSet((char *) htup, 0, sizeof(HeapTupleHeaderData));
        /* PG73FORMAT: get bitmap [+ padding] [+ oid] + data */
        memcpy((char *) htup + offsetof(HeapTupleHeaderData, t_bits),
               (char *) recdata,
               newlen);
        recdata += newlen;

        newlen += offsetof(HeapTupleHeaderData, t_bits);
        htup->t_infomask2 = xlhdr->t_infomask2;
        htup->t_infomask = xlhdr->t_infomask;
        htup->t_hoff = xlhdr->t_hoff;
        HeapTupleHeaderSetXmin(htup, record->xl_xid);
        HeapTupleHeaderSetCmin(htup, FirstCommandId);
        ItemPointerSetBlockNumber(&htup->t_ctid, blkno);
        ItemPointerSetOffsetNumber(&htup->t_ctid, offnum);

        offnum = PageAddItem(page, (Item) htup, newlen, offnum, true, true);
        if (offnum == InvalidOffsetNumber)
            elog(PANIC, "heap_multi_insert_redo: failed to add tuple");
    }

    freespace = PageGetHeapFreeSpace(page);     /* needed to update FSM below */

    PageSetLSN(page, lsn);

    if (xlrec->all_visible_cleared)
        PageClearAllVisible(page);

    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);

    /*
     * If the page is running low on free space, update the FSM as well.
     * Arbitrarily, our definition of "low" is less than 20%. We can't do much
     * better than that without knowing the fill-factor for the table.
     *
     * XXX: We don't get here if the page was restored from full page image.
     * We don't bother to update the FSM in that case, it doesn't need to be
     * totally accurate anyway.
     */
    if (freespace < BLCKSZ / 5)
        XLogRecordPageWithFreeSpace(xlrec->node, blkno, freespace);
}

/*
 * Handles UPDATE and HOT_UPDATE
 */
static void
heap_xlog_update(XLogRecPtr lsn, XLogRecord *record, bool hot_update)
{
    xl_heap_update *xlrec = (xl_heap_update *) XLogRecGetData(record);
    bool        samepage = (ItemPointerGetBlockNumber(&(xlrec->newtid)) ==
                            ItemPointerGetBlockNumber(&(xlrec->target.tid)));
    Buffer      obuffer,
                nbuffer;
    Page        page;
    OffsetNumber offnum;
    ItemId      lp = NULL;
    HeapTupleHeader htup;
    struct
    {
        HeapTupleHeaderData hdr;
        char        data[MaxHeapTupleSize];
    }           tbuf;
    xl_heap_header xlhdr;
    int         hsize;
    uint32      newlen;
    Size        freespace;

    /*
     * The visibility map may need to be fixed even if the heap page is
     * already up-to-date.
     */
    if (xlrec->all_visible_cleared)
    {
        Relation    reln = CreateFakeRelcacheEntry(xlrec->target.node);
        BlockNumber block = ItemPointerGetBlockNumber(&xlrec->target.tid);
        Buffer      vmbuffer = InvalidBuffer;

        visibilitymap_pin(reln, block, &vmbuffer);
        visibilitymap_clear(reln, block, vmbuffer);
        ReleaseBuffer(vmbuffer);
        FreeFakeRelcacheEntry(reln);
    }

    /*
     * In normal operation, it is important to lock the two pages in
     * page-number order, to avoid possible deadlocks against other update
     * operations going the other way.  However, during WAL replay there can
     * be no other update happening, so we don't need to worry about that. But
     * we *do* need to worry that we don't expose an inconsistent state to Hot
     * Standby queries --- so the original page can't be unlocked before we've
     * added the new tuple to the new page.
     */

    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        obuffer = RestoreBackupBlock(lsn, record, 0, false, true);
        if (samepage)
        {
            /* backup block covered both changes, so we're done */
            UnlockReleaseBuffer(obuffer);
            return;
        }
        goto newt;
    }

