hashovfl.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * hashovfl.c
 *    Overflow page management code for the Postgres hash access method
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/hash/hashovfl.c
 *
 * NOTES
 *    Overflow pages look like ordinary relation pages.
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/hash.h"
#include "access/hash_xlog.h"
#include "miscadmin.h"
#include "utils/rel.h"


static uint32 _hash_firstfreebit(uint32 map);


/*
 * Convert overflow page bit number (its index in the free-page bitmaps)
 * to block number within the index.
 */
static BlockNumber
bitno_to_blkno(HashMetaPage metap, uint32 ovflbitnum)
{
    uint32      splitnum = metap->hashm_ovflpoint;
    uint32      i;

    /* Convert zero-based bitnumber to 1-based page number */
    ovflbitnum += 1;

    /* Determine the split number for this page (must be >= 1) */
    for (i = 1;
         i < splitnum && ovflbitnum > metap->hashm_spares[i];
         i++)
         /* loop */ ;

    /*
     * Convert to absolute page number by adding the number of bucket pages
     * that exist before this split point.
     */
    return (BlockNumber) (_hash_get_totalbuckets(i) + ovflbitnum);
}

/*
 * _hash_ovflblkno_to_bitno
 *
 * Convert overflow page block number to bit number for free-page bitmap.
 */
uint32
_hash_ovflblkno_to_bitno(HashMetaPage metap, BlockNumber ovflblkno)
{
    uint32      splitnum = metap->hashm_ovflpoint;
    uint32      i;
    uint32      bitnum;

    /* Determine the split number containing this page */
    for (i = 1; i <= splitnum; i++)
    {
        if (ovflblkno <= (BlockNumber) _hash_get_totalbuckets(i))
            break;              /* oops */
        bitnum = ovflblkno - _hash_get_totalbuckets(i);

        /*
         * bitnum has to be greater than number of overflow page added in
         * previous split point. The overflow page at this splitnum (i) if any
         * should start from (_hash_get_totalbuckets(i) +
         * metap->hashm_spares[i - 1] + 1).
         */
        if (bitnum > metap->hashm_spares[i - 1] &&
            bitnum <= metap->hashm_spares[i])
            return bitnum - 1;  /* -1 to convert 1-based to 0-based */
    }

    ereport(ERROR,
            (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
             errmsg("invalid overflow block number %u", ovflblkno)));
    return 0;                   /* keep compiler quiet */
}

/*
 *  _hash_addovflpage
 *
 *  Add an overflow page to the bucket whose last page is pointed to by 'buf'.
 *
 *  On entry, the caller must hold a pin but no lock on 'buf'.  The pin is
 *  dropped before exiting (we assume the caller is not interested in 'buf'
 *  anymore) if not asked to retain.  The pin will be retained only for the
 *  primary bucket.  The returned overflow page will be pinned and
 *  write-locked; it is guaranteed to be empty.
 *
 *  The caller must hold a pin, but no lock, on the metapage buffer.
 *  That buffer is returned in the same state.
 *
 * NB: since this could be executed concurrently by multiple processes,
 * one should not assume that the returned overflow page will be the
 * immediate successor of the originally passed 'buf'.  Additional overflow
 * pages might have been added to the bucket chain in between.
 */
Buffer
_hash_addovflpage(Relation rel, Buffer metabuf, Buffer buf, bool retain_pin)
{
    Buffer      ovflbuf;
    Page        page;
    Page        ovflpage;
    HashPageOpaque pageopaque;
    HashPageOpaque ovflopaque;
    HashMetaPage metap;
    Buffer      mapbuf = InvalidBuffer;
    Buffer      newmapbuf = InvalidBuffer;
    BlockNumber blkno;
    uint32      orig_firstfree;
    uint32      splitnum;
    uint32     *freep = NULL;
    uint32      max_ovflpg;
    uint32      bit;
    uint32      bitmap_page_bit;
    uint32      first_page;
    uint32      last_bit;
    uint32      last_page;
    uint32      i,
                j;
    bool        page_found = false;

    /*
     * Write-lock the tail page.  Here, we need to maintain locking order such
     * that, first acquire the lock on tail page of bucket, then on meta page
     * to find and lock the bitmap page and if it is found, then lock on meta
     * page is released, then finally acquire the lock on new overflow buffer.
     * We need this locking order to avoid deadlock with backends that are
     * doing inserts.
     *
     * Note: We could have avoided locking many buffers here if we made two
     * WAL records for acquiring an overflow page (one to allocate an overflow
     * page and another to add it to overflow bucket chain).  However, doing
     * so can leak an overflow page, if the system crashes after allocation.
     * Needless to say, it is better to have a single record from a
     * performance point of view as well.
     */
    LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);

    /* probably redundant... */
    _hash_checkpage(rel, buf, LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);

