hashpage.c

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/*-------------------------------------------------------------------------
 *
 * hashpage.c
 *    Hash table page management code for the Postgres hash access method
 *
 * Portions Copyright (c) 1996-2024, 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 "access/hash_xlog.h"
#include "access/xloginsert.h"
#include "miscadmin.h"
#include "port/pg_bitutils.h"
#include "storage/predicate.h"
#include "storage/smgr.h"
#include "utils/rel.h"
 
static bool _hash_alloc_buckets(Relation rel, BlockNumber firstblock,
                                uint32 nblocks);
static void _hash_splitbucket(Relation rel, Buffer metabuf,
                              Bucket obucket, Bucket nbucket,
                              Buffer obuf,
                              Buffer nbuf,
                              HTAB *htab,
                              uint32 maxbucket,
                              uint32 highmask, uint32 lowmask);
static void log_split_page(Relation rel, Buffer buf);
 
 
/*
 *  _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_getbuf_with_condlock_cleanup() -- Try to get a buffer for cleanup.
 *
 *      We read the page and try to acquire a cleanup lock.  If we get it,
 *      we return the buffer; otherwise, we return InvalidBuffer.
 */
Buffer
_hash_getbuf_with_condlock_cleanup(Relation rel, BlockNumber blkno, int flags)
{
    Buffer      buf;
 
    if (blkno == P_NEW)
        elog(ERROR, "hash AM does not use P_NEW");
 
    buf = ReadBuffer(rel, blkno);
 
    if (!ConditionalLockBufferForCleanup(buf))
    {
        ReleaseBuffer(buf);
        return InvalidBuffer;
    }
 
    /* 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_AND_LOCK,
                             NULL);
 
    /* ref count and lock type are correct */
 
    /* initialize the page */
    _hash_pageinit(BufferGetPage(buf), BufferGetPageSize(buf));
 
    return buf;
}
 
/*
 *  _hash_initbuf() -- Get and initialize a buffer by bucket number.
 */
void
_hash_initbuf(Buffer buf, uint32 max_bucket, uint32 num_bucket, uint32 flag,
              bool initpage)
{
    HashPageOpaque pageopaque;
    Page        page;
 
    page = BufferGetPage(buf);
 
    /* initialize the page */
    if (initpage)
        _hash_pageinit(page, BufferGetPageSize(buf));
 
    pageopaque = HashPageGetOpaque(page);
 
    /*
     * Set hasho_prevblkno with current hashm_maxbucket. This value will be
     * used to validate cached HashMetaPageData. See
     * _hash_getbucketbuf_from_hashkey().
     */
    pageopaque->hasho_prevblkno = max_bucket;
    pageopaque->hasho_nextblkno = InvalidBlockNumber;
    pageopaque->hasho_bucket = num_bucket;
    pageopaque->hasho_flag = flag;
    pageopaque->hasho_page_id = HASHO_PAGE_ID;
}
 
/*
 *  _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.  In practice, this function is called
 *      only while holding write lock on the metapage, because adding a page
 *      is always associated with an update of metapage data.
 */
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 explicitly extend the relation */
    if (blkno == nblocks)
    {
        buf = ExtendBufferedRel(BMR_REL(rel), forkNum, NULL,
                                EB_LOCK_FIRST | EB_SKIP_EXTENSION_LOCK);
        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_AND_LOCK,
                                 NULL);
    }
 
    /* 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_dropscanbuf() -- release buffers used in scan.
 *
 * This routine unpins the buffers used during scan on which we
 * hold no lock.
 */
void
_hash_dropscanbuf(Relation rel, HashScanOpaque so)
{
    /* release pin we hold on primary bucket page */
    if (BufferIsValid(so->hashso_bucket_buf) &&
        so->hashso_bucket_buf != so->currPos.buf)
        _hash_dropbuf(rel, so->hashso_bucket_buf);
    so->hashso_bucket_buf = InvalidBuffer;
 
    /* release pin we hold on primary bucket page  of bucket being split */
    if (BufferIsValid(so->hashso_split_bucket_buf) &&
        so->hashso_split_bucket_buf != so->currPos.buf)
        _hash_dropbuf(rel, so->hashso_split_bucket_buf);
    so->hashso_split_bucket_buf = InvalidBuffer;
 
