vacuum.c

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/*-------------------------------------------------------------------------
 *
 * vacuum.c
 *    The postgres vacuum cleaner.
 *
 * This file now includes only control and dispatch code for VACUUM and
 * ANALYZE commands.  Regular VACUUM is implemented in vacuumlazy.c,
 * ANALYZE in analyze.c, and VACUUM FULL is a variant of CLUSTER, handled
 * in cluster.c.
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/commands/vacuum.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <math.h>

#include "access/clog.h"
#include "access/genam.h"
#include "access/heapam.h"
#include "access/htup_details.h"
#include "access/multixact.h"
#include "access/transam.h"
#include "access/xact.h"
#include "catalog/namespace.h"
#include "catalog/pg_database.h"
#include "catalog/pg_namespace.h"
#include "commands/cluster.h"
#include "commands/vacuum.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "storage/bufmgr.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/acl.h"
#include "utils/fmgroids.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"
#include "utils/tqual.h"


/*
 * GUC parameters
 */
int         vacuum_freeze_min_age;
int         vacuum_freeze_table_age;


/* A few variables that don't seem worth passing around as parameters */
static MemoryContext vac_context = NULL;
static BufferAccessStrategy vac_strategy;


/* non-export function prototypes */
static List *get_rel_oids(Oid relid, const RangeVar *vacrel);
static void vac_truncate_clog(TransactionId frozenXID, MultiXactId frozenMulti);
static bool vacuum_rel(Oid relid, VacuumStmt *vacstmt, bool do_toast,
           bool for_wraparound);


/*
 * Primary entry point for VACUUM and ANALYZE commands.
 *
 * relid is normally InvalidOid; if it is not, then it provides the relation
 * OID to be processed, and vacstmt->relation is ignored.  (The non-invalid
 * case is currently only used by autovacuum.)
 *
 * do_toast is passed as FALSE by autovacuum, because it processes TOAST
 * tables separately.
 *
 * for_wraparound is used by autovacuum to let us know when it's forcing
 * a vacuum for wraparound, which should not be auto-canceled.
 *
 * bstrategy is normally given as NULL, but in autovacuum it can be passed
 * in to use the same buffer strategy object across multiple vacuum() calls.
 *
 * isTopLevel should be passed down from ProcessUtility.
 *
 * It is the caller's responsibility that vacstmt and bstrategy
 * (if given) be allocated in a memory context that won't disappear
 * at transaction commit.
 */
void
vacuum(VacuumStmt *vacstmt, Oid relid, bool do_toast,
       BufferAccessStrategy bstrategy, bool for_wraparound, bool isTopLevel)
{
    const char *stmttype;
    volatile bool in_outer_xact,
                use_own_xacts;
    List       *relations;

    /* sanity checks on options */
    Assert(vacstmt->options & (VACOPT_VACUUM | VACOPT_ANALYZE));
    Assert((vacstmt->options & VACOPT_VACUUM) ||
           !(vacstmt->options & (VACOPT_FULL | VACOPT_FREEZE)));
    Assert((vacstmt->options & VACOPT_ANALYZE) || vacstmt->va_cols == NIL);

    stmttype = (vacstmt->options & VACOPT_VACUUM) ? "VACUUM" : "ANALYZE";

    /*
     * We cannot run VACUUM inside a user transaction block; if we were inside
     * a transaction, then our commit- and start-transaction-command calls
     * would not have the intended effect!  There are numerous other subtle
     * dependencies on this, too.
     *
     * ANALYZE (without VACUUM) can run either way.
     */
    if (vacstmt->options & VACOPT_VACUUM)
    {
        PreventTransactionChain(isTopLevel, stmttype);
        in_outer_xact = false;
    }
    else
        in_outer_xact = IsInTransactionChain(isTopLevel);

    /*
     * Send info about dead objects to the statistics collector, unless we are
     * in autovacuum --- autovacuum.c does this for itself.
     */
    if ((vacstmt->options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess())
        pgstat_vacuum_stat();

    /*
     * Create special memory context for cross-transaction storage.
     *
     * Since it is a child of PortalContext, it will go away eventually even
     * if we suffer an error; there's no need for special abort cleanup logic.
     */
    vac_context = AllocSetContextCreate(PortalContext,
                                        "Vacuum",
                                        ALLOCSET_DEFAULT_MINSIZE,
                                        ALLOCSET_DEFAULT_INITSIZE,
                                        ALLOCSET_DEFAULT_MAXSIZE);

    /*
     * If caller didn't give us a buffer strategy object, make one in the
     * cross-transaction memory context.
     */
    if (bstrategy == NULL)
    {
        MemoryContext old_context = MemoryContextSwitchTo(vac_context);

        bstrategy = GetAccessStrategy(BAS_VACUUM);
        MemoryContextSwitchTo(old_context);
    }
    vac_strategy = bstrategy;

    /*
     * Build list of relations to process, unless caller gave us one. (If we
     * build one, we put it in vac_context for safekeeping.)
     */
    relations = get_rel_oids(relid, vacstmt->relation);