    /* loop to find current tail page, in case someone else inserted too */
    for (;;)
    {
        BlockNumber nextblkno;

        page = BufferGetPage(buf);
        pageopaque = (HashPageOpaque) PageGetSpecialPointer(page);
        nextblkno = pageopaque->hasho_nextblkno;

        if (!BlockNumberIsValid(nextblkno))
            break;

        /* we assume we do not need to write the unmodified page */
        if (retain_pin)
        {
            /* pin will be retained only for the primary bucket page */
            Assert((pageopaque->hasho_flag & LH_PAGE_TYPE) == LH_BUCKET_PAGE);
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        }
        else
            _hash_relbuf(rel, buf);

        retain_pin = false;

        buf = _hash_getbuf(rel, nextblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
    }

    /* Get exclusive lock on the meta page */
    LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);

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

    /* start search at hashm_firstfree */
    orig_firstfree = metap->hashm_firstfree;
    first_page = orig_firstfree >> BMPG_SHIFT(metap);
    bit = orig_firstfree & BMPG_MASK(metap);
    i = first_page;
    j = bit / BITS_PER_MAP;
    bit &= ~(BITS_PER_MAP - 1);

    /* outer loop iterates once per bitmap page */
    for (;;)
    {
        BlockNumber mapblkno;
        Page        mappage;
        uint32      last_inpage;

        /* want to end search with the last existing overflow page */
        splitnum = metap->hashm_ovflpoint;
        max_ovflpg = metap->hashm_spares[splitnum] - 1;
        last_page = max_ovflpg >> BMPG_SHIFT(metap);
        last_bit = max_ovflpg & BMPG_MASK(metap);

        if (i > last_page)
            break;

        Assert(i < metap->hashm_nmaps);
        mapblkno = metap->hashm_mapp[i];

        if (i == last_page)
            last_inpage = last_bit;
        else
            last_inpage = BMPGSZ_BIT(metap) - 1;

        /* Release exclusive lock on metapage while reading bitmap page */
        LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);

        mapbuf = _hash_getbuf(rel, mapblkno, HASH_WRITE, LH_BITMAP_PAGE);
        mappage = BufferGetPage(mapbuf);
        freep = HashPageGetBitmap(mappage);

        for (; bit <= last_inpage; j++, bit += BITS_PER_MAP)
        {
            if (freep[j] != ALL_SET)
            {
                page_found = true;

                /* Reacquire exclusive lock on the meta page */
                LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);

                /* convert bit to bit number within page */
                bit += _hash_firstfreebit(freep[j]);
                bitmap_page_bit = bit;

                /* convert bit to absolute bit number */
                bit += (i << BMPG_SHIFT(metap));
                /* Calculate address of the recycled overflow page */
                blkno = bitno_to_blkno(metap, bit);

                /* Fetch and init the recycled page */
                ovflbuf = _hash_getinitbuf(rel, blkno);

                goto found;
            }
        }

        /* No free space here, try to advance to next map page */
        _hash_relbuf(rel, mapbuf);
        mapbuf = InvalidBuffer;
        i++;
        j = 0;                  /* scan from start of next map page */
        bit = 0;

        /* Reacquire exclusive lock on the meta page */
        LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);
    }

    /*
     * No free pages --- have to extend the relation to add an overflow page.
     * First, check to see if we have to add a new bitmap page too.
     */
    if (last_bit == (uint32) (BMPGSZ_BIT(metap) - 1))
    {
        /*
         * We create the new bitmap page with all pages marked "in use".
         * Actually two pages in the new bitmap's range will exist
         * immediately: the bitmap page itself, and the following page which
         * is the one we return to the caller.  Both of these are correctly
         * marked "in use".  Subsequent pages do not exist yet, but it is
         * convenient to pre-mark them as "in use" too.
         */
        bit = metap->hashm_spares[splitnum];

        /* metapage already has a write lock */
        if (metap->hashm_nmaps >= HASH_MAX_BITMAPS)
            ereport(ERROR,
                    (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                     errmsg("out of overflow pages in hash index \"%s\"",
                            RelationGetRelationName(rel))));

        newmapbuf = _hash_getnewbuf(rel, bitno_to_blkno(metap, bit), MAIN_FORKNUM);
    }
    else
    {
        /*
         * Nothing to do here; since the page will be past the last used page,
         * we know its bitmap bit was preinitialized to "in use".
         */
    }

    /* Calculate address of the new overflow page */
    bit = BufferIsValid(newmapbuf) ?
        metap->hashm_spares[splitnum] + 1 : metap->hashm_spares[splitnum];
    blkno = bitno_to_blkno(metap, bit);