    /* release any pin we still hold */
    if (BufferIsValid(so->currPos.buf))
        _hash_dropbuf(rel, so->currPos.buf);
    so->currPos.buf = InvalidBuffer;
 
    /* reset split scan */
    so->hashso_buc_populated = false;
    so->hashso_buc_split = false;
}
 
 
/*
 *  _hash_init() -- 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_init(Relation rel, double num_tuples, ForkNumber forkNum)
{
    Buffer      metabuf;
    Buffer      buf;
    Buffer      bitmapbuf;
    Page        pg;
    HashMetaPage metap;
    RegProcedure procid;
    int32       data_width;
    int32       item_width;
    int32       ffactor;
    uint32      num_buckets;
    uint32      i;
    bool        use_wal;
 
    /* safety check */
    if (RelationGetNumberOfBlocksInFork(rel, forkNum) != 0)
        elog(ERROR, "cannot initialize non-empty hash index \"%s\"",
             RelationGetRelationName(rel));
 
    /*
     * WAL log creation of pages if the relation is persistent, or this is the
     * init fork.  Init forks for unlogged relations always need to be WAL
     * logged.
     */
    use_wal = RelationNeedsWAL(rel) || forkNum == INIT_FORKNUM;
 
    /*
     * 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 = HashGetTargetPageUsage(rel) / item_width;
    /* keep to a sane range */
    if (ffactor < 10)
        ffactor = 10;
 
    procid = index_getprocid(rel, 1, HASHSTANDARD_PROC);
 
    /*
     * 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.
     *
     * Critical section not required, because on error the creation of the
     * whole relation will be rolled back.
     */
    metabuf = _hash_getnewbuf(rel, HASH_METAPAGE, forkNum);
    _hash_init_metabuffer(metabuf, num_tuples, procid, ffactor, false);
    MarkBufferDirty(metabuf);
 
    pg = BufferGetPage(metabuf);
    metap = HashPageGetMeta(pg);
 
    /* XLOG stuff */
    if (use_wal)
    {
        xl_hash_init_meta_page xlrec;
        XLogRecPtr  recptr;
 
        xlrec.num_tuples = num_tuples;
        xlrec.procid = metap->hashm_procid;
        xlrec.ffactor = metap->hashm_ffactor;
 
        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfHashInitMetaPage);
        XLogRegisterBuffer(0, metabuf, REGBUF_WILL_INIT | REGBUF_STANDARD);
 
        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_INIT_META_PAGE);
 
        PageSetLSN(BufferGetPage(metabuf), recptr);
    }
 
    num_buckets = metap->hashm_maxbucket + 1;
 
    /*
     * 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.
     */
    LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
 
    /*
     * Initialize and WAL Log the first N buckets
     */
    for (i = 0; i < num_buckets; i++)
    {
        BlockNumber blkno;
 
        /* Allow interrupts, in case N is huge */
        CHECK_FOR_INTERRUPTS();
 
        blkno = BUCKET_TO_BLKNO(metap, i);
        buf = _hash_getnewbuf(rel, blkno, forkNum);
        _hash_initbuf(buf, metap->hashm_maxbucket, i, LH_BUCKET_PAGE, false);
        MarkBufferDirty(buf);
 
        if (use_wal)
            log_newpage(&rel->rd_locator,
                        forkNum,
                        blkno,
                        BufferGetPage(buf),
                        true);
        _hash_relbuf(rel, buf);
    }
 
    /* Now reacquire buffer lock on metapage */
    LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);
 
    /*
     * Initialize bitmap page
     */
    bitmapbuf = _hash_getnewbuf(rel, num_buckets + 1, forkNum);
    _hash_initbitmapbuffer(bitmapbuf, metap->hashm_bmsize, false);
    MarkBufferDirty(bitmapbuf);
 
    /* add the new bitmap page to the metapage's list of bitmaps */
    /* 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))));
 
    metap->hashm_mapp[metap->hashm_nmaps] = num_buckets + 1;
 
    metap->hashm_nmaps++;
    MarkBufferDirty(metabuf);
 