    /* Deal with old tuple version */

    obuffer = XLogReadBuffer(xlrec->target.node,
                             ItemPointerGetBlockNumber(&(xlrec->target.tid)),
                             false);
    if (!BufferIsValid(obuffer))
        goto newt;
    page = (Page) BufferGetPage(obuffer);

    if (lsn <= PageGetLSN(page))        /* changes are applied */
    {
        if (samepage)
        {
            UnlockReleaseBuffer(obuffer);
            return;
        }
        goto newt;
    }

    offnum = ItemPointerGetOffsetNumber(&(xlrec->target.tid));
    if (PageGetMaxOffsetNumber(page) >= offnum)
        lp = PageGetItemId(page, offnum);

    if (PageGetMaxOffsetNumber(page) < offnum || !ItemIdIsNormal(lp))
        elog(PANIC, "heap_update_redo: invalid lp");

    htup = (HeapTupleHeader) PageGetItem(page, lp);

    htup->t_infomask &= ~(HEAP_XMAX_BITS | HEAP_MOVED);
    htup->t_infomask2 &= ~HEAP_KEYS_UPDATED;
    if (hot_update)
        HeapTupleHeaderSetHotUpdated(htup);
    else
        HeapTupleHeaderClearHotUpdated(htup);
    fix_infomask_from_infobits(xlrec->old_infobits_set, &htup->t_infomask,
                               &htup->t_infomask2);
    HeapTupleHeaderSetXmax(htup, xlrec->old_xmax);
    HeapTupleHeaderSetCmax(htup, FirstCommandId, false);
    /* Set forward chain link in t_ctid */
    htup->t_ctid = xlrec->newtid;

    /* Mark the page as a candidate for pruning */
    PageSetPrunable(page, record->xl_xid);

    if (xlrec->all_visible_cleared)
        PageClearAllVisible(page);

    /*
     * this test is ugly, but necessary to avoid thinking that insert change
     * is already applied
     */
    if (samepage)
    {
        nbuffer = obuffer;
        goto newsame;
    }

    PageSetLSN(page, lsn);
    MarkBufferDirty(obuffer);

    /* Deal with new tuple */

newt:;

    /*
     * The visibility map may need to be fixed even if the heap page is
     * already up-to-date.
     */
    if (xlrec->new_all_visible_cleared)
    {
        Relation    reln = CreateFakeRelcacheEntry(xlrec->target.node);
        BlockNumber block = ItemPointerGetBlockNumber(&xlrec->newtid);
        Buffer      vmbuffer = InvalidBuffer;

        visibilitymap_pin(reln, block, &vmbuffer);
        visibilitymap_clear(reln, block, vmbuffer);
        ReleaseBuffer(vmbuffer);
        FreeFakeRelcacheEntry(reln);
    }

    if (record->xl_info & XLR_BKP_BLOCK(1))
    {
        (void) RestoreBackupBlock(lsn, record, 1, false, false);
        if (BufferIsValid(obuffer))
            UnlockReleaseBuffer(obuffer);
        return;
    }

    if (record->xl_info & XLOG_HEAP_INIT_PAGE)
    {
        nbuffer = XLogReadBuffer(xlrec->target.node,
                                 ItemPointerGetBlockNumber(&(xlrec->newtid)),
                                 true);
        Assert(BufferIsValid(nbuffer));
        page = (Page) BufferGetPage(nbuffer);

        PageInit(page, BufferGetPageSize(nbuffer), 0);
    }
    else
    {
        nbuffer = XLogReadBuffer(xlrec->target.node,
                                 ItemPointerGetBlockNumber(&(xlrec->newtid)),
                                 false);
        if (!BufferIsValid(nbuffer))
        {
            if (BufferIsValid(obuffer))
                UnlockReleaseBuffer(obuffer);
            return;
        }
        page = (Page) BufferGetPage(nbuffer);