    /*
     * Fetch the page with _hash_getnewbuf to ensure smgr's idea of the
     * relation length stays in sync with ours.  XXX It's annoying to do this
     * with metapage write lock held; would be better to use a lock that
     * doesn't block incoming searches.
     *
     * It is okay to hold two buffer locks here (one on tail page of bucket
     * and other on new overflow page) since there cannot be anyone else
     * contending for access to ovflbuf.
     */
    ovflbuf = _hash_getnewbuf(rel, blkno, MAIN_FORKNUM);

found:

    /*
     * Do the update.  No ereport(ERROR) until changes are logged. We want to
     * log the changes for bitmap page and overflow page together to avoid
     * loss of pages in case the new page is added.
     */
    START_CRIT_SECTION();

    if (page_found)
    {
        Assert(BufferIsValid(mapbuf));

        /* mark page "in use" in the bitmap */
        SETBIT(freep, bitmap_page_bit);
        MarkBufferDirty(mapbuf);
    }
    else
    {
        /* update the count to indicate new overflow page is added */
        metap->hashm_spares[splitnum]++;

        if (BufferIsValid(newmapbuf))
        {
            _hash_initbitmapbuffer(newmapbuf, metap->hashm_bmsize, false);
            MarkBufferDirty(newmapbuf);

            /* add the new bitmap page to the metapage's list of bitmaps */
            metap->hashm_mapp[metap->hashm_nmaps] = BufferGetBlockNumber(newmapbuf);
            metap->hashm_nmaps++;
            metap->hashm_spares[splitnum]++;
        }

        MarkBufferDirty(metabuf);

        /*
         * for new overflow page, we don't need to explicitly set the bit in
         * bitmap page, as by default that will be set to "in use".
         */
    }

    /*
     * Adjust hashm_firstfree to avoid redundant searches.  But don't risk
     * changing it if someone moved it while we were searching bitmap pages.
     */
    if (metap->hashm_firstfree == orig_firstfree)
    {
        metap->hashm_firstfree = bit + 1;
        MarkBufferDirty(metabuf);
    }

    /* initialize new overflow page */
    ovflpage = BufferGetPage(ovflbuf);
    ovflopaque = (HashPageOpaque) PageGetSpecialPointer(ovflpage);
    ovflopaque->hasho_prevblkno = BufferGetBlockNumber(buf);
    ovflopaque->hasho_nextblkno = InvalidBlockNumber;
    ovflopaque->hasho_bucket = pageopaque->hasho_bucket;
    ovflopaque->hasho_flag = LH_OVERFLOW_PAGE;
    ovflopaque->hasho_page_id = HASHO_PAGE_ID;

    MarkBufferDirty(ovflbuf);

    /* logically chain overflow page to previous page */
    pageopaque->hasho_nextblkno = BufferGetBlockNumber(ovflbuf);

    MarkBufferDirty(buf);

    /* XLOG stuff */
    if (RelationNeedsWAL(rel))
    {
        XLogRecPtr  recptr;
        xl_hash_add_ovfl_page xlrec;

        xlrec.bmpage_found = page_found;
        xlrec.bmsize = metap->hashm_bmsize;

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfHashAddOvflPage);

        XLogRegisterBuffer(0, ovflbuf, REGBUF_WILL_INIT);
        XLogRegisterBufData(0, (char *) &pageopaque->hasho_bucket, sizeof(Bucket));

        XLogRegisterBuffer(1, buf, REGBUF_STANDARD);

        if (BufferIsValid(mapbuf))
        {
            XLogRegisterBuffer(2, mapbuf, REGBUF_STANDARD);
            XLogRegisterBufData(2, (char *) &bitmap_page_bit, sizeof(uint32));
        }

        if (BufferIsValid(newmapbuf))
            XLogRegisterBuffer(3, newmapbuf, REGBUF_WILL_INIT);

        XLogRegisterBuffer(4, metabuf, REGBUF_STANDARD);
        XLogRegisterBufData(4, (char *) &metap->hashm_firstfree, sizeof(uint32));

        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_ADD_OVFL_PAGE);

        PageSetLSN(BufferGetPage(ovflbuf), recptr);
        PageSetLSN(BufferGetPage(buf), recptr);

        if (BufferIsValid(mapbuf))
            PageSetLSN(BufferGetPage(mapbuf), recptr);

        if (BufferIsValid(newmapbuf))
            PageSetLSN(BufferGetPage(newmapbuf), recptr);

        PageSetLSN(BufferGetPage(metabuf), recptr);
    }

    END_CRIT_SECTION();

    if (retain_pin)
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
    else
        _hash_relbuf(rel, buf);

    if (BufferIsValid(mapbuf))
        _hash_relbuf(rel, mapbuf);

    LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);

    if (BufferIsValid(newmapbuf))
        _hash_relbuf(rel, newmapbuf);

    return ovflbuf;
}

/*
 *  _hash_firstfreebit()
 *
 *  Return the number of the first bit that is not set in the word 'map'.
 */
static uint32
_hash_firstfreebit(uint32 map)
{
    uint32      i,
                mask;

    mask = 0x1;
    for (i = 0; i < BITS_PER_MAP; i++)
    {
        if (!(mask & map))
            return i;
        mask <<= 1;
    }

    elog(ERROR, "firstfreebit found no free bit");

    return 0;                   /* keep compiler quiet */
}

/*
 *  _hash_freeovflpage() -
 *
 *  Remove this overflow page from its bucket's chain, and mark the page as
 *  free.  On entry, ovflbuf is write-locked; it is released before exiting.
 *
 *  Add the tuples (itups) to wbuf in this function.  We could do that in the
 *  caller as well, but the advantage of doing it here is we can easily write
 *  the WAL for XLOG_HASH_SQUEEZE_PAGE operation.  Addition of tuples and
 *  removal of overflow page has to done as an atomic operation, otherwise
 *  during replay on standby users might find duplicate records.
 *
 *  Since this function is invoked in VACUUM, we provide an access strategy
 *  parameter that controls fetches of the bucket pages.
 *
 *  Returns the block number of the page that followed the given page
 *  in the bucket, or InvalidBlockNumber if no following page.
 *
 *  NB: caller must not hold lock on metapage, nor on page, that's next to
 *  ovflbuf in the bucket chain.  We don't acquire the lock on page that's
 *  prior to ovflbuf in chain if it is same as wbuf because the caller already
 *  has a lock on same.
 */
BlockNumber
_hash_freeovflpage(Relation rel, Buffer bucketbuf, Buffer ovflbuf,
                   Buffer wbuf, IndexTuple *itups, OffsetNumber *itup_offsets,
                   Size *tups_size, uint16 nitups,
                   BufferAccessStrategy bstrategy)
{
    HashMetaPage metap;
    Buffer      metabuf;
    Buffer      mapbuf;
    BlockNumber ovflblkno;
    BlockNumber prevblkno;
    BlockNumber blkno;
    BlockNumber nextblkno;
    BlockNumber writeblkno;
    HashPageOpaque ovflopaque;
    Page        ovflpage;
    Page        mappage;
    uint32     *freep;
    uint32      ovflbitno;
    int32       bitmappage,
                bitmapbit;
    Bucket      bucket PG_USED_FOR_ASSERTS_ONLY;
    Buffer      prevbuf = InvalidBuffer;
    Buffer      nextbuf = InvalidBuffer;
    bool        update_metap = false;

    /* Get information from the doomed page */
    _hash_checkpage(rel, ovflbuf, LH_OVERFLOW_PAGE);
    ovflblkno = BufferGetBlockNumber(ovflbuf);
    ovflpage = BufferGetPage(ovflbuf);
    ovflopaque = (HashPageOpaque) PageGetSpecialPointer(ovflpage);
    nextblkno = ovflopaque->hasho_nextblkno;
    prevblkno = ovflopaque->hasho_prevblkno;
    writeblkno = BufferGetBlockNumber(wbuf);
    bucket = ovflopaque->hasho_bucket;

    /*
     * Fix up the bucket chain.  this is a doubly-linked list, so we must fix
     * up the bucket chain members behind and ahead of the overflow page being
     * deleted.  Concurrency issues are avoided by using lock chaining as
     * described atop hashbucketcleanup.
     */
    if (BlockNumberIsValid(prevblkno))
    {
        if (prevblkno == writeblkno)
            prevbuf = wbuf;
        else
            prevbuf = _hash_getbuf_with_strategy(rel,
                                                 prevblkno,
                                                 HASH_WRITE,
                                                 LH_BUCKET_PAGE | LH_OVERFLOW_PAGE,
                                                 bstrategy);
    }
    if (BlockNumberIsValid(nextblkno))
        nextbuf = _hash_getbuf_with_strategy(rel,
                                             nextblkno,
                                             HASH_WRITE,
                                             LH_OVERFLOW_PAGE,
                                             bstrategy);

    /* Note: bstrategy is intentionally not used for metapage and bitmap */

    /* Read the metapage so we can determine which bitmap page to use */
    metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ, LH_META_PAGE);
    metap = HashPageGetMeta(BufferGetPage(metabuf));

    /* Identify which bit to set */
    ovflbitno = _hash_ovflblkno_to_bitno(metap, ovflblkno);

    bitmappage = ovflbitno >> BMPG_SHIFT(metap);
    bitmapbit = ovflbitno & BMPG_MASK(metap);

    if (bitmappage >= metap->hashm_nmaps)
        elog(ERROR, "invalid overflow bit number %u", ovflbitno);
    blkno = metap->hashm_mapp[bitmappage];

