hashpage.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * hashpage.c
 *    Hash table page management code for the Postgres hash access method
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/hash/hashpage.c
 *
 * NOTES
 *    Postgres hash pages look like ordinary relation pages.  The opaque
 *    data at high addresses includes information about the page including
 *    whether a page is an overflow page or a true bucket, the bucket
 *    number, and the block numbers of the preceding and following pages
 *    in the same bucket.
 *
 *    The first page in a hash relation, page zero, is special -- it stores
 *    information describing the hash table; it is referred to as the
 *    "meta page." Pages one and higher store the actual data.
 *
 *    There are also bitmap pages, which are not manipulated here;
 *    see hashovfl.c.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/hash.h"
#include "miscadmin.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"


static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
                    uint32 nblocks);
static void _hash_splitbucket(Relation rel, Buffer metabuf,
                  Bucket obucket, Bucket nbucket,
                  BlockNumber start_oblkno,
                  BlockNumber start_nblkno,
                  uint32 maxbucket,
                  uint32 highmask, uint32 lowmask);


/*
 * We use high-concurrency locking on hash indexes (see README for an overview
 * of the locking rules).  However, we can skip taking lmgr locks when the
 * index is local to the current backend (ie, either temp or new in the
 * current transaction).  No one else can see it, so there's no reason to
 * take locks.  We still take buffer-level locks, but not lmgr locks.
 */
#define USELOCKING(rel)     (!RELATION_IS_LOCAL(rel))


/*
 * _hash_getlock() -- Acquire an lmgr lock.
 *
 * 'whichlock' should the block number of a bucket's primary bucket page to
 * acquire the per-bucket lock.  (See README for details of the use of these
 * locks.)
 *
 * 'access' must be HASH_SHARE or HASH_EXCLUSIVE.
 */
void
_hash_getlock(Relation rel, BlockNumber whichlock, int access)
{
    if (USELOCKING(rel))
        LockPage(rel, whichlock, access);
}

/*
 * _hash_try_getlock() -- Acquire an lmgr lock, but only if it's free.
 *
 * Same as above except we return FALSE without blocking if lock isn't free.
 */
bool
_hash_try_getlock(Relation rel, BlockNumber whichlock, int access)
{
    if (USELOCKING(rel))
        return ConditionalLockPage(rel, whichlock, access);
    else
        return true;
}

/*
 * _hash_droplock() -- Release an lmgr lock.
 */
void
_hash_droplock(Relation rel, BlockNumber whichlock, int access)
{
    if (USELOCKING(rel))
        UnlockPage(rel, whichlock, access);
}

/*
 *  _hash_getbuf() -- Get a buffer by block number for read or write.
 *
 *      'access' must be HASH_READ, HASH_WRITE, or HASH_NOLOCK.
 *      'flags' is a bitwise OR of the allowed page types.
 *
 *      This must be used only to fetch pages that are expected to be valid
 *      already.  _hash_checkpage() is applied using the given flags.
 *
 *      When this routine returns, the appropriate lock is set on the
 *      requested buffer and its reference count has been incremented
 *      (ie, the buffer is "locked and pinned").
 *
 *      P_NEW is disallowed because this routine can only be used
 *      to access pages that are known to be before the filesystem EOF.
 *      Extending the index should be done with _hash_getnewbuf.
 */
Buffer
_hash_getbuf(Relation rel, BlockNumber blkno, int access, int flags)
{
    Buffer      buf;

    if (blkno == P_NEW)
        elog(ERROR, "hash AM does not use P_NEW");

    buf = ReadBuffer(rel, blkno);

    if (access != HASH_NOLOCK)
        LockBuffer(buf, access);

    /* ref count and lock type are correct */

    _hash_checkpage(rel, buf, flags);

    return buf;
}

/*
 *  _hash_getinitbuf() -- Get and initialize a buffer by block number.
 *
 *      This must be used only to fetch pages that are known to be before
 *      the index's filesystem EOF, but are to be filled from scratch.
 *      _hash_pageinit() is applied automatically.  Otherwise it has
 *      effects similar to _hash_getbuf() with access = HASH_WRITE.
 *
 *      When this routine returns, a write lock is set on the
 *      requested buffer and its reference count has been incremented
 *      (ie, the buffer is "locked and pinned").
 *
 *      P_NEW is disallowed because this routine can only be used
 *      to access pages that are known to be before the filesystem EOF.
 *      Extending the index should be done with _hash_getnewbuf.
 */
Buffer
_hash_getinitbuf(Relation rel, BlockNumber blkno)
{
    Buffer      buf;

    if (blkno == P_NEW)
        elog(ERROR, "hash AM does not use P_NEW");

    buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_ZERO, NULL);