        if (lsn <= PageGetLSN(page))    /* changes are applied */
        {
            UnlockReleaseBuffer(nbuffer);
            if (BufferIsValid(obuffer))
                UnlockReleaseBuffer(obuffer);
            return;
        }
    }

newsame:;

    offnum = ItemPointerGetOffsetNumber(&(xlrec->newtid));
    if (PageGetMaxOffsetNumber(page) + 1 < offnum)
        elog(PANIC, "heap_update_redo: invalid max offset number");

    hsize = SizeOfHeapUpdate + SizeOfHeapHeader;

    newlen = record->xl_len - hsize;
    Assert(newlen <= MaxHeapTupleSize);
    memcpy((char *) &xlhdr,
           (char *) xlrec + SizeOfHeapUpdate,
           SizeOfHeapHeader);
    htup = &tbuf.hdr;
    MemSet((char *) htup, 0, sizeof(HeapTupleHeaderData));
    /* PG73FORMAT: get bitmap [+ padding] [+ oid] + data */
    memcpy((char *) htup + offsetof(HeapTupleHeaderData, t_bits),
           (char *) xlrec + hsize,
           newlen);
    newlen += offsetof(HeapTupleHeaderData, t_bits);
    htup->t_infomask2 = xlhdr.t_infomask2;
    htup->t_infomask = xlhdr.t_infomask;
    htup->t_hoff = xlhdr.t_hoff;

    HeapTupleHeaderSetXmin(htup, record->xl_xid);
    HeapTupleHeaderSetCmin(htup, FirstCommandId);
    HeapTupleHeaderSetXmax(htup, xlrec->new_xmax);
    /* Make sure there is no forward chain link in t_ctid */
    htup->t_ctid = xlrec->newtid;

    offnum = PageAddItem(page, (Item) htup, newlen, offnum, true, true);
    if (offnum == InvalidOffsetNumber)
        elog(PANIC, "heap_update_redo: failed to add tuple");

    if (xlrec->new_all_visible_cleared)
        PageClearAllVisible(page);

    freespace = PageGetHeapFreeSpace(page);     /* needed to update FSM below */

    PageSetLSN(page, lsn);
    MarkBufferDirty(nbuffer);
    UnlockReleaseBuffer(nbuffer);

    if (BufferIsValid(obuffer) && obuffer != nbuffer)
        UnlockReleaseBuffer(obuffer);

    /*
     * If the new page is running low on free space, update the FSM as well.
     * Arbitrarily, our definition of "low" is less than 20%. We can't do much
     * better than that without knowing the fill-factor for the table.
     *
     * However, don't update the FSM on HOT updates, because after crash
     * recovery, either the old or the new tuple will certainly be dead and
     * prunable. After pruning, the page will have roughly as much free space
     * as it did before the update, assuming the new tuple is about the same
     * size as the old one.
     *
     * XXX: We don't get here if the page was restored from full page image.
     * We don't bother to update the FSM in that case, it doesn't need to be
     * totally accurate anyway.
     */
    if (!hot_update && freespace < BLCKSZ / 5)
        XLogRecordPageWithFreeSpace(xlrec->target.node,
                                 ItemPointerGetBlockNumber(&(xlrec->newtid)),
                                    freespace);
}

static void
heap_xlog_lock(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_lock *xlrec = (xl_heap_lock *) XLogRecGetData(record);
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    ItemId      lp = NULL;
    HeapTupleHeader htup;

    /* If we have a full-page image, restore it and we're done */
    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        (void) RestoreBackupBlock(lsn, record, 0, false, false);
        return;
    }

    buffer = XLogReadBuffer(xlrec->target.node,
                            ItemPointerGetBlockNumber(&(xlrec->target.tid)),
                            false);
    if (!BufferIsValid(buffer))
        return;
    page = (Page) BufferGetPage(buffer);

    if (lsn <= PageGetLSN(page))        /* changes are applied */
    {
        UnlockReleaseBuffer(buffer);
        return;
    }