    /*
     * Decide whether we need to start/commit our own transactions.
     *
     * For VACUUM (with or without ANALYZE): always do so, so that we can
     * release locks as soon as possible.  (We could possibly use the outer
     * transaction for a one-table VACUUM, but handling TOAST tables would be
     * problematic.)
     *
     * For ANALYZE (no VACUUM): if inside a transaction block, we cannot
     * start/commit our own transactions.  Also, there's no need to do so if
     * only processing one relation.  For multiple relations when not within a
     * transaction block, and also in an autovacuum worker, use own
     * transactions so we can release locks sooner.
     */
    if (vacstmt->options & VACOPT_VACUUM)
        use_own_xacts = true;
    else
    {
        Assert(vacstmt->options & VACOPT_ANALYZE);
        if (IsAutoVacuumWorkerProcess())
            use_own_xacts = true;
        else if (in_outer_xact)
            use_own_xacts = false;
        else if (list_length(relations) > 1)
            use_own_xacts = true;
        else
            use_own_xacts = false;
    }

    /*
     * vacuum_rel expects to be entered with no transaction active; it will
     * start and commit its own transaction.  But we are called by an SQL
     * command, and so we are executing inside a transaction already. We
     * commit the transaction started in PostgresMain() here, and start
     * another one before exiting to match the commit waiting for us back in
     * PostgresMain().
     */
    if (use_own_xacts)
    {
        /* ActiveSnapshot is not set by autovacuum */
        if (ActiveSnapshotSet())
            PopActiveSnapshot();

        /* matches the StartTransaction in PostgresMain() */
        CommitTransactionCommand();
    }

    /* Turn vacuum cost accounting on or off */
    PG_TRY();
    {
        ListCell   *cur;

        VacuumCostActive = (VacuumCostDelay > 0);
        VacuumCostBalance = 0;
        VacuumPageHit = 0;
        VacuumPageMiss = 0;
        VacuumPageDirty = 0;

        /*
         * Loop to process each selected relation.
         */
        foreach(cur, relations)
        {
            Oid         relid = lfirst_oid(cur);

            if (vacstmt->options & VACOPT_VACUUM)
            {
                if (!vacuum_rel(relid, vacstmt, do_toast, for_wraparound))
                    continue;
            }

            if (vacstmt->options & VACOPT_ANALYZE)
            {
                /*
                 * If using separate xacts, start one for analyze. Otherwise,
                 * we can use the outer transaction.
                 */
                if (use_own_xacts)
                {
                    StartTransactionCommand();
                    /* functions in indexes may want a snapshot set */
                    PushActiveSnapshot(GetTransactionSnapshot());
                }

                analyze_rel(relid, vacstmt, vac_strategy);

                if (use_own_xacts)
                {
                    PopActiveSnapshot();
                    CommitTransactionCommand();
                }
            }
        }
    }
    PG_CATCH();
    {
        /* Make sure cost accounting is turned off after error */
        VacuumCostActive = false;
        PG_RE_THROW();
    }
    PG_END_TRY();

    /* Turn off vacuum cost accounting */
    VacuumCostActive = false;

    /*
     * Finish up processing.
     */
    if (use_own_xacts)
    {
        /* here, we are not in a transaction */

        /*
         * This matches the CommitTransaction waiting for us in
         * PostgresMain().
         */
        StartTransactionCommand();
    }

    if ((vacstmt->options & VACOPT_VACUUM) && !IsAutoVacuumWorkerProcess())
    {
        /*
         * Update pg_database.datfrozenxid, and truncate pg_clog if possible.
         * (autovacuum.c does this for itself.)
         */
        vac_update_datfrozenxid();
    }

    /*
     * Clean up working storage --- note we must do this after
     * StartTransactionCommand, else we might be trying to delete the active
     * context!
     */
    MemoryContextDelete(vac_context);
    vac_context = NULL;
}

/*
 * Build a list of Oids for each relation to be processed
 *
 * The list is built in vac_context so that it will survive across our
 * per-relation transactions.
 */
static List *
get_rel_oids(Oid relid, const RangeVar *vacrel)
{
    List       *oid_list = NIL;
    MemoryContext oldcontext;

    /* OID supplied by VACUUM's caller? */
    if (OidIsValid(relid))
    {
        oldcontext = MemoryContextSwitchTo(vac_context);
        oid_list = lappend_oid(oid_list, relid);
        MemoryContextSwitchTo(oldcontext);
    }
    else if (vacrel)
    {
        /* Process a specific relation */
        Oid         relid;

        /*
         * Since we don't take a lock here, the relation might be gone, or the
         * RangeVar might no longer refer to the OID we look up here.  In the
         * former case, VACUUM will do nothing; in the latter case, it will
         * process the OID we looked up here, rather than the new one.
         * Neither is ideal, but there's little practical alternative, since
         * we're going to commit this transaction and begin a new one between
         * now and then.
         */
        relid = RangeVarGetRelid(vacrel, NoLock, false);

        /* Make a relation list entry for this guy */
        oldcontext = MemoryContextSwitchTo(vac_context);
        oid_list = lappend_oid(oid_list, relid);
        MemoryContextSwitchTo(oldcontext);
    }
    else
    {
        /*
         * Process all plain relations and materialized views listed in
         * pg_class
         */
        Relation    pgclass;
        HeapScanDesc scan;
        HeapTuple   tuple;

        pgclass = heap_open(RelationRelationId, AccessShareLock);

        scan = heap_beginscan(pgclass, SnapshotNow, 0, NULL);

        while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
        {
            Form_pg_class classForm = (Form_pg_class) GETSTRUCT(tuple);

            if (classForm->relkind != RELKIND_RELATION &&
                classForm->relkind != RELKIND_MATVIEW)
                continue;