    /* Release metapage lock while we access the bitmap page */
    LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);

    /* read the bitmap page to clear the bitmap bit */
    mapbuf = _hash_getbuf(rel, blkno, HASH_WRITE, LH_BITMAP_PAGE);
    mappage = BufferGetPage(mapbuf);
    freep = HashPageGetBitmap(mappage);
    Assert(ISSET(freep, bitmapbit));

    /* Get write-lock on metapage to update firstfree */
    LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);

    /* This operation needs to log multiple tuples, prepare WAL for that */
    if (RelationNeedsWAL(rel))
        XLogEnsureRecordSpace(HASH_XLOG_FREE_OVFL_BUFS, 4 + nitups);

    START_CRIT_SECTION();

    /*
     * we have to insert tuples on the "write" page, being careful to preserve
     * hashkey ordering.  (If we insert many tuples into the same "write" page
     * it would be worth qsort'ing them).
     */
    if (nitups > 0)
    {
        _hash_pgaddmultitup(rel, wbuf, itups, itup_offsets, nitups);
        MarkBufferDirty(wbuf);
    }

    /*
     * Reinitialize the freed overflow page.  Just zeroing the page won't
     * work, because WAL replay routines expect pages to be initialized. See
     * explanation of RBM_NORMAL mode atop XLogReadBufferExtended.  We are
     * careful to make the special space valid here so that tools like
     * pageinspect won't get confused.
     */
    _hash_pageinit(ovflpage, BufferGetPageSize(ovflbuf));

    ovflopaque = (HashPageOpaque) PageGetSpecialPointer(ovflpage);

    ovflopaque->hasho_prevblkno = InvalidBlockNumber;
    ovflopaque->hasho_nextblkno = InvalidBlockNumber;
    ovflopaque->hasho_bucket = -1;
    ovflopaque->hasho_flag = LH_UNUSED_PAGE;
    ovflopaque->hasho_page_id = HASHO_PAGE_ID;

    MarkBufferDirty(ovflbuf);

    if (BufferIsValid(prevbuf))
    {
        Page        prevpage = BufferGetPage(prevbuf);
        HashPageOpaque prevopaque = (HashPageOpaque) PageGetSpecialPointer(prevpage);

        Assert(prevopaque->hasho_bucket == bucket);
        prevopaque->hasho_nextblkno = nextblkno;
        MarkBufferDirty(prevbuf);
    }
    if (BufferIsValid(nextbuf))
    {
        Page        nextpage = BufferGetPage(nextbuf);
        HashPageOpaque nextopaque = (HashPageOpaque) PageGetSpecialPointer(nextpage);

        Assert(nextopaque->hasho_bucket == bucket);
        nextopaque->hasho_prevblkno = prevblkno;
        MarkBufferDirty(nextbuf);
    }

    /* Clear the bitmap bit to indicate that this overflow page is free */
    CLRBIT(freep, bitmapbit);
    MarkBufferDirty(mapbuf);

    /* if this is now the first free page, update hashm_firstfree */
    if (ovflbitno < metap->hashm_firstfree)
    {
        metap->hashm_firstfree = ovflbitno;
        update_metap = true;
        MarkBufferDirty(metabuf);
    }

    /* XLOG stuff */
    if (RelationNeedsWAL(rel))
    {
        xl_hash_squeeze_page xlrec;
        XLogRecPtr  recptr;
        int         i;

        xlrec.prevblkno = prevblkno;
        xlrec.nextblkno = nextblkno;
        xlrec.ntups = nitups;
        xlrec.is_prim_bucket_same_wrt = (wbuf == bucketbuf);
        xlrec.is_prev_bucket_same_wrt = (wbuf == prevbuf);

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfHashSqueezePage);

        /*
         * bucket buffer needs to be registered to ensure that we can acquire
         * a cleanup lock on it during replay.
         */
        if (!xlrec.is_prim_bucket_same_wrt)
            XLogRegisterBuffer(0, bucketbuf, REGBUF_STANDARD | REGBUF_NO_IMAGE);

        XLogRegisterBuffer(1, wbuf, REGBUF_STANDARD);
        if (xlrec.ntups > 0)
        {
            XLogRegisterBufData(1, (char *) itup_offsets,
                                nitups * sizeof(OffsetNumber));
            for (i = 0; i < nitups; i++)
                XLogRegisterBufData(1, (char *) itups[i], tups_size[i]);
        }

        XLogRegisterBuffer(2, ovflbuf, REGBUF_STANDARD);