    LockBuffer(buf, HASH_WRITE);

    /* ref count and lock type are correct */

    /* initialize the page */
    _hash_pageinit(BufferGetPage(buf), BufferGetPageSize(buf));

    return buf;
}

/*
 *  _hash_getnewbuf() -- Get a new page at the end of the index.
 *
 *      This has the same API as _hash_getinitbuf, except that we are adding
 *      a page to the index, and hence expect the page to be past the
 *      logical EOF.  (However, we have to support the case where it isn't,
 *      since a prior try might have crashed after extending the filesystem
 *      EOF but before updating the metapage to reflect the added page.)
 *
 *      It is caller's responsibility to ensure that only one process can
 *      extend the index at a time.
 */
Buffer
_hash_getnewbuf(Relation rel, BlockNumber blkno, ForkNumber forkNum)
{
    BlockNumber nblocks = RelationGetNumberOfBlocksInFork(rel, forkNum);
    Buffer      buf;

    if (blkno == P_NEW)
        elog(ERROR, "hash AM does not use P_NEW");
    if (blkno > nblocks)
        elog(ERROR, "access to noncontiguous page in hash index \"%s\"",
             RelationGetRelationName(rel));

    /* smgr insists we use P_NEW to extend the relation */
    if (blkno == nblocks)
    {
        buf = ReadBufferExtended(rel, forkNum, P_NEW, RBM_NORMAL, NULL);
        if (BufferGetBlockNumber(buf) != blkno)
            elog(ERROR, "unexpected hash relation size: %u, should be %u",
                 BufferGetBlockNumber(buf), blkno);
    }
    else
        buf = ReadBufferExtended(rel, forkNum, blkno, RBM_ZERO, NULL);

    LockBuffer(buf, HASH_WRITE);

    /* ref count and lock type are correct */

    /* initialize the page */
    _hash_pageinit(BufferGetPage(buf), BufferGetPageSize(buf));

    return buf;
}

/*
 *  _hash_getbuf_with_strategy() -- Get a buffer with nondefault strategy.
 *
 *      This is identical to _hash_getbuf() but also allows a buffer access
 *      strategy to be specified.  We use this for VACUUM operations.
 */
Buffer
_hash_getbuf_with_strategy(Relation rel, BlockNumber blkno,
                           int access, int flags,
                           BufferAccessStrategy bstrategy)
{
    Buffer      buf;

    if (blkno == P_NEW)
        elog(ERROR, "hash AM does not use P_NEW");

    buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, bstrategy);

    if (access != HASH_NOLOCK)
        LockBuffer(buf, access);

    /* ref count and lock type are correct */

    _hash_checkpage(rel, buf, flags);

    return buf;
}

/*
 *  _hash_relbuf() -- release a locked buffer.
 *
 * Lock and pin (refcount) are both dropped.
 */
void
_hash_relbuf(Relation rel, Buffer buf)
{
    UnlockReleaseBuffer(buf);
}

/*
 *  _hash_dropbuf() -- release an unlocked buffer.
 *
 * This is used to unpin a buffer on which we hold no lock.
 */
void
_hash_dropbuf(Relation rel, Buffer buf)
{
    ReleaseBuffer(buf);
}

/*
 *  _hash_wrtbuf() -- write a hash page to disk.
 *
 *      This routine releases the lock held on the buffer and our refcount
 *      for it.  It is an error to call _hash_wrtbuf() without a write lock
 *      and a pin on the buffer.
 *
 * NOTE: this routine should go away when/if hash indexes are WAL-ified.
 * The correct sequence of operations is to mark the buffer dirty, then
 * write the WAL record, then release the lock and pin; so marking dirty
 * can't be combined with releasing.
 */
void
_hash_wrtbuf(Relation rel, Buffer buf)
{
    MarkBufferDirty(buf);
    UnlockReleaseBuffer(buf);
}