    /* XLOG stuff */
    if (use_wal)
    {
        xl_hash_init_bitmap_page xlrec;
        XLogRecPtr  recptr;
 
        xlrec.bmsize = metap->hashm_bmsize;
 
        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfHashInitBitmapPage);
        XLogRegisterBuffer(0, bitmapbuf, REGBUF_WILL_INIT);
 
        /*
         * This is safe only because nobody else can be modifying the index at
         * this stage; it's only visible to the transaction that is creating
         * it.
         */
        XLogRegisterBuffer(1, metabuf, REGBUF_STANDARD);
 
        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_INIT_BITMAP_PAGE);
 
        PageSetLSN(BufferGetPage(bitmapbuf), recptr);
        PageSetLSN(BufferGetPage(metabuf), recptr);
    }
 
    /* all done */
    _hash_relbuf(rel, bitmapbuf);
    _hash_relbuf(rel, metabuf);
 
    return num_buckets;
}
 
/*
 *  _hash_init_metabuffer() -- Initialize the metadata page of a hash index.
 */
void
_hash_init_metabuffer(Buffer buf, double num_tuples, RegProcedure procid,
                      uint16 ffactor, bool initpage)
{
    HashMetaPage metap;
    HashPageOpaque pageopaque;
    Page        page;
    double      dnumbuckets;
    uint32      num_buckets;
    uint32      spare_index;
    uint32      lshift;
 
    /*
     * 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
     * total number of buckets which has to be allocated before using its
     * hashm_spares element. However 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 = _hash_get_totalbuckets(_hash_spareindex(dnumbuckets));
 
    spare_index = _hash_spareindex(num_buckets);
    Assert(spare_index < HASH_MAX_SPLITPOINTS);
 
    page = BufferGetPage(buf);
    if (initpage)
        _hash_pageinit(page, BufferGetPageSize(buf));
 
    pageopaque = HashPageGetOpaque(page);
    pageopaque->hasho_prevblkno = InvalidBlockNumber;
    pageopaque->hasho_nextblkno = InvalidBlockNumber;
    pageopaque->hasho_bucket = InvalidBucket;
    pageopaque->hasho_flag = LH_META_PAGE;
    pageopaque->hasho_page_id = HASHO_PAGE_ID;
 
    metap = HashPageGetMeta(page);
 
    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(page);
 
    /* find largest bitmap array size that will fit in page size */
    lshift = pg_leftmost_one_pos32(metap->hashm_bsize);
    Assert(lshift > 0);
    metap->hashm_bmsize = 1 << lshift;
    metap->hashm_bmshift = lshift + 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 = procid;
 
    /*
     * 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.
     */
    metap->hashm_maxbucket = num_buckets - 1;
 
    /*
     * Set highmask as next immediate ((2 ^ x) - 1), which should be
     * sufficient to cover num_buckets.
     */
    metap->hashm_highmask = pg_nextpower2_32(num_buckets + 1) - 1;
    metap->hashm_lowmask = (metap->hashm_highmask >> 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[spare_index] = 1;
    metap->hashm_ovflpoint = spare_index;
    metap->hashm_firstfree = 0;
 
    /*
     * Set pd_lower just past the end of the metadata.  This is essential,
     * because without doing so, metadata will be lost if xlog.c compresses
     * the page.
     */
    ((PageHeader) page)->pd_lower =
        ((char *) metap + sizeof(HashMetaPageData)) - (char *) page;
}
 
/*
 *  _hash_pageinit() -- Initialize a new hash index page.
 */
void
_hash_pageinit(Page page, Size size)
{
    PageInit(page, size, sizeof(HashPageOpaqueData));
}
 
/*
 * Attempt to expand the hash table by creating one new bucket.
 *
 * This will silently do nothing if we don't get cleanup lock on old or
 * new bucket.
 *
 * Complete the pending splits and remove the tuples from old bucket,
 * if there are any left over from the previous split.
 *
 * 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;
    Buffer      buf_nblkno;
    Buffer      buf_oblkno;
    Page        opage;
    Page        npage;
    HashPageOpaque oopaque;
    HashPageOpaque nopaque;
    uint32      maxbucket;
    uint32      highmask;
    uint32      lowmask;
    bool        metap_update_masks = false;
    bool        metap_update_splitpoint = false;
 
restart_expand:
 