        /*
         * If prevpage and the writepage (block in which we are moving tuples
         * from overflow) are same, then no need to separately register
         * prevpage.  During replay, we can directly update the nextblock in
         * writepage.
         */
        if (BufferIsValid(prevbuf) && !xlrec.is_prev_bucket_same_wrt)
            XLogRegisterBuffer(3, prevbuf, REGBUF_STANDARD);

        if (BufferIsValid(nextbuf))
            XLogRegisterBuffer(4, nextbuf, REGBUF_STANDARD);

        XLogRegisterBuffer(5, mapbuf, REGBUF_STANDARD);
        XLogRegisterBufData(5, (char *) &bitmapbit, sizeof(uint32));

        if (update_metap)
        {
            XLogRegisterBuffer(6, metabuf, REGBUF_STANDARD);
            XLogRegisterBufData(6, (char *) &metap->hashm_firstfree, sizeof(uint32));
        }

        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SQUEEZE_PAGE);

        PageSetLSN(BufferGetPage(wbuf), recptr);
        PageSetLSN(BufferGetPage(ovflbuf), recptr);

        if (BufferIsValid(prevbuf) && !xlrec.is_prev_bucket_same_wrt)
            PageSetLSN(BufferGetPage(prevbuf), recptr);
        if (BufferIsValid(nextbuf))
            PageSetLSN(BufferGetPage(nextbuf), recptr);

        PageSetLSN(BufferGetPage(mapbuf), recptr);

        if (update_metap)
            PageSetLSN(BufferGetPage(metabuf), recptr);
    }

    END_CRIT_SECTION();

    /* release previous bucket if it is not same as write bucket */
    if (BufferIsValid(prevbuf) && prevblkno != writeblkno)
        _hash_relbuf(rel, prevbuf);

    if (BufferIsValid(ovflbuf))
        _hash_relbuf(rel, ovflbuf);

    if (BufferIsValid(nextbuf))
        _hash_relbuf(rel, nextbuf);

    _hash_relbuf(rel, mapbuf);
    _hash_relbuf(rel, metabuf);

    return nextblkno;
}


/*
 *  _hash_initbitmapbuffer()
 *
 *   Initialize a new bitmap page.  All bits in the new bitmap page are set to
 *   "1", indicating "in use".
 */
void
_hash_initbitmapbuffer(Buffer buf, uint16 bmsize, bool initpage)
{
    Page        pg;
    HashPageOpaque op;
    uint32     *freep;

    pg = BufferGetPage(buf);

    /* initialize the page */
    if (initpage)
        _hash_pageinit(pg, BufferGetPageSize(buf));

    /* initialize the page's special space */
    op = (HashPageOpaque) PageGetSpecialPointer(pg);
    op->hasho_prevblkno = InvalidBlockNumber;
    op->hasho_nextblkno = InvalidBlockNumber;
    op->hasho_bucket = -1;
    op->hasho_flag = LH_BITMAP_PAGE;
    op->hasho_page_id = HASHO_PAGE_ID;

    /* set all of the bits to 1 */
    freep = HashPageGetBitmap(pg);
    MemSet(freep, 0xFF, bmsize);

    /*
     * Set pd_lower just past the end of the bitmap page data.  We could even
     * set pd_lower equal to pd_upper, but this is more precise and makes the
     * page look compressible to xlog.c.
     */
    ((PageHeader) pg)->pd_lower = ((char *) freep + bmsize) - (char *) pg;
}


/*
 *  _hash_squeezebucket(rel, bucket)
 *
 *  Try to squeeze the tuples onto pages occurring earlier in the
 *  bucket chain in an attempt to free overflow pages. When we start
 *  the "squeezing", the page from which we start taking tuples (the
 *  "read" page) is the last bucket in the bucket chain and the page
 *  onto which we start squeezing tuples (the "write" page) is the
 *  first page in the bucket chain.  The read page works backward and
 *  the write page works forward; the procedure terminates when the
 *  read page and write page are the same page.
 *
 *  At completion of this procedure, it is guaranteed that all pages in
 *  the bucket are nonempty, unless the bucket is totally empty (in
 *  which case all overflow pages will be freed).  The original implementation
 *  required that to be true on entry as well, but it's a lot easier for
 *  callers to leave empty overflow pages and let this guy clean it up.
 *
 *  Caller must acquire cleanup lock on the primary page of the target
 *  bucket to exclude any scans that are in progress, which could easily
 *  be confused into returning the same tuple more than once or some tuples
 *  not at all by the rearrangement we are performing here.  To prevent
 *  any concurrent scan to cross the squeeze scan we use lock chaining
 *  similar to hashbucketcleanup.  Refer comments atop hashbucketcleanup.
 *
 *  We need to retain a pin on the primary bucket to ensure that no concurrent
 *  split can start.
 *
 *  Since this function is invoked in VACUUM, we provide an access strategy
 *  parameter that controls fetches of the bucket pages.
 */
void
_hash_squeezebucket(Relation rel,
                    Bucket bucket,
                    BlockNumber bucket_blkno,
                    Buffer bucket_buf,
                    BufferAccessStrategy bstrategy)
{
    BlockNumber wblkno;
    BlockNumber rblkno;
    Buffer      wbuf;
    Buffer      rbuf;
    Page        wpage;
    Page        rpage;
    HashPageOpaque wopaque;
    HashPageOpaque ropaque;