/*
 * _hash_chgbufaccess() -- Change the lock type on a buffer, without
 *          dropping our pin on it.
 *
 * from_access and to_access may be HASH_READ, HASH_WRITE, or HASH_NOLOCK,
 * the last indicating that no buffer-level lock is held or wanted.
 *
 * When from_access == HASH_WRITE, we assume the buffer is dirty and tell
 * bufmgr it must be written out.  If the caller wants to release a write
 * lock on a page that's not been modified, it's okay to pass from_access
 * as HASH_READ (a bit ugly, but handy in some places).
 */
void
_hash_chgbufaccess(Relation rel,
                   Buffer buf,
                   int from_access,
                   int to_access)
{
    if (from_access == HASH_WRITE)
        MarkBufferDirty(buf);
    if (from_access != HASH_NOLOCK)
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
    if (to_access != HASH_NOLOCK)
        LockBuffer(buf, to_access);
}


/*
 *  _hash_metapinit() -- Initialize the metadata page of a hash index,
 *              the initial buckets, and the initial bitmap page.
 *
 * The initial number of buckets is dependent on num_tuples, an estimate
 * of the number of tuples to be loaded into the index initially.  The
 * chosen number of buckets is returned.
 *
 * We are fairly cavalier about locking here, since we know that no one else
 * could be accessing this index.  In particular the rule about not holding
 * multiple buffer locks is ignored.
 */
uint32
_hash_metapinit(Relation rel, double num_tuples, ForkNumber forkNum)
{
    HashMetaPage metap;
    HashPageOpaque pageopaque;
    Buffer      metabuf;
    Buffer      buf;
    Page        pg;
    int32       data_width;
    int32       item_width;
    int32       ffactor;
    double      dnumbuckets;
    uint32      num_buckets;
    uint32      log2_num_buckets;
    uint32      i;

    /* safety check */
    if (RelationGetNumberOfBlocksInFork(rel, forkNum) != 0)
        elog(ERROR, "cannot initialize non-empty hash index \"%s\"",
             RelationGetRelationName(rel));

    /*
     * Determine the target fill factor (in tuples per bucket) for this index.
     * The idea is to make the fill factor correspond to pages about as full
     * as the user-settable fillfactor parameter says.  We can compute it
     * exactly since the index datatype (i.e. uint32 hash key) is fixed-width.
     */
    data_width = sizeof(uint32);
    item_width = MAXALIGN(sizeof(IndexTupleData)) + MAXALIGN(data_width) +
        sizeof(ItemIdData);     /* include the line pointer */
    ffactor = RelationGetTargetPageUsage(rel, HASH_DEFAULT_FILLFACTOR) / item_width;
    /* keep to a sane range */
    if (ffactor < 10)
        ffactor = 10;

    /*
     * Choose the number of initial bucket pages to match the fill factor
     * given the estimated number of tuples.  We round up the result to the
     * next power of 2, however, and always force at least 2 bucket pages. The
     * upper limit is determined by considerations explained in
     * _hash_expandtable().
     */
    dnumbuckets = num_tuples / ffactor;
    if (dnumbuckets <= 2.0)
        num_buckets = 2;
    else if (dnumbuckets >= (double) 0x40000000)
        num_buckets = 0x40000000;
    else
        num_buckets = ((uint32) 1) << _hash_log2((uint32) dnumbuckets);

    log2_num_buckets = _hash_log2(num_buckets);
    Assert(num_buckets == (((uint32) 1) << log2_num_buckets));
    Assert(log2_num_buckets < HASH_MAX_SPLITPOINTS);

    /*
     * We initialize the metapage, the first N bucket pages, and the first
     * bitmap page in sequence, using _hash_getnewbuf to cause smgrextend()
     * calls to occur.  This ensures that the smgr level has the right idea of
     * the physical index length.
     */
    metabuf = _hash_getnewbuf(rel, HASH_METAPAGE, forkNum);
    pg = BufferGetPage(metabuf);

    pageopaque = (HashPageOpaque) PageGetSpecialPointer(pg);
    pageopaque->hasho_prevblkno = InvalidBlockNumber;
    pageopaque->hasho_nextblkno = InvalidBlockNumber;
    pageopaque->hasho_bucket = -1;
    pageopaque->hasho_flag = LH_META_PAGE;
    pageopaque->hasho_page_id = HASHO_PAGE_ID;

    metap = HashPageGetMeta(pg);

    metap->hashm_magic = HASH_MAGIC;
    metap->hashm_version = HASH_VERSION;
    metap->hashm_ntuples = 0;
    metap->hashm_nmaps = 0;
    metap->hashm_ffactor = ffactor;
    metap->hashm_bsize = HashGetMaxBitmapSize(pg);
    /* find largest bitmap array size that will fit in page size */
    for (i = _hash_log2(metap->hashm_bsize); i > 0; --i)
    {
        if ((1 << i) <= metap->hashm_bsize)
            break;
    }
    Assert(i > 0);
    metap->hashm_bmsize = 1 << i;
    metap->hashm_bmshift = i + BYTE_TO_BIT;
    Assert((1 << BMPG_SHIFT(metap)) == (BMPG_MASK(metap) + 1));