    /*
     * 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.
     */
    LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);
 
    _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 to prevent failure of pg_ceil_log2_32() 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_init().
     */
    if (metap->hashm_maxbucket >= (uint32) 0x7FFFFFFE)
        goto fail;
 
    /*
     * Determine which bucket is to be split, and attempt to take cleanup lock
     * on the old bucket.  If we can't get the lock, give up.
     *
     * The cleanup lock protects us not only against other backends, but
     * against our own backend as well.
     *
     * The cleanup lock is mainly to protect the split from concurrent
     * inserts. See src/backend/access/hash/README, Lock Definitions for
     * further details.  Due to this locking restriction, if there is any
     * pending scan, the split will give up which is not good, but harmless.
     */
    new_bucket = metap->hashm_maxbucket + 1;
 
    old_bucket = (new_bucket & metap->hashm_lowmask);
 
    start_oblkno = BUCKET_TO_BLKNO(metap, old_bucket);
 
    buf_oblkno = _hash_getbuf_with_condlock_cleanup(rel, start_oblkno, LH_BUCKET_PAGE);
    if (!buf_oblkno)
        goto fail;
 
    opage = BufferGetPage(buf_oblkno);
    oopaque = HashPageGetOpaque(opage);
 
    /*
     * We want to finish the split from a bucket as there is no apparent
     * benefit by not doing so and it will make the code complicated to finish
     * the split that involves multiple buckets considering the case where new
     * split also fails.  We don't need to consider the new bucket for
     * completing the split here as it is not possible that a re-split of new
     * bucket starts when there is still a pending split from old bucket.
     */
    if (H_BUCKET_BEING_SPLIT(oopaque))
    {
        /*
         * Copy bucket mapping info now; refer the comment in code below where
         * we copy this information before calling _hash_splitbucket to see
         * why this is okay.
         */
        maxbucket = metap->hashm_maxbucket;
        highmask = metap->hashm_highmask;
        lowmask = metap->hashm_lowmask;
 
        /*
         * Release the lock on metapage and old_bucket, before completing the
         * split.
         */
        LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
        LockBuffer(buf_oblkno, BUFFER_LOCK_UNLOCK);
 
        _hash_finish_split(rel, metabuf, buf_oblkno, old_bucket, maxbucket,
                           highmask, lowmask);
 
        /* release the pin on old buffer and retry for expand. */
        _hash_dropbuf(rel, buf_oblkno);
 
        goto restart_expand;
    }
 
    /*
     * Clean the tuples remained from the previous split.  This operation
     * requires cleanup lock and we already have one on the old bucket, so
     * let's do it. We also don't want to allow further splits from the bucket
     * till the garbage of previous split is cleaned.  This has two
     * advantages; first, it helps in avoiding the bloat due to garbage and
     * second is, during cleanup of bucket, we are always sure that the
     * garbage tuples belong to most recently split bucket.  On the contrary,
     * if we allow cleanup of bucket after meta page is updated to indicate
     * the new split and before the actual split, the cleanup operation won't
     * be able to decide whether the tuple has been moved to the newly created
     * bucket and ended up deleting such tuples.
     */
    if (H_NEEDS_SPLIT_CLEANUP(oopaque))
    {
        /*
         * Copy bucket mapping info now; refer to the comment in code below
         * where we copy this information before calling _hash_splitbucket to
         * see why this is okay.
         */
        maxbucket = metap->hashm_maxbucket;
        highmask = metap->hashm_highmask;
        lowmask = metap->hashm_lowmask;
 
        /* Release the metapage lock. */
        LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
 
        hashbucketcleanup(rel, old_bucket, buf_oblkno, start_oblkno, NULL,
                          maxbucket, highmask, lowmask, NULL, NULL, true,
                          NULL, NULL);
 