    offnum = ItemPointerGetOffsetNumber(&(xlrec->target.tid));
    if (PageGetMaxOffsetNumber(page) >= offnum)
        lp = PageGetItemId(page, offnum);

    if (PageGetMaxOffsetNumber(page) < offnum || !ItemIdIsNormal(lp))
        elog(PANIC, "heap_lock_redo: invalid lp");

    htup = (HeapTupleHeader) PageGetItem(page, lp);

    fix_infomask_from_infobits(xlrec->infobits_set, &htup->t_infomask,
                               &htup->t_infomask2);
    HeapTupleHeaderClearHotUpdated(htup);
    HeapTupleHeaderSetXmax(htup, xlrec->locking_xid);
    HeapTupleHeaderSetCmax(htup, FirstCommandId, false);
    /* Make sure there is no forward chain link in t_ctid */
    htup->t_ctid = xlrec->target.tid;
    PageSetLSN(page, lsn);
    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);
}

static void
heap_xlog_lock_updated(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_lock_updated *xlrec =
        (xl_heap_lock_updated *) XLogRecGetData(record);
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    ItemId      lp = NULL;
    HeapTupleHeader htup;

    /* If we have a full-page image, restore it and we're done */
    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        (void) RestoreBackupBlock(lsn, record, 0, false, false);
        return;
    }

    buffer = XLogReadBuffer(xlrec->target.node,
                            ItemPointerGetBlockNumber(&(xlrec->target.tid)),
                            false);
    if (!BufferIsValid(buffer))
        return;
    page = (Page) BufferGetPage(buffer);

    if (lsn <= PageGetLSN(page))        /* changes are applied */
    {
        UnlockReleaseBuffer(buffer);
        return;
    }

    offnum = ItemPointerGetOffsetNumber(&(xlrec->target.tid));
    if (PageGetMaxOffsetNumber(page) >= offnum)
        lp = PageGetItemId(page, offnum);

    if (PageGetMaxOffsetNumber(page) < offnum || !ItemIdIsNormal(lp))
        elog(PANIC, "heap_xlog_lock_updated: invalid lp");

    htup = (HeapTupleHeader) PageGetItem(page, lp);

    fix_infomask_from_infobits(xlrec->infobits_set, &htup->t_infomask,
                               &htup->t_infomask2);
    HeapTupleHeaderSetXmax(htup, xlrec->xmax);

    PageSetLSN(page, lsn);
    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);
}

static void
heap_xlog_inplace(XLogRecPtr lsn, XLogRecord *record)
{
    xl_heap_inplace *xlrec = (xl_heap_inplace *) XLogRecGetData(record);
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    ItemId      lp = NULL;
    HeapTupleHeader htup;
    uint32      oldlen;
    uint32      newlen;

    /* If we have a full-page image, restore it and we're done */
    if (record->xl_info & XLR_BKP_BLOCK(0))
    {
        (void) RestoreBackupBlock(lsn, record, 0, false, false);
        return;
    }

    buffer = XLogReadBuffer(xlrec->target.node,
                            ItemPointerGetBlockNumber(&(xlrec->target.tid)),
                            false);
    if (!BufferIsValid(buffer))
        return;
    page = (Page) BufferGetPage(buffer);

    if (lsn <= PageGetLSN(page))        /* changes are applied */
    {
        UnlockReleaseBuffer(buffer);
        return;
    }

    offnum = ItemPointerGetOffsetNumber(&(xlrec->target.tid));
    if (PageGetMaxOffsetNumber(page) >= offnum)
        lp = PageGetItemId(page, offnum);

    if (PageGetMaxOffsetNumber(page) < offnum || !ItemIdIsNormal(lp))
        elog(PANIC, "heap_inplace_redo: invalid lp");

    htup = (HeapTupleHeader) PageGetItem(page, lp);

    oldlen = ItemIdGetLength(lp) - htup->t_hoff;
    newlen = record->xl_len - SizeOfHeapInplace;
    if (oldlen != newlen)
        elog(PANIC, "heap_inplace_redo: wrong tuple length");

    memcpy((char *) htup + htup->t_hoff,
           (char *) xlrec + SizeOfHeapInplace,
           newlen);