    /*
     * Label the index with its primary hash support function's OID.  This is
     * pretty useless for normal operation (in fact, hashm_procid is not used
     * anywhere), but it might be handy for forensic purposes so we keep it.
     */
    metap->hashm_procid = index_getprocid(rel, 1, HASHPROC);

    /*
     * We initialize the index with N buckets, 0 .. N-1, occupying physical
     * blocks 1 to N.  The first freespace bitmap page is in block N+1. Since
     * N is a power of 2, we can set the masks this way:
     */
    metap->hashm_maxbucket = metap->hashm_lowmask = num_buckets - 1;
    metap->hashm_highmask = (num_buckets << 1) - 1;

    MemSet(metap->hashm_spares, 0, sizeof(metap->hashm_spares));
    MemSet(metap->hashm_mapp, 0, sizeof(metap->hashm_mapp));

    /* Set up mapping for one spare page after the initial splitpoints */
    metap->hashm_spares[log2_num_buckets] = 1;
    metap->hashm_ovflpoint = log2_num_buckets;
    metap->hashm_firstfree = 0;

    /*
     * Release buffer lock on the metapage while we initialize buckets.
     * Otherwise, we'll be in interrupt holdoff and the CHECK_FOR_INTERRUPTS
     * won't accomplish anything.  It's a bad idea to hold buffer locks for
     * long intervals in any case, since that can block the bgwriter.
     */
    _hash_chgbufaccess(rel, metabuf, HASH_WRITE, HASH_NOLOCK);

    /*
     * Initialize the first N buckets
     */
    for (i = 0; i < num_buckets; i++)
    {
        /* Allow interrupts, in case N is huge */
        CHECK_FOR_INTERRUPTS();

        buf = _hash_getnewbuf(rel, BUCKET_TO_BLKNO(metap, i), forkNum);
        pg = BufferGetPage(buf);
        pageopaque = (HashPageOpaque) PageGetSpecialPointer(pg);
        pageopaque->hasho_prevblkno = InvalidBlockNumber;
        pageopaque->hasho_nextblkno = InvalidBlockNumber;
        pageopaque->hasho_bucket = i;
        pageopaque->hasho_flag = LH_BUCKET_PAGE;
        pageopaque->hasho_page_id = HASHO_PAGE_ID;
        _hash_wrtbuf(rel, buf);
    }

    /* Now reacquire buffer lock on metapage */
    _hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_WRITE);

    /*
     * Initialize first bitmap page
     */
    _hash_initbitmap(rel, metap, num_buckets + 1, forkNum);

    /* all done */
    _hash_wrtbuf(rel, metabuf);

    return num_buckets;
}

/*
 *  _hash_pageinit() -- Initialize a new hash index page.
 */
void
_hash_pageinit(Page page, Size size)
{
    Assert(PageIsNew(page));
    PageInit(page, size, sizeof(HashPageOpaqueData));
}

/*
 * Attempt to expand the hash table by creating one new bucket.
 *
 * This will silently do nothing if it cannot get the needed locks.
 *
 * The caller should hold no locks on the hash index.
 *
 * The caller must hold a pin, but no lock, on the metapage buffer.
 * The buffer is returned in the same state.
 */
void
_hash_expandtable(Relation rel, Buffer metabuf)
{
    HashMetaPage metap;
    Bucket      old_bucket;
    Bucket      new_bucket;
    uint32      spare_ndx;
    BlockNumber start_oblkno;
    BlockNumber start_nblkno;
    uint32      maxbucket;
    uint32      highmask;
    uint32      lowmask;

    /*
     * Write-lock the meta page.  It used to be necessary to acquire a
     * heavyweight lock to begin a split, but that is no longer required.
     */
    _hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_WRITE);