        _hash_dropbuf(rel, buf_oblkno);
 
        goto restart_expand;
    }
 
    /*
     * There shouldn't be any active scan on new bucket.
     *
     * 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 the split point is increasing we need to allocate a new batch of
     * bucket pages.
     */
    spare_ndx = _hash_spareindex(new_bucket + 1);
    if (spare_ndx > metap->hashm_ovflpoint)
    {
        uint32      buckets_to_add;
 
        Assert(spare_ndx == metap->hashm_ovflpoint + 1);
 
        /*
         * We treat allocation of buckets as a separate WAL-logged action.
         * Even if we fail after this operation, won't leak bucket pages;
         * rather, the next split will consume this space. In any case, even
         * without failure we don't use all the space in one split operation.
         */
        buckets_to_add = _hash_get_totalbuckets(spare_ndx) - new_bucket;
        if (!_hash_alloc_buckets(rel, start_nblkno, buckets_to_add))
        {
            /* can't split due to BlockNumber overflow */
            _hash_relbuf(rel, buf_oblkno);
            goto fail;
        }
    }
 
    /*
     * Physically allocate the new bucket's primary page.  We want to do this
     * before changing the metapage's mapping info, in case we can't get the
     * disk space.
     *
     * XXX It doesn't make sense to call _hash_getnewbuf first, zeroing the
     * buffer, and then only afterwards check whether we have a cleanup lock.
     * However, since no scan can be accessing the buffer yet, any concurrent
     * accesses will just be from processes like the bgwriter or checkpointer
     * which don't care about its contents, so it doesn't really matter.
     */
    buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
    if (!IsBufferCleanupOK(buf_nblkno))
    {
        _hash_relbuf(rel, buf_oblkno);
        _hash_relbuf(rel, buf_nblkno);
        goto fail;
    }
 
    /*
     * Since we are scribbling on the pages in the shared buffers, establish a
     * critical section.  Any failure in this next code leaves us with a big
     * problem: the metapage is effectively corrupt but could get written back
     * to disk.
     */
    START_CRIT_SECTION();
 
    /*
     * Okay to proceed with split.  Update the metapage bucket mapping info.
     */
    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;
        metap_update_masks = true;
    }
 
    /*
     * If the split point is increasing 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;
        metap_update_splitpoint = true;
    }
 
    MarkBufferDirty(metabuf);
 
    /*
     * 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;
 
    opage = BufferGetPage(buf_oblkno);
    oopaque = HashPageGetOpaque(opage);
 
    /*
     * Mark the old bucket to indicate that split is in progress.  (At
     * operation end, we will clear the split-in-progress flag.)  Also, for a
     * primary bucket page, hasho_prevblkno stores the number of buckets that
     * existed as of the last split, so we must update that value here.
     */
    oopaque->hasho_flag |= LH_BUCKET_BEING_SPLIT;
    oopaque->hasho_prevblkno = maxbucket;
 
    MarkBufferDirty(buf_oblkno);
 
    npage = BufferGetPage(buf_nblkno);
 
    /*
     * initialize the new bucket's primary page and mark it to indicate that
     * split is in progress.
     */
    nopaque = HashPageGetOpaque(npage);
    nopaque->hasho_prevblkno = maxbucket;
    nopaque->hasho_nextblkno = InvalidBlockNumber;
    nopaque->hasho_bucket = new_bucket;
    nopaque->hasho_flag = LH_BUCKET_PAGE | LH_BUCKET_BEING_POPULATED;
    nopaque->hasho_page_id = HASHO_PAGE_ID;
 
    MarkBufferDirty(buf_nblkno);
 
    /* XLOG stuff */
    if (RelationNeedsWAL(rel))
    {
        xl_hash_split_allocate_page xlrec;
        XLogRecPtr  recptr;
 
        xlrec.new_bucket = maxbucket;
        xlrec.old_bucket_flag = oopaque->hasho_flag;
        xlrec.new_bucket_flag = nopaque->hasho_flag;
        xlrec.flags = 0;
 
        XLogBeginInsert();
 