            /* Make a relation list entry for this guy */
            oldcontext = MemoryContextSwitchTo(vac_context);
            oid_list = lappend_oid(oid_list, HeapTupleGetOid(tuple));
            MemoryContextSwitchTo(oldcontext);
        }

        heap_endscan(scan);
        heap_close(pgclass, AccessShareLock);
    }

    return oid_list;
}

/*
 * vacuum_set_xid_limits() -- compute oldest-Xmin and freeze cutoff points
 */
void
vacuum_set_xid_limits(int freeze_min_age,
                      int freeze_table_age,
                      bool sharedRel,
                      TransactionId *oldestXmin,
                      TransactionId *freezeLimit,
                      TransactionId *freezeTableLimit,
                      MultiXactId *multiXactFrzLimit)
{
    int         freezemin;
    TransactionId limit;
    TransactionId safeLimit;

    /*
     * We can always ignore processes running lazy vacuum.  This is because we
     * use these values only for deciding which tuples we must keep in the
     * tables.  Since lazy vacuum doesn't write its XID anywhere, it's safe to
     * ignore it.  In theory it could be problematic to ignore lazy vacuums in
     * a full vacuum, but keep in mind that only one vacuum process can be
     * working on a particular table at any time, and that each vacuum is
     * always an independent transaction.
     */
    *oldestXmin = GetOldestXmin(sharedRel, true);

    Assert(TransactionIdIsNormal(*oldestXmin));

    /*
     * Determine the minimum freeze age to use: as specified by the caller, or
     * vacuum_freeze_min_age, but in any case not more than half
     * autovacuum_freeze_max_age, so that autovacuums to prevent XID
     * wraparound won't occur too frequently.
     */
    freezemin = freeze_min_age;
    if (freezemin < 0)
        freezemin = vacuum_freeze_min_age;
    freezemin = Min(freezemin, autovacuum_freeze_max_age / 2);
    Assert(freezemin >= 0);

    /*
     * Compute the cutoff XID, being careful not to generate a "permanent" XID
     */
    limit = *oldestXmin - freezemin;
    if (!TransactionIdIsNormal(limit))
        limit = FirstNormalTransactionId;

    /*
     * If oldestXmin is very far back (in practice, more than
     * autovacuum_freeze_max_age / 2 XIDs old), complain and force a minimum
     * freeze age of zero.
     */
    safeLimit = ReadNewTransactionId() - autovacuum_freeze_max_age;
    if (!TransactionIdIsNormal(safeLimit))
        safeLimit = FirstNormalTransactionId;

    if (TransactionIdPrecedes(limit, safeLimit))
    {
        ereport(WARNING,
                (errmsg("oldest xmin is far in the past"),
                 errhint("Close open transactions soon to avoid wraparound problems.")));
        limit = *oldestXmin;
    }

    *freezeLimit = limit;

    if (freezeTableLimit != NULL)
    {
        int         freezetable;

        /*
         * Determine the table freeze age to use: as specified by the caller,
         * or vacuum_freeze_table_age, but in any case not more than
         * autovacuum_freeze_max_age * 0.95, so that if you have e.g nightly
         * VACUUM schedule, the nightly VACUUM gets a chance to freeze tuples
         * before anti-wraparound autovacuum is launched.
         */
        freezetable = freeze_table_age;
        if (freezetable < 0)
            freezetable = vacuum_freeze_table_age;
        freezetable = Min(freezetable, autovacuum_freeze_max_age * 0.95);
        Assert(freezetable >= 0);

        /*
         * Compute the cutoff XID, being careful not to generate a "permanent"
         * XID.
         */
        limit = ReadNewTransactionId() - freezetable;
        if (!TransactionIdIsNormal(limit))
            limit = FirstNormalTransactionId;

        *freezeTableLimit = limit;
    }

    if (multiXactFrzLimit != NULL)
    {
        MultiXactId mxLimit;

        /*
         * simplistic multixactid freezing: use the same freezing policy as
         * for Xids
         */
        mxLimit = GetOldestMultiXactId() - freezemin;
        if (mxLimit < FirstMultiXactId)
            mxLimit = FirstMultiXactId;

        *multiXactFrzLimit = mxLimit;
    }
}

/*
 * vac_estimate_reltuples() -- estimate the new value for pg_class.reltuples
 *
 *      If we scanned the whole relation then we should just use the count of
 *      live tuples seen; but if we did not, we should not trust the count
 *      unreservedly, especially not in VACUUM, which may have scanned a quite
 *      nonrandom subset of the table.  When we have only partial information,
 *      we take the old value of pg_class.reltuples as a measurement of the
 *      tuple density in the unscanned pages.
 *
 *      This routine is shared by VACUUM and ANALYZE.
 */
double
vac_estimate_reltuples(Relation relation, bool is_analyze,
                       BlockNumber total_pages,
                       BlockNumber scanned_pages,
                       double scanned_tuples)
{
    BlockNumber old_rel_pages = relation->rd_rel->relpages;
    double      old_rel_tuples = relation->rd_rel->reltuples;
    double      old_density;
    double      new_density;
    double      multiplier;
    double      updated_density;

    /* If we did scan the whole table, just use the count as-is */
    if (scanned_pages >= total_pages)
        return scanned_tuples;