    _hash_checkpage(rel, metabuf, LH_META_PAGE);
    metap = HashPageGetMeta(BufferGetPage(metabuf));

    /*
     * Check to see if split is still needed; someone else might have already
     * done one while we waited for the lock.
     *
     * Make sure this stays in sync with _hash_doinsert()
     */
    if (metap->hashm_ntuples <=
        (double) metap->hashm_ffactor * (metap->hashm_maxbucket + 1))
        goto fail;

    /*
     * Can't split anymore if maxbucket has reached its maximum possible
     * value.
     *
     * Ideally we'd allow bucket numbers up to UINT_MAX-1 (no higher because
     * the calculation maxbucket+1 mustn't overflow).  Currently we restrict
     * to half that because of overflow looping in _hash_log2() and
     * insufficient space in hashm_spares[].  It's moot anyway because an
     * index with 2^32 buckets would certainly overflow BlockNumber and hence
     * _hash_alloc_buckets() would fail, but if we supported buckets smaller
     * than a disk block then this would be an independent constraint.
     *
     * If you change this, see also the maximum initial number of buckets in
     * _hash_metapinit().
     */
    if (metap->hashm_maxbucket >= (uint32) 0x7FFFFFFE)
        goto fail;

    /*
     * Determine which bucket is to be split, and attempt to lock the old
     * bucket.  If we can't get the lock, give up.
     *
     * The lock protects us against other backends, but not against our own
     * backend.  Must check for active scans separately.
     */
    new_bucket = metap->hashm_maxbucket + 1;

    old_bucket = (new_bucket & metap->hashm_lowmask);

    start_oblkno = BUCKET_TO_BLKNO(metap, old_bucket);

    if (_hash_has_active_scan(rel, old_bucket))
        goto fail;

    if (!_hash_try_getlock(rel, start_oblkno, HASH_EXCLUSIVE))
        goto fail;

    /*
     * Likewise lock the new bucket (should never fail).
     *
     * Note: it is safe to compute the new bucket's blkno here, even though we
     * may still need to update the BUCKET_TO_BLKNO mapping.  This is because
     * the current value of hashm_spares[hashm_ovflpoint] correctly shows
     * where we are going to put a new splitpoint's worth of buckets.
     */
    start_nblkno = BUCKET_TO_BLKNO(metap, new_bucket);

    if (_hash_has_active_scan(rel, new_bucket))
        elog(ERROR, "scan in progress on supposedly new bucket");

    if (!_hash_try_getlock(rel, start_nblkno, HASH_EXCLUSIVE))
        elog(ERROR, "could not get lock on supposedly new bucket");

    /*
     * If the split point is increasing (hashm_maxbucket's log base 2
     * increases), we need to allocate a new batch of bucket pages.
     */
    spare_ndx = _hash_log2(new_bucket + 1);
    if (spare_ndx > metap->hashm_ovflpoint)
    {
        Assert(spare_ndx == metap->hashm_ovflpoint + 1);

        /*
         * The number of buckets in the new splitpoint is equal to the total
         * number already in existence, i.e. new_bucket.  Currently this maps
         * one-to-one to blocks required, but someday we may need a more
         * complicated calculation here.
         */
        if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
        {
            /* can't split due to BlockNumber overflow */
            _hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
            _hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
            goto fail;
        }
    }

    /*
     * Okay to proceed with split.  Update the metapage bucket mapping info.
     *
     * Since we are scribbling on the metapage data right in the shared
     * buffer, any failure in this next little bit leaves us with a big
     * problem: the metapage is effectively corrupt but could get written back
     * to disk.  We don't really expect any failure, but just to be sure,
     * establish a critical section.
     */
    START_CRIT_SECTION();

    metap->hashm_maxbucket = new_bucket;

    if (new_bucket > metap->hashm_highmask)
    {
        /* Starting a new doubling */
        metap->hashm_lowmask = metap->hashm_highmask;
        metap->hashm_highmask = new_bucket | metap->hashm_lowmask;
    }

    /*
     * If the split point is increasing (hashm_maxbucket's log base 2
     * increases), we need to adjust the hashm_spares[] array and
     * hashm_ovflpoint so that future overflow pages will be created beyond
     * this new batch of bucket pages.
     */
    if (spare_ndx > metap->hashm_ovflpoint)
    {
        metap->hashm_spares[spare_ndx] = metap->hashm_spares[metap->hashm_ovflpoint];
        metap->hashm_ovflpoint = spare_ndx;
    }