        XLogRegisterBuffer(0, buf_oblkno, REGBUF_STANDARD);
        XLogRegisterBuffer(1, buf_nblkno, REGBUF_WILL_INIT);
        XLogRegisterBuffer(2, metabuf, REGBUF_STANDARD);
 
        if (metap_update_masks)
        {
            xlrec.flags |= XLH_SPLIT_META_UPDATE_MASKS;
            XLogRegisterBufData(2, (char *) &metap->hashm_lowmask, sizeof(uint32));
            XLogRegisterBufData(2, (char *) &metap->hashm_highmask, sizeof(uint32));
        }
 
        if (metap_update_splitpoint)
        {
            xlrec.flags |= XLH_SPLIT_META_UPDATE_SPLITPOINT;
            XLogRegisterBufData(2, (char *) &metap->hashm_ovflpoint,
                                sizeof(uint32));
            XLogRegisterBufData(2,
                                (char *) &metap->hashm_spares[metap->hashm_ovflpoint],
                                sizeof(uint32));
        }
 
        XLogRegisterData((char *) &xlrec, SizeOfHashSplitAllocPage);
 
        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SPLIT_ALLOCATE_PAGE);
 
        PageSetLSN(BufferGetPage(buf_oblkno), recptr);
        PageSetLSN(BufferGetPage(buf_nblkno), recptr);
        PageSetLSN(BufferGetPage(metabuf), recptr);
    }
 
    END_CRIT_SECTION();
 
    /* drop lock, but keep pin */
    LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
 
    /* Relocate records to the new bucket */
    _hash_splitbucket(rel, metabuf,
                      old_bucket, new_bucket,
                      buf_oblkno, buf_nblkno, NULL,
                      maxbucket, highmask, lowmask);
 
    /* all done, now release the pins on primary buckets. */
    _hash_dropbuf(rel, buf_oblkno);
    _hash_dropbuf(rel, buf_nblkno);
 
    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 */
    LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
}
 
 
/*
 * _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_addovflpage() 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;
    PGIOAlignedBlock zerobuf;
    Page        page;
    HashPageOpaque ovflopaque;
 
    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;
 
    page = (Page) zerobuf.data;
 
    /*
     * Initialize the page.  Just zeroing the page won't work; see
     * _hash_freeovflpage for similar usage.  We take care to make the special
     * space valid for the benefit of tools such as pageinspect.
     */
    _hash_pageinit(page, BLCKSZ);
 
    ovflopaque = HashPageGetOpaque(page);
 
    ovflopaque->hasho_prevblkno = InvalidBlockNumber;
    ovflopaque->hasho_nextblkno = InvalidBlockNumber;
    ovflopaque->hasho_bucket = InvalidBucket;
    ovflopaque->hasho_flag = LH_UNUSED_PAGE;
    ovflopaque->hasho_page_id = HASHO_PAGE_ID;
 
    if (RelationNeedsWAL(rel))
        log_newpage(&rel->rd_locator,
                    MAIN_FORKNUM,
                    lastblock,
                    zerobuf.data,
                    true);
 
    PageSetChecksumInplace(page, lastblock);
    smgrextend(RelationGetSmgr(rel), MAIN_FORKNUM, lastblock, zerobuf.data,
               false);
 
    return true;
}
 
 
/*
 * _hash_splitbucket -- split 'obucket' into 'obucket' and 'nbucket'
 *
 * This routine is used to partition the tuples between old and new bucket and
 * is used to finish the incomplete split operations.  To finish the previously
 * interrupted split operation, the caller needs to fill htab.  If htab is set,
 * then we skip the movement of tuples that exists in htab, otherwise NULL
 * value of htab indicates movement of all the tuples that belong to the new
 * bucket.
 *
 * 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.
 *
 * The caller must hold cleanup 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.)
 *
 * Split needs to retain pin on primary bucket pages of both old and new
 * buckets till end of operation.  This is to prevent vacuum from starting
 * while a split is in progress.
 *
 * In addition, the caller must have created the new bucket's base page,
 * which is passed in buffer nbuf, pinned and write-locked.  The lock will be
 * released here and pin must be released by the caller.  (The API is set up
 * this way because we must do _hash_getnewbuf() before releasing the metapage
 * write lock.  So instead of passing the new bucket's start block number, we
 * pass an actual buffer.)
 */
static void
_hash_splitbucket(Relation rel,
                  Buffer metabuf,
                  Bucket obucket,
                  Bucket nbucket,
                  Buffer obuf,
                  Buffer nbuf,
                  HTAB *htab,
                  uint32 maxbucket,
                  uint32 highmask,
                  uint32 lowmask)
{
    Buffer      bucket_obuf;
    Buffer      bucket_nbuf;
    Page        opage;
    Page        npage;
    HashPageOpaque oopaque;
    HashPageOpaque nopaque;
    OffsetNumber itup_offsets[MaxIndexTuplesPerPage];
    IndexTuple  itups[MaxIndexTuplesPerPage];
    Size        all_tups_size = 0;
    int         i;
    uint16      nitups = 0;
 