    /*
     * If scanned_pages is zero but total_pages isn't, keep the existing value
     * of reltuples.  (Note: callers should avoid updating the pg_class
     * statistics in this situation, since no new information has been
     * provided.)
     */
    if (scanned_pages == 0)
        return old_rel_tuples;

    /*
     * If old value of relpages is zero, old density is indeterminate; we
     * can't do much except scale up scanned_tuples to match total_pages.
     */
    if (old_rel_pages == 0)
        return floor((scanned_tuples / scanned_pages) * total_pages + 0.5);

    /*
     * Okay, we've covered the corner cases.  The normal calculation is to
     * convert the old measurement to a density (tuples per page), then update
     * the density using an exponential-moving-average approach, and finally
     * compute reltuples as updated_density * total_pages.
     *
     * For ANALYZE, the moving average multiplier is just the fraction of the
     * table's pages we scanned.  This is equivalent to assuming that the
     * tuple density in the unscanned pages didn't change.  Of course, it
     * probably did, if the new density measurement is different. But over
     * repeated cycles, the value of reltuples will converge towards the
     * correct value, if repeated measurements show the same new density.
     *
     * For VACUUM, the situation is a bit different: we have looked at a
     * nonrandom sample of pages, but we know for certain that the pages we
     * didn't look at are precisely the ones that haven't changed lately.
     * Thus, there is a reasonable argument for doing exactly the same thing
     * as for the ANALYZE case, that is use the old density measurement as the
     * value for the unscanned pages.
     *
     * This logic could probably use further refinement.
     */
    old_density = old_rel_tuples / old_rel_pages;
    new_density = scanned_tuples / scanned_pages;
    multiplier = (double) scanned_pages / (double) total_pages;
    updated_density = old_density + (new_density - old_density) * multiplier;
    return floor(updated_density * total_pages + 0.5);
}


/*
 *  vac_update_relstats() -- update statistics for one relation
 *
 *      Update the whole-relation statistics that are kept in its pg_class
 *      row.  There are additional stats that will be updated if we are
 *      doing ANALYZE, but we always update these stats.  This routine works
 *      for both index and heap relation entries in pg_class.
 *
 *      We violate transaction semantics here by overwriting the rel's
 *      existing pg_class tuple with the new values.  This is reasonably
 *      safe since the new values are correct whether or not this transaction
 *      commits.  The reason for this is that if we updated these tuples in
 *      the usual way, vacuuming pg_class itself wouldn't work very well ---
 *      by the time we got done with a vacuum cycle, most of the tuples in
 *      pg_class would've been obsoleted.  Of course, this only works for
 *      fixed-size never-null columns, but these are.
 *
 *      Note another assumption: that two VACUUMs/ANALYZEs on a table can't
 *      run in parallel, nor can VACUUM/ANALYZE run in parallel with a
 *      schema alteration such as adding an index, rule, or trigger.  Otherwise
 *      our updates of relhasindex etc might overwrite uncommitted updates.
 *
 *      Another reason for doing it this way is that when we are in a lazy
 *      VACUUM and have PROC_IN_VACUUM set, we mustn't do any updates ---
 *      somebody vacuuming pg_class might think they could delete a tuple
 *      marked with xmin = our xid.
 *
 *      This routine is shared by VACUUM and ANALYZE.
 */
void
vac_update_relstats(Relation relation,
                    BlockNumber num_pages, double num_tuples,
                    BlockNumber num_all_visible_pages,
                    bool hasindex, TransactionId frozenxid,
                    MultiXactId minmulti)
{
    Oid         relid = RelationGetRelid(relation);
    Relation    rd;
    HeapTuple   ctup;
    Form_pg_class pgcform;
    bool        dirty;

    rd = heap_open(RelationRelationId, RowExclusiveLock);

    /* Fetch a copy of the tuple to scribble on */
    ctup = SearchSysCacheCopy1(RELOID, ObjectIdGetDatum(relid));
    if (!HeapTupleIsValid(ctup))
        elog(ERROR, "pg_class entry for relid %u vanished during vacuuming",
             relid);
    pgcform = (Form_pg_class) GETSTRUCT(ctup);

    /* Apply required updates, if any, to copied tuple */

    dirty = false;
    if (pgcform->relpages != (int32) num_pages)
    {
        pgcform->relpages = (int32) num_pages;
        dirty = true;
    }
    if (pgcform->reltuples != (float4) num_tuples)
    {
        pgcform->reltuples = (float4) num_tuples;
        dirty = true;
    }
    if (pgcform->relallvisible != (int32) num_all_visible_pages)
    {
        pgcform->relallvisible = (int32) num_all_visible_pages;
        dirty = true;
    }
    if (pgcform->relhasindex != hasindex)
    {
        pgcform->relhasindex = hasindex;
        dirty = true;
    }

    /*
     * If we have discovered that there are no indexes, then there's no
     * primary key either.  This could be done more thoroughly...
     */
    if (pgcform->relhaspkey && !hasindex)
    {
        pgcform->relhaspkey = false;
        dirty = true;
    }

    /* We also clear relhasrules and relhastriggers if needed */
    if (pgcform->relhasrules && relation->rd_rules == NULL)
    {
        pgcform->relhasrules = false;
        dirty = true;
    }
    if (pgcform->relhastriggers && relation->trigdesc == NULL)
    {
        pgcform->relhastriggers = false;
        dirty = true;
    }