    PageSetLSN(page, lsn);
    MarkBufferDirty(buffer);
    UnlockReleaseBuffer(buffer);
}

void
heap_redo(XLogRecPtr lsn, XLogRecord *record)
{
    uint8       info = record->xl_info & ~XLR_INFO_MASK;

    /*
     * These operations don't overwrite MVCC data so no conflict processing is
     * required. The ones in heap2 rmgr do.
     */

    switch (info & XLOG_HEAP_OPMASK)
    {
        case XLOG_HEAP_INSERT:
            heap_xlog_insert(lsn, record);
            break;
        case XLOG_HEAP_DELETE:
            heap_xlog_delete(lsn, record);
            break;
        case XLOG_HEAP_UPDATE:
            heap_xlog_update(lsn, record, false);
            break;
        case XLOG_HEAP_HOT_UPDATE:
            heap_xlog_update(lsn, record, true);
            break;
        case XLOG_HEAP_NEWPAGE:
            heap_xlog_newpage(lsn, record);
            break;
        case XLOG_HEAP_LOCK:
            heap_xlog_lock(lsn, record);
            break;
        case XLOG_HEAP_INPLACE:
            heap_xlog_inplace(lsn, record);
            break;
        default:
            elog(PANIC, "heap_redo: unknown op code %u", info);
    }
}

void
heap2_redo(XLogRecPtr lsn, XLogRecord *record)
{
    uint8       info = record->xl_info & ~XLR_INFO_MASK;

    switch (info & XLOG_HEAP_OPMASK)
    {
        case XLOG_HEAP2_FREEZE:
            heap_xlog_freeze(lsn, record);
            break;
        case XLOG_HEAP2_CLEAN:
            heap_xlog_clean(lsn, record);
            break;
        case XLOG_HEAP2_CLEANUP_INFO:
            heap_xlog_cleanup_info(lsn, record);
            break;
        case XLOG_HEAP2_VISIBLE:
            heap_xlog_visible(lsn, record);
            break;
        case XLOG_HEAP2_MULTI_INSERT:
            heap_xlog_multi_insert(lsn, record);
            break;
        case XLOG_HEAP2_LOCK_UPDATED:
            heap_xlog_lock_updated(lsn, record);
            break;
        default:
            elog(PANIC, "heap2_redo: unknown op code %u", info);
    }
}

/*
 *  heap_sync       - sync a heap, for use when no WAL has been written
 *
 * This forces the heap contents (including TOAST heap if any) down to disk.
 * If we skipped using WAL, and WAL is otherwise needed, we must force the
 * relation down to disk before it's safe to commit the transaction.  This
 * requires writing out any dirty buffers and then doing a forced fsync.
 *
 * Indexes are not touched.  (Currently, index operations associated with
 * the commands that use this are WAL-logged and so do not need fsync.
 * That behavior might change someday, but in any case it's likely that
 * any fsync decisions required would be per-index and hence not appropriate
 * to be done here.)
 */
void
heap_sync(Relation rel)
{
    /* non-WAL-logged tables never need fsync */
    if (!RelationNeedsWAL(rel))
        return;

    /* main heap */
    FlushRelationBuffers(rel);
    /* FlushRelationBuffers will have opened rd_smgr */
    smgrimmedsync(rel->rd_smgr, MAIN_FORKNUM);

    /* FSM is not critical, don't bother syncing it */

    /* toast heap, if any */
    if (OidIsValid(rel->rd_rel->reltoastrelid))
    {
        Relation    toastrel;

        toastrel = heap_open(rel->rd_rel->reltoastrelid, AccessShareLock);
        FlushRelationBuffers(toastrel);
        smgrimmedsync(toastrel->rd_smgr, MAIN_FORKNUM);
        heap_close(toastrel, AccessShareLock);
    }
}