    bucket_obuf = obuf;
    opage = BufferGetPage(obuf);
    oopaque = HashPageGetOpaque(opage);
 
    bucket_nbuf = nbuf;
    npage = BufferGetPage(nbuf);
    nopaque = HashPageGetOpaque(npage);
 
    /* Copy the predicate locks from old bucket to new bucket. */
    PredicateLockPageSplit(rel,
                           BufferGetBlockNumber(bucket_obuf),
                           BufferGetBlockNumber(bucket_nbuf));
 
    /*
     * 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 (;;)
    {
        BlockNumber oblkno;
        OffsetNumber ooffnum;
        OffsetNumber omaxoffnum;
 
        /* Scan each tuple in old page */
        omaxoffnum = PageGetMaxOffsetNumber(opage);
        for (ooffnum = FirstOffsetNumber;
             ooffnum <= omaxoffnum;
             ooffnum = OffsetNumberNext(ooffnum))
        {
            IndexTuple  itup;
            Size        itemsz;
            Bucket      bucket;
            bool        found = false;
 
            /* skip dead tuples */
            if (ItemIdIsDead(PageGetItemId(opage, ooffnum)))
                continue;
 
            /*
             * Before inserting a tuple, probe the hash table containing TIDs
             * of tuples belonging to new bucket, if we find a match, then
             * skip that tuple, else 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));
 
            if (htab)
                (void) hash_search(htab, &itup->t_tid, HASH_FIND, &found);
 
            if (found)
                continue;
 
            bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
                                          maxbucket, highmask, lowmask);
 
            if (bucket == nbucket)
            {
                IndexTuple  new_itup;
 
                /*
                 * make a copy of index tuple as we have to scribble on it.
                 */
                new_itup = CopyIndexTuple(itup);
 
                /*
                 * mark the index tuple as moved by split, such tuples are
                 * skipped by scan if there is split in progress for a bucket.
                 */
                new_itup->t_info |= INDEX_MOVED_BY_SPLIT_MASK;
 
                /*
                 * 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 = IndexTupleSize(new_itup);
                itemsz = MAXALIGN(itemsz);
 
                if (PageGetFreeSpaceForMultipleTuples(npage, nitups + 1) < (all_tups_size + itemsz))
                {
                    /*
                     * Change the shared buffer state in critical section,
                     * otherwise any error could make it unrecoverable.
                     */
                    START_CRIT_SECTION();
 
                    _hash_pgaddmultitup(rel, nbuf, itups, itup_offsets, nitups);
                    MarkBufferDirty(nbuf);
                    /* log the split operation before releasing the lock */
                    log_split_page(rel, nbuf);
 
                    END_CRIT_SECTION();
 
                    /* drop lock, but keep pin */
                    LockBuffer(nbuf, BUFFER_LOCK_UNLOCK);
 
                    /* be tidy */
                    for (i = 0; i < nitups; i++)
                        pfree(itups[i]);
                    nitups = 0;
                    all_tups_size = 0;
 
                    /* chain to a new overflow page */
                    nbuf = _hash_addovflpage(rel, metabuf, nbuf, (nbuf == bucket_nbuf));
                    npage = BufferGetPage(nbuf);
                    nopaque = HashPageGetOpaque(npage);
                }
 
                itups[nitups++] = new_itup;
                all_tups_size += itemsz;
            }
            else
            {
                /*
                 * the tuple stays on this page, so nothing to do.
                 */
                Assert(bucket == obucket);
            }
        }
 
        oblkno = oopaque->hasho_nextblkno;
 