    /*
     * relfrozenxid should never go backward.  Caller can pass
     * InvalidTransactionId if it has no new data.
     */
    if (TransactionIdIsNormal(frozenxid) &&
        TransactionIdPrecedes(pgcform->relfrozenxid, frozenxid))
    {
        pgcform->relfrozenxid = frozenxid;
        dirty = true;
    }

    /* relminmxid must never go backward, either */
    if (MultiXactIdIsValid(minmulti) &&
        MultiXactIdPrecedes(pgcform->relminmxid, minmulti))
    {
        pgcform->relminmxid = minmulti;
        dirty = true;
    }

    /* If anything changed, write out the tuple. */
    if (dirty)
        heap_inplace_update(rd, ctup);

    heap_close(rd, RowExclusiveLock);
}


/*
 *  vac_update_datfrozenxid() -- update pg_database.datfrozenxid for our DB
 *
 *      Update pg_database's datfrozenxid entry for our database to be the
 *      minimum of the pg_class.relfrozenxid values.
 *
 *      Similarly, update our datfrozenmulti to be the minimum of the
 *      pg_class.relfrozenmulti values.
 *
 *      If we are able to advance either pg_database value, also try to
 *      truncate pg_clog and pg_multixact.
 *
 *      We violate transaction semantics here by overwriting the database's
 *      existing pg_database tuple with the new value.  This is reasonably
 *      safe since the new value is correct whether or not this transaction
 *      commits.  As with vac_update_relstats, this avoids leaving dead tuples
 *      behind after a VACUUM.
 */
void
vac_update_datfrozenxid(void)
{
    HeapTuple   tuple;
    Form_pg_database dbform;
    Relation    relation;
    SysScanDesc scan;
    HeapTuple   classTup;
    TransactionId newFrozenXid;
    MultiXactId newFrozenMulti;
    bool        dirty = false;

    /*
     * Initialize the "min" calculation with GetOldestXmin, which is a
     * reasonable approximation to the minimum relfrozenxid for not-yet-
     * committed pg_class entries for new tables; see AddNewRelationTuple().
     * So we cannot produce a wrong minimum by starting with this.
     */
    newFrozenXid = GetOldestXmin(true, true);

    /*
     * Similarly, initialize the MultiXact "min" with the value that would
     * be used on pg_class for new tables.  See AddNewRelationTuple().
     */
    newFrozenMulti = GetOldestMultiXactId();

    /*
     * We must seqscan pg_class to find the minimum Xid, because there is no
     * index that can help us here.
     */
    relation = heap_open(RelationRelationId, AccessShareLock);

    scan = systable_beginscan(relation, InvalidOid, false,
                              SnapshotNow, 0, NULL);

    while ((classTup = systable_getnext(scan)) != NULL)
    {
        Form_pg_class classForm = (Form_pg_class) GETSTRUCT(classTup);

        /*
         * Only consider heap and TOAST tables (anything else should have
         * InvalidTransactionId in relfrozenxid anyway.)
         */
        if (classForm->relkind != RELKIND_RELATION &&
            classForm->relkind != RELKIND_MATVIEW &&
            classForm->relkind != RELKIND_TOASTVALUE)
            continue;

        Assert(TransactionIdIsNormal(classForm->relfrozenxid));
        Assert(MultiXactIdIsValid(classForm->relminmxid));

        if (TransactionIdPrecedes(classForm->relfrozenxid, newFrozenXid))
            newFrozenXid = classForm->relfrozenxid;

        if (MultiXactIdPrecedes(classForm->relminmxid, newFrozenMulti))
            newFrozenMulti = classForm->relminmxid;
    }

    /* we're done with pg_class */
    systable_endscan(scan);
    heap_close(relation, AccessShareLock);

    Assert(TransactionIdIsNormal(newFrozenXid));
    Assert(MultiXactIdIsValid(newFrozenMulti));

    /* Now fetch the pg_database tuple we need to update. */
    relation = heap_open(DatabaseRelationId, RowExclusiveLock);

    /* Fetch a copy of the tuple to scribble on */
    tuple = SearchSysCacheCopy1(DATABASEOID, ObjectIdGetDatum(MyDatabaseId));
    if (!HeapTupleIsValid(tuple))
        elog(ERROR, "could not find tuple for database %u", MyDatabaseId);
    dbform = (Form_pg_database) GETSTRUCT(tuple);

    /*
     * Don't allow datfrozenxid to go backward (probably can't happen anyway);
     * and detect the common case where it doesn't go forward either.
     */
    if (TransactionIdPrecedes(dbform->datfrozenxid, newFrozenXid))
    {
        dbform->datfrozenxid = newFrozenXid;
        dirty = true;
    }

    /* ditto */
    if (MultiXactIdPrecedes(dbform->datminmxid, newFrozenMulti))
    {
        dbform->datminmxid = newFrozenMulti;
        dirty = true;
    }

    if (dirty)
        heap_inplace_update(relation, tuple);

    heap_freetuple(tuple);
    heap_close(relation, RowExclusiveLock);

    /*
     * If we were able to advance datfrozenxid, see if we can truncate
     * pg_clog. Also do it if the shared XID-wrap-limit info is stale, since
     * this action will update that too.
     */
    if (dirty || ForceTransactionIdLimitUpdate())
        vac_truncate_clog(newFrozenXid, newFrozenMulti);
}