    /*
     * start squeezing into the primary bucket page.
     */
    wblkno = bucket_blkno;
    wbuf = bucket_buf;
    wpage = BufferGetPage(wbuf);
    wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);

    /*
     * if there aren't any overflow pages, there's nothing to squeeze. caller
     * is responsible for releasing the pin on primary bucket page.
     */
    if (!BlockNumberIsValid(wopaque->hasho_nextblkno))
    {
        LockBuffer(wbuf, BUFFER_LOCK_UNLOCK);
        return;
    }

    /*
     * Find the last page in the bucket chain by starting at the base bucket
     * page and working forward.  Note: we assume that a hash bucket chain is
     * usually smaller than the buffer ring being used by VACUUM, else using
     * the access strategy here would be counterproductive.
     */
    rbuf = InvalidBuffer;
    ropaque = wopaque;
    do
    {
        rblkno = ropaque->hasho_nextblkno;
        if (rbuf != InvalidBuffer)
            _hash_relbuf(rel, rbuf);
        rbuf = _hash_getbuf_with_strategy(rel,
                                          rblkno,
                                          HASH_WRITE,
                                          LH_OVERFLOW_PAGE,
                                          bstrategy);
        rpage = BufferGetPage(rbuf);
        ropaque = (HashPageOpaque) PageGetSpecialPointer(rpage);
        Assert(ropaque->hasho_bucket == bucket);
    } while (BlockNumberIsValid(ropaque->hasho_nextblkno));

    /*
     * squeeze the tuples.
     */
    for (;;)
    {
        OffsetNumber roffnum;
        OffsetNumber maxroffnum;
        OffsetNumber deletable[MaxOffsetNumber];
        IndexTuple  itups[MaxIndexTuplesPerPage];
        Size        tups_size[MaxIndexTuplesPerPage];
        OffsetNumber itup_offsets[MaxIndexTuplesPerPage];
        uint16      ndeletable = 0;
        uint16      nitups = 0;
        Size        all_tups_size = 0;
        int         i;
        bool        retain_pin = false;

readpage:
        /* Scan each tuple in "read" page */
        maxroffnum = PageGetMaxOffsetNumber(rpage);
        for (roffnum = FirstOffsetNumber;
             roffnum <= maxroffnum;
             roffnum = OffsetNumberNext(roffnum))
        {
            IndexTuple  itup;
            Size        itemsz;

            /* skip dead tuples */
            if (ItemIdIsDead(PageGetItemId(rpage, roffnum)))
                continue;

            itup = (IndexTuple) PageGetItem(rpage,
                                            PageGetItemId(rpage, roffnum));
            itemsz = IndexTupleSize(itup);
            itemsz = MAXALIGN(itemsz);

            /*
             * Walk up the bucket chain, looking for a page big enough for
             * this item and all other accumulated items.  Exit if we reach
             * the read page.
             */
            while (PageGetFreeSpaceForMultipleTuples(wpage, nitups + 1) < (all_tups_size + itemsz))
            {
                Buffer      next_wbuf = InvalidBuffer;
                bool        tups_moved = false;

                Assert(!PageIsEmpty(wpage));

                if (wblkno == bucket_blkno)
                    retain_pin = true;

                wblkno = wopaque->hasho_nextblkno;
                Assert(BlockNumberIsValid(wblkno));

                /* don't need to move to next page if we reached the read page */
                if (wblkno != rblkno)
                    next_wbuf = _hash_getbuf_with_strategy(rel,
                                                           wblkno,
                                                           HASH_WRITE,
                                                           LH_OVERFLOW_PAGE,
                                                           bstrategy);

                if (nitups > 0)
                {
                    Assert(nitups == ndeletable);

                    /*
                     * This operation needs to log multiple tuples, prepare
                     * WAL for that.
                     */
                    if (RelationNeedsWAL(rel))
                        XLogEnsureRecordSpace(0, 3 + nitups);

                    START_CRIT_SECTION();

                    /*
                     * we have to insert tuples on the "write" page, being
                     * careful to preserve hashkey ordering.  (If we insert
                     * many tuples into the same "write" page it would be
                     * worth qsort'ing them).
                     */
                    _hash_pgaddmultitup(rel, wbuf, itups, itup_offsets, nitups);
                    MarkBufferDirty(wbuf);