    /* Done mucking with metapage */
    END_CRIT_SECTION();

    /*
     * Copy bucket mapping info now; this saves re-accessing the meta page
     * inside _hash_splitbucket's inner loop.  Note that once we drop the
     * split lock, other splits could begin, so these values might be out of
     * date before _hash_splitbucket finishes.  That's okay, since all it
     * needs is to tell which of these two buckets to map hashkeys into.
     */
    maxbucket = metap->hashm_maxbucket;
    highmask = metap->hashm_highmask;
    lowmask = metap->hashm_lowmask;

    /* Write out the metapage and drop lock, but keep pin */
    _hash_chgbufaccess(rel, metabuf, HASH_WRITE, HASH_NOLOCK);

    /* Relocate records to the new bucket */
    _hash_splitbucket(rel, metabuf, old_bucket, new_bucket,
                      start_oblkno, start_nblkno,
                      maxbucket, highmask, lowmask);

    /* Release bucket locks, allowing others to access them */
    _hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
    _hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);

    return;

    /* Here if decide not to split or fail to acquire old bucket lock */
fail:

    /* We didn't write the metapage, so just drop lock */
    _hash_chgbufaccess(rel, metabuf, HASH_READ, HASH_NOLOCK);
}


/*
 * _hash_alloc_buckets -- allocate a new splitpoint's worth of bucket pages
 *
 * This does not need to initialize the new bucket pages; we'll do that as
 * each one is used by _hash_expandtable().  But we have to extend the logical
 * EOF to the end of the splitpoint; this keeps smgr's idea of the EOF in
 * sync with ours, so that we don't get complaints from smgr.
 *
 * We do this by writing a page of zeroes at the end of the splitpoint range.
 * We expect that the filesystem will ensure that the intervening pages read
 * as zeroes too.  On many filesystems this "hole" will not be allocated
 * immediately, which means that the index file may end up more fragmented
 * than if we forced it all to be allocated now; but since we don't scan
 * hash indexes sequentially anyway, that probably doesn't matter.
 *
 * XXX It's annoying that this code is executed with the metapage lock held.
 * We need to interlock against _hash_getovflpage() adding a new overflow page
 * concurrently, but it'd likely be better to use LockRelationForExtension
 * for the purpose.  OTOH, adding a splitpoint is a very infrequent operation,
 * so it may not be worth worrying about.
 *
 * Returns TRUE if successful, or FALSE if allocation failed due to
 * BlockNumber overflow.
 */
static bool
_hash_alloc_buckets(Relation rel, BlockNumber firstblock, uint32 nblocks)
{
    BlockNumber lastblock;
    char        zerobuf[BLCKSZ];

    lastblock = firstblock + nblocks - 1;

    /*
     * Check for overflow in block number calculation; if so, we cannot extend
     * the index anymore.
     */
    if (lastblock < firstblock || lastblock == InvalidBlockNumber)
        return false;

    MemSet(zerobuf, 0, sizeof(zerobuf));

    RelationOpenSmgr(rel);
    smgrextend(rel->rd_smgr, MAIN_FORKNUM, lastblock, zerobuf, false);

    return true;
}


/*
 * _hash_splitbucket -- split 'obucket' into 'obucket' and 'nbucket'
 *
 * We are splitting a bucket that consists of a base bucket page and zero
 * or more overflow (bucket chain) pages.  We must relocate tuples that
 * belong in the new bucket, and compress out any free space in the old
 * bucket.
 *
 * The caller must hold exclusive locks on both buckets to ensure that
 * no one else is trying to access them (see README).
 *
 * The caller must hold a pin, but no lock, on the metapage buffer.
 * The buffer is returned in the same state.  (The metapage is only
 * touched if it becomes necessary to add or remove overflow pages.)
 */
static void
_hash_splitbucket(Relation rel,
                  Buffer metabuf,
                  Bucket obucket,
                  Bucket nbucket,
                  BlockNumber start_oblkno,
                  BlockNumber start_nblkno,
                  uint32 maxbucket,
                  uint32 highmask,
                  uint32 lowmask)
{
    BlockNumber oblkno;
    BlockNumber nblkno;
    Buffer      obuf;
    Buffer      nbuf;
    Page        opage;
    Page        npage;
    HashPageOpaque oopaque;
    HashPageOpaque nopaque;