/*
 *  vac_truncate_clog() -- attempt to truncate the commit log
 *
 *      Scan pg_database to determine the system-wide oldest datfrozenxid,
 *      and use it to truncate the transaction commit log (pg_clog).
 *      Also update the XID wrap limit info maintained by varsup.c.
 *
 *      The passed XID is simply the one I just wrote into my pg_database
 *      entry.  It's used to initialize the "min" calculation.
 *
 *      This routine is only invoked when we've managed to change our
 *      DB's datfrozenxid entry, or we found that the shared XID-wrap-limit
 *      info is stale.
 */
static void
vac_truncate_clog(TransactionId frozenXID, MultiXactId frozenMulti)
{
    TransactionId myXID = GetCurrentTransactionId();
    Relation    relation;
    HeapScanDesc scan;
    HeapTuple   tuple;
    Oid         oldestxid_datoid;
    Oid         oldestmulti_datoid;
    bool        frozenAlreadyWrapped = false;

    /* init oldest datoids to sync with my frozen values */
    oldestxid_datoid = MyDatabaseId;
    oldestmulti_datoid = MyDatabaseId;

    /*
     * Scan pg_database to compute the minimum datfrozenxid
     *
     * Note: we need not worry about a race condition with new entries being
     * inserted by CREATE DATABASE.  Any such entry will have a copy of some
     * existing DB's datfrozenxid, and that source DB cannot be ours because
     * of the interlock against copying a DB containing an active backend.
     * Hence the new entry will not reduce the minimum.  Also, if two VACUUMs
     * concurrently modify the datfrozenxid's of different databases, the
     * worst possible outcome is that pg_clog is not truncated as aggressively
     * as it could be.
     */
    relation = heap_open(DatabaseRelationId, AccessShareLock);

    scan = heap_beginscan(relation, SnapshotNow, 0, NULL);

    while ((tuple = heap_getnext(scan, ForwardScanDirection)) != NULL)
    {
        Form_pg_database dbform = (Form_pg_database) GETSTRUCT(tuple);

        Assert(TransactionIdIsNormal(dbform->datfrozenxid));
        Assert(MultiXactIdIsValid(dbform->datminmxid));

        if (TransactionIdPrecedes(myXID, dbform->datfrozenxid))
            frozenAlreadyWrapped = true;
        else if (TransactionIdPrecedes(dbform->datfrozenxid, frozenXID))
        {
            frozenXID = dbform->datfrozenxid;
            oldestxid_datoid = HeapTupleGetOid(tuple);
        }

        if (MultiXactIdPrecedes(dbform->datminmxid, frozenMulti))
        {
            frozenMulti = dbform->datminmxid;
            oldestmulti_datoid = HeapTupleGetOid(tuple);
        }
    }

    heap_endscan(scan);

    heap_close(relation, AccessShareLock);

    /*
     * Do not truncate CLOG if we seem to have suffered wraparound already;
     * the computed minimum XID might be bogus.  This case should now be
     * impossible due to the defenses in GetNewTransactionId, but we keep the
     * test anyway.
     */
    if (frozenAlreadyWrapped)
    {
        ereport(WARNING,
                (errmsg("some databases have not been vacuumed in over 2 billion transactions"),
                 errdetail("You might have already suffered transaction-wraparound data loss.")));
        return;
    }

    /* Truncate CLOG and Multi to the oldest computed value */
    TruncateCLOG(frozenXID);
    TruncateMultiXact(frozenMulti);

    /*
     * Update the wrap limit for GetNewTransactionId and creation of new
     * MultiXactIds.  Note: these functions will also signal the postmaster for
     * an(other) autovac cycle if needed.   XXX should we avoid possibly
     * signalling twice?
     */
    SetTransactionIdLimit(frozenXID, oldestxid_datoid);
    MultiXactAdvanceOldest(frozenMulti, oldestmulti_datoid);
}


/*
 *  vacuum_rel() -- vacuum one heap relation
 *
 *      Doing one heap at a time incurs extra overhead, since we need to
 *      check that the heap exists again just before we vacuum it.  The
 *      reason that we do this is so that vacuuming can be spread across
 *      many small transactions.  Otherwise, two-phase locking would require
 *      us to lock the entire database during one pass of the vacuum cleaner.
 *
 *      At entry and exit, we are not inside a transaction.
 */
static bool
vacuum_rel(Oid relid, VacuumStmt *vacstmt, bool do_toast, bool for_wraparound)
{
    LOCKMODE    lmode;
    Relation    onerel;
    LockRelId   onerelid;
    Oid         toast_relid;
    Oid         save_userid;
    int         save_sec_context;
    int         save_nestlevel;

    /* Begin a transaction for vacuuming this relation */
    StartTransactionCommand();

    /*
     * Functions in indexes may want a snapshot set.  Also, setting a snapshot
     * ensures that RecentGlobalXmin is kept truly recent.
     */
    PushActiveSnapshot(GetTransactionSnapshot());

    if (!(vacstmt->options & VACOPT_FULL))
    {
        /*
         * In lazy vacuum, we can set the PROC_IN_VACUUM flag, which lets
         * other concurrent VACUUMs know that they can ignore this one while
         * determining their OldestXmin.  (The reason we don't set it during a
         * full VACUUM is exactly that we may have to run user-defined
         * functions for functional indexes, and we want to make sure that if
         * they use the snapshot set above, any tuples it requires can't get
         * removed from other tables.  An index function that depends on the
         * contents of other tables is arguably broken, but we won't break it
         * here by violating transaction semantics.)
         *
         * We also set the VACUUM_FOR_WRAPAROUND flag, which is passed down by
         * autovacuum; it's used to avoid canceling a vacuum that was invoked
         * in an emergency.
         *
         * Note: these flags remain set until CommitTransaction or
         * AbortTransaction.  We don't want to clear them until we reset
         * MyPgXact->xid/xmin, else OldestXmin might appear to go backwards,
         * which is probably Not Good.
         */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
        MyPgXact->vacuumFlags |= PROC_IN_VACUUM;
        if (for_wraparound)
            MyPgXact->vacuumFlags |= PROC_VACUUM_FOR_WRAPAROUND;
        LWLockRelease(ProcArrayLock);
    }