                    /* Delete tuples we already moved off read page */
                    PageIndexMultiDelete(rpage, deletable, ndeletable);
                    MarkBufferDirty(rbuf);

                    /* XLOG stuff */
                    if (RelationNeedsWAL(rel))
                    {
                        XLogRecPtr  recptr;
                        xl_hash_move_page_contents xlrec;

                        xlrec.ntups = nitups;
                        xlrec.is_prim_bucket_same_wrt = (wbuf == bucket_buf) ? true : false;

                        XLogBeginInsert();
                        XLogRegisterData((char *) &xlrec, SizeOfHashMovePageContents);

                        /*
                         * bucket buffer needs to be registered to ensure that
                         * we can acquire a cleanup lock on it during replay.
                         */
                        if (!xlrec.is_prim_bucket_same_wrt)
                            XLogRegisterBuffer(0, bucket_buf, REGBUF_STANDARD | REGBUF_NO_IMAGE);

                        XLogRegisterBuffer(1, wbuf, REGBUF_STANDARD);
                        XLogRegisterBufData(1, (char *) itup_offsets,
                                            nitups * sizeof(OffsetNumber));
                        for (i = 0; i < nitups; i++)
                            XLogRegisterBufData(1, (char *) itups[i], tups_size[i]);

                        XLogRegisterBuffer(2, rbuf, REGBUF_STANDARD);
                        XLogRegisterBufData(2, (char *) deletable,
                                            ndeletable * sizeof(OffsetNumber));

                        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_MOVE_PAGE_CONTENTS);

                        PageSetLSN(BufferGetPage(wbuf), recptr);
                        PageSetLSN(BufferGetPage(rbuf), recptr);
                    }

                    END_CRIT_SECTION();

                    tups_moved = true;
                }

                /*
                 * release the lock on previous page after acquiring the lock
                 * on next page
                 */
                if (retain_pin)
                    LockBuffer(wbuf, BUFFER_LOCK_UNLOCK);
                else
                    _hash_relbuf(rel, wbuf);

                /* nothing more to do if we reached the read page */
                if (rblkno == wblkno)
                {
                    _hash_relbuf(rel, rbuf);
                    return;
                }

                wbuf = next_wbuf;
                wpage = BufferGetPage(wbuf);
                wopaque = (HashPageOpaque) PageGetSpecialPointer(wpage);
                Assert(wopaque->hasho_bucket == bucket);
                retain_pin = false;

                /* be tidy */
                for (i = 0; i < nitups; i++)
                    pfree(itups[i]);
                nitups = 0;
                all_tups_size = 0;
                ndeletable = 0;

                /*
                 * after moving the tuples, rpage would have been compacted,
                 * so we need to rescan it.
                 */
                if (tups_moved)
                    goto readpage;
            }

            /* remember tuple for deletion from "read" page */
            deletable[ndeletable++] = roffnum;

            /*
             * we need a copy of index tuples as they can be freed as part of
             * overflow page, however we need them to write a WAL record in
             * _hash_freeovflpage.
             */
            itups[nitups] = CopyIndexTuple(itup);
            tups_size[nitups++] = itemsz;
            all_tups_size += itemsz;
        }

        /*
         * If we reach here, there are no live tuples on the "read" page ---
         * it was empty when we got to it, or we moved them all.  So we can
         * just free the page without bothering with deleting tuples
         * individually.  Then advance to the previous "read" page.
         *
         * Tricky point here: if our read and write pages are adjacent in the
         * bucket chain, our write lock on wbuf will conflict with
         * _hash_freeovflpage's attempt to update the sibling links of the
         * removed page.  In that case, we don't need to lock it again.
         */
        rblkno = ropaque->hasho_prevblkno;
        Assert(BlockNumberIsValid(rblkno));

        /* free this overflow page (releases rbuf) */
        _hash_freeovflpage(rel, bucket_buf, rbuf, wbuf, itups, itup_offsets,
                           tups_size, nitups, bstrategy);

        /* be tidy */
        for (i = 0; i < nitups; i++)
            pfree(itups[i]);

        /* are we freeing the page adjacent to wbuf? */
        if (rblkno == wblkno)
        {
            /* retain the pin on primary bucket page till end of bucket scan */
            if (wblkno == bucket_blkno)
                LockBuffer(wbuf, BUFFER_LOCK_UNLOCK);
            else
                _hash_relbuf(rel, wbuf);
            return;
        }

        rbuf = _hash_getbuf_with_strategy(rel,
                                          rblkno,
                                          HASH_WRITE,
                                          LH_OVERFLOW_PAGE,
                                          bstrategy);
        rpage = BufferGetPage(rbuf);
        ropaque = (HashPageOpaque) PageGetSpecialPointer(rpage);
        Assert(ropaque->hasho_bucket == bucket);
    }

    /* NOTREACHED */
}