    /*
     * It should be okay to simultaneously write-lock pages from each bucket,
     * since no one else can be trying to acquire buffer lock on pages of
     * either bucket.
     */
    oblkno = start_oblkno;
    obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_BUCKET_PAGE);
    opage = BufferGetPage(obuf);
    oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);

    nblkno = start_nblkno;
    nbuf = _hash_getnewbuf(rel, nblkno, MAIN_FORKNUM);
    npage = BufferGetPage(nbuf);

    /* initialize the new bucket's primary page */
    nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
    nopaque->hasho_prevblkno = InvalidBlockNumber;
    nopaque->hasho_nextblkno = InvalidBlockNumber;
    nopaque->hasho_bucket = nbucket;
    nopaque->hasho_flag = LH_BUCKET_PAGE;
    nopaque->hasho_page_id = HASHO_PAGE_ID;

    /*
     * Partition the tuples in the old bucket between the old bucket and the
     * new bucket, advancing along the old bucket's overflow bucket chain and
     * adding overflow pages to the new bucket as needed.  Outer loop iterates
     * once per page in old bucket.
     */
    for (;;)
    {
        OffsetNumber ooffnum;
        OffsetNumber omaxoffnum;
        OffsetNumber deletable[MaxOffsetNumber];
        int         ndeletable = 0;

        /* Scan each tuple in old page */
        omaxoffnum = PageGetMaxOffsetNumber(opage);
        for (ooffnum = FirstOffsetNumber;
             ooffnum <= omaxoffnum;
             ooffnum = OffsetNumberNext(ooffnum))
        {
            IndexTuple  itup;
            Size        itemsz;
            Bucket      bucket;

            /*
             * Fetch the item's hash key (conveniently stored in the item) and
             * determine which bucket it now belongs in.
             */
            itup = (IndexTuple) PageGetItem(opage,
                                            PageGetItemId(opage, ooffnum));
            bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
                                          maxbucket, highmask, lowmask);

            if (bucket == nbucket)
            {
                /*
                 * insert the tuple into the new bucket.  if it doesn't fit on
                 * the current page in the new bucket, we must allocate a new
                 * overflow page and place the tuple on that page instead.
                 */
                itemsz = IndexTupleDSize(*itup);
                itemsz = MAXALIGN(itemsz);

                if (PageGetFreeSpace(npage) < itemsz)
                {
                    /* write out nbuf and drop lock, but keep pin */
                    _hash_chgbufaccess(rel, nbuf, HASH_WRITE, HASH_NOLOCK);
                    /* chain to a new overflow page */
                    nbuf = _hash_addovflpage(rel, metabuf, nbuf);
                    npage = BufferGetPage(nbuf);
                    /* we don't need nblkno or nopaque within the loop */
                }

                /*
                 * Insert tuple on new page, using _hash_pgaddtup to ensure
                 * correct ordering by hashkey.  This is a tad inefficient
                 * since we may have to shuffle itempointers repeatedly.
                 * Possible future improvement: accumulate all the items for
                 * the new page and qsort them before insertion.
                 */
                (void) _hash_pgaddtup(rel, nbuf, itemsz, itup);

                /*
                 * Mark tuple for deletion from old page.
                 */
                deletable[ndeletable++] = ooffnum;
            }
            else
            {
                /*
                 * the tuple stays on this page, so nothing to do.
                 */
                Assert(bucket == obucket);
            }
        }

        oblkno = oopaque->hasho_nextblkno;

        /*
         * Done scanning this old page.  If we moved any tuples, delete them
         * from the old page.
         */
        if (ndeletable > 0)
        {
            PageIndexMultiDelete(opage, deletable, ndeletable);
            _hash_wrtbuf(rel, obuf);
        }
        else
            _hash_relbuf(rel, obuf);

        /* Exit loop if no more overflow pages in old bucket */
        if (!BlockNumberIsValid(oblkno))
            break;

        /* Else, advance to next old page */
        obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
        opage = BufferGetPage(obuf);
        oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
    }

    /*
     * We're at the end of the old bucket chain, so we're done partitioning
     * the tuples.  Before quitting, call _hash_squeezebucket to ensure the
     * tuples remaining in the old bucket (including the overflow pages) are
     * packed as tightly as possible.  The new bucket is already tight.
     */
    _hash_wrtbuf(rel, nbuf);

    _hash_squeezebucket(rel, obucket, start_oblkno, NULL);
}