    /*
     * Check for user-requested abort.  Note we want this to be inside a
     * transaction, so xact.c doesn't issue useless WARNING.
     */
    CHECK_FOR_INTERRUPTS();

    /*
     * Determine the type of lock we want --- hard exclusive lock for a FULL
     * vacuum, but just ShareUpdateExclusiveLock for concurrent vacuum. Either
     * way, we can be sure that no other backend is vacuuming the same table.
     */
    lmode = (vacstmt->options & VACOPT_FULL) ? AccessExclusiveLock : ShareUpdateExclusiveLock;

    /*
     * Open the relation and get the appropriate lock on it.
     *
     * There's a race condition here: the rel may have gone away since the
     * last time we saw it.  If so, we don't need to vacuum it.
     *
     * If we've been asked not to wait for the relation lock, acquire it first
     * in non-blocking mode, before calling try_relation_open().
     */
    if (!(vacstmt->options & VACOPT_NOWAIT))
        onerel = try_relation_open(relid, lmode);
    else if (ConditionalLockRelationOid(relid, lmode))
        onerel = try_relation_open(relid, NoLock);
    else
    {
        onerel = NULL;
        if (IsAutoVacuumWorkerProcess() && Log_autovacuum_min_duration >= 0)
            ereport(LOG,
                    (errcode(ERRCODE_LOCK_NOT_AVAILABLE),
                   errmsg("skipping vacuum of \"%s\" --- lock not available",
                          vacstmt->relation->relname)));
    }

    if (!onerel)
    {
        PopActiveSnapshot();
        CommitTransactionCommand();
        return false;
    }

    /*
     * Check permissions.
     *
     * We allow the user to vacuum a table if he is superuser, the table
     * owner, or the database owner (but in the latter case, only if it's not
     * a shared relation).  pg_class_ownercheck includes the superuser case.
     *
     * Note we choose to treat permissions failure as a WARNING and keep
     * trying to vacuum the rest of the DB --- is this appropriate?
     */
    if (!(pg_class_ownercheck(RelationGetRelid(onerel), GetUserId()) ||
          (pg_database_ownercheck(MyDatabaseId, GetUserId()) && !onerel->rd_rel->relisshared)))
    {
        if (onerel->rd_rel->relisshared)
            ereport(WARNING,
                  (errmsg("skipping \"%s\" --- only superuser can vacuum it",
                          RelationGetRelationName(onerel))));
        else if (onerel->rd_rel->relnamespace == PG_CATALOG_NAMESPACE)
            ereport(WARNING,
                    (errmsg("skipping \"%s\" --- only superuser or database owner can vacuum it",
                            RelationGetRelationName(onerel))));
        else
            ereport(WARNING,
                    (errmsg("skipping \"%s\" --- only table or database owner can vacuum it",
                            RelationGetRelationName(onerel))));
        relation_close(onerel, lmode);
        PopActiveSnapshot();
        CommitTransactionCommand();
        return false;
    }

    /*
     * Check that it's a vacuumable table; we used to do this in
     * get_rel_oids() but seems safer to check after we've locked the
     * relation.
     */
    if (onerel->rd_rel->relkind != RELKIND_RELATION &&
        onerel->rd_rel->relkind != RELKIND_MATVIEW &&
        onerel->rd_rel->relkind != RELKIND_TOASTVALUE)
    {
        ereport(WARNING,
                (errmsg("skipping \"%s\" --- cannot vacuum non-tables or special system tables",
                        RelationGetRelationName(onerel))));
        relation_close(onerel, lmode);
        PopActiveSnapshot();
        CommitTransactionCommand();
        return false;
    }

    /*
     * Silently ignore tables that are temp tables of other backends ---
     * trying to vacuum these will lead to great unhappiness, since their
     * contents are probably not up-to-date on disk.  (We don't throw a
     * warning here; it would just lead to chatter during a database-wide
     * VACUUM.)
     */
    if (RELATION_IS_OTHER_TEMP(onerel))
    {
        relation_close(onerel, lmode);
        PopActiveSnapshot();
        CommitTransactionCommand();
        return false;
    }

    /*
     * Get a session-level lock too. This will protect our access to the
     * relation across multiple transactions, so that we can vacuum the
     * relation's TOAST table (if any) secure in the knowledge that no one is
     * deleting the parent relation.
     *
     * NOTE: this cannot block, even if someone else is waiting for access,
     * because the lock manager knows that both lock requests are from the
     * same process.
     */
    onerelid = onerel->rd_lockInfo.lockRelId;
    LockRelationIdForSession(&onerelid, lmode);

    /*
     * Remember the relation's TOAST relation for later, if the caller asked
     * us to process it.  In VACUUM FULL, though, the toast table is
     * automatically rebuilt by cluster_rel so we shouldn't recurse to it.
     */
    if (do_toast && !(vacstmt->options & VACOPT_FULL))
        toast_relid = onerel->rd_rel->reltoastrelid;
    else
        toast_relid = InvalidOid;

    /*
     * Switch to the table owner's userid, so that any index functions are run
     * as that user.  Also lock down security-restricted operations and
     * arrange to make GUC variable changes local to this command. (This is
     * unnecessary, but harmless, for lazy VACUUM.)
     */
    GetUserIdAndSecContext(&save_userid, &save_sec_context);
    SetUserIdAndSecContext(onerel->rd_rel->relowner,
                           save_sec_context | SECURITY_RESTRICTED_OPERATION);
    save_nestlevel = NewGUCNestLevel();

    /*
     * Do the actual work --- either FULL or "lazy" vacuum
     */
    if (vacstmt->options & VACOPT_FULL)
    {
        /* close relation before vacuuming, but hold lock until commit */
        relation_close(onerel, NoLock);
        onerel = NULL;

        /* VACUUM FULL is now a variant of CLUSTER; see cluster.c */
        cluster_rel(relid, InvalidOid, false,
                    (vacstmt->options & VACOPT_VERBOSE) != 0,
                    vacstmt->freeze_min_age, vacstmt->freeze_table_age);
    }
    else
        lazy_vacuum_rel(onerel, vacstmt, vac_strategy);

    /* Roll back any GUC changes executed by index functions */
    AtEOXact_GUC(false, save_nestlevel);

    /* Restore userid and security context */
    SetUserIdAndSecContext(save_userid, save_sec_context);

    /* all done with this class, but hold lock until commit */
    if (onerel)
        relation_close(onerel, NoLock);

    /*
     * Complete the transaction and free all temporary memory used.
     */
    PopActiveSnapshot();
    CommitTransactionCommand();

    /*
     * If the relation has a secondary toast rel, vacuum that too while we
     * still hold the session lock on the master table.  Note however that
     * "analyze" will not get done on the toast table.  This is good, because
     * the toaster always uses hardcoded index access and statistics are
     * totally unimportant for toast relations.
     */
    if (toast_relid != InvalidOid)
        vacuum_rel(toast_relid, vacstmt, false, for_wraparound);

    /*
     * Now release the session-level lock on the master table.
     */
    UnlockRelationIdForSession(&onerelid, lmode);

    /* Report that we really did it. */
    return true;
}


/*
 * Open all the vacuumable indexes of the given relation, obtaining the
 * specified kind of lock on each.  Return an array of Relation pointers for
 * the indexes into *Irel, and the number of indexes into *nindexes.
 *
 * We consider an index vacuumable if it is marked insertable (IndexIsReady).
 * If it isn't, probably a CREATE INDEX CONCURRENTLY command failed early in
 * execution, and what we have is too corrupt to be processable.  We will
 * vacuum even if the index isn't indisvalid; this is important because in a
 * unique index, uniqueness checks will be performed anyway and had better not
 * hit dangling index pointers.
 */
void
vac_open_indexes(Relation relation, LOCKMODE lockmode,
                 int *nindexes, Relation **Irel)
{
    List       *indexoidlist;
    ListCell   *indexoidscan;
    int         i;

    Assert(lockmode != NoLock);

    indexoidlist = RelationGetIndexList(relation);

    /* allocate enough memory for all indexes */
    i = list_length(indexoidlist);

    if (i > 0)
        *Irel = (Relation *) palloc(i * sizeof(Relation));
    else
        *Irel = NULL;

    /* collect just the ready indexes */
    i = 0;
    foreach(indexoidscan, indexoidlist)
    {
        Oid         indexoid = lfirst_oid(indexoidscan);
        Relation    indrel;

        indrel = index_open(indexoid, lockmode);
        if (IndexIsReady(indrel->rd_index))
            (*Irel)[i++] = indrel;
        else
            index_close(indrel, lockmode);
    }

    *nindexes = i;

    list_free(indexoidlist);
}

/*
 * Release the resources acquired by vac_open_indexes.  Optionally release
 * the locks (say NoLock to keep 'em).
 */
void
vac_close_indexes(int nindexes, Relation *Irel, LOCKMODE lockmode)
{
    if (Irel == NULL)
        return;

    while (nindexes--)
    {
        Relation    ind = Irel[nindexes];

        index_close(ind, lockmode);
    }
    pfree(Irel);
}

/*
 * vacuum_delay_point --- check for interrupts and cost-based delay.
 *
 * This should be called in each major loop of VACUUM processing,
 * typically once per page processed.
 */
void
vacuum_delay_point(void)
{
    /* Always check for interrupts */
    CHECK_FOR_INTERRUPTS();

    /* Nap if appropriate */
    if (VacuumCostActive && !InterruptPending &&
        VacuumCostBalance >= VacuumCostLimit)
    {
        int         msec;

        msec = VacuumCostDelay * VacuumCostBalance / VacuumCostLimit;
        if (msec > VacuumCostDelay * 4)
            msec = VacuumCostDelay * 4;

        pg_usleep(msec * 1000L);

        VacuumCostBalance = 0;

        /* update balance values for workers */
        AutoVacuumUpdateDelay();

        /* Might have gotten an interrupt while sleeping */
        CHECK_FOR_INTERRUPTS();
    }
}