        /* retain the pin on the old primary bucket */
        if (obuf == bucket_obuf)
            LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
        else
            _hash_relbuf(rel, obuf);
 
        /* Exit loop if no more overflow pages in old bucket */
        if (!BlockNumberIsValid(oblkno))
        {
            /*
             * Change the shared buffer state in critical section, otherwise
             * any error could make it unrecoverable.
             */
            START_CRIT_SECTION();
 
            _hash_pgaddmultitup(rel, nbuf, itups, itup_offsets, nitups);
            MarkBufferDirty(nbuf);
            /* log the split operation before releasing the lock */
            log_split_page(rel, nbuf);
 
            END_CRIT_SECTION();
 
            if (nbuf == bucket_nbuf)
                LockBuffer(nbuf, BUFFER_LOCK_UNLOCK);
            else
                _hash_relbuf(rel, nbuf);
 
            /* be tidy */
            for (i = 0; i < nitups; i++)
                pfree(itups[i]);
            break;
        }
 
        /* Else, advance to next old page */
        obuf = _hash_getbuf(rel, oblkno, HASH_READ, LH_OVERFLOW_PAGE);
        opage = BufferGetPage(obuf);
        oopaque = HashPageGetOpaque(opage);
    }
 
    /*
     * We're at the end of the old bucket chain, so we're done partitioning
     * the tuples.  Mark the old and new buckets to indicate split is
     * finished.
     *
     * To avoid deadlocks due to locking order of buckets, first lock the old
     * bucket and then the new bucket.
     */
    LockBuffer(bucket_obuf, BUFFER_LOCK_EXCLUSIVE);
    opage = BufferGetPage(bucket_obuf);
    oopaque = HashPageGetOpaque(opage);
 
    LockBuffer(bucket_nbuf, BUFFER_LOCK_EXCLUSIVE);
    npage = BufferGetPage(bucket_nbuf);
    nopaque = HashPageGetOpaque(npage);
 
    START_CRIT_SECTION();
 
    oopaque->hasho_flag &= ~LH_BUCKET_BEING_SPLIT;
    nopaque->hasho_flag &= ~LH_BUCKET_BEING_POPULATED;
 
    /*
     * After the split is finished, mark the old bucket to indicate that it
     * contains deletable tuples.  We will clear split-cleanup flag after
     * deleting such tuples either at the end of split or at the next split
     * from old bucket or at the time of vacuum.
     */
    oopaque->hasho_flag |= LH_BUCKET_NEEDS_SPLIT_CLEANUP;
 
    /*
     * now write the buffers, here we don't release the locks as caller is
     * responsible to release locks.
     */
    MarkBufferDirty(bucket_obuf);
    MarkBufferDirty(bucket_nbuf);
 
    if (RelationNeedsWAL(rel))
    {
        XLogRecPtr  recptr;
        xl_hash_split_complete xlrec;
 
        xlrec.old_bucket_flag = oopaque->hasho_flag;
        xlrec.new_bucket_flag = nopaque->hasho_flag;
 
        XLogBeginInsert();
 
        XLogRegisterData((char *) &xlrec, SizeOfHashSplitComplete);
 
        XLogRegisterBuffer(0, bucket_obuf, REGBUF_STANDARD);
        XLogRegisterBuffer(1, bucket_nbuf, REGBUF_STANDARD);
 
        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SPLIT_COMPLETE);
 
        PageSetLSN(BufferGetPage(bucket_obuf), recptr);
        PageSetLSN(BufferGetPage(bucket_nbuf), recptr);
    }
 
    END_CRIT_SECTION();
 
    /*
     * If possible, clean up the old bucket.  We might not be able to do this
     * if someone else has a pin on it, but if not then we can go ahead.  This
     * isn't absolutely necessary, but it reduces bloat; if we don't do it
     * now, VACUUM will do it eventually, but maybe not until new overflow
     * pages have been allocated.  Note that there's no need to clean up the
     * new bucket.
     */
    if (IsBufferCleanupOK(bucket_obuf))
    {
        LockBuffer(bucket_nbuf, BUFFER_LOCK_UNLOCK);
        hashbucketcleanup(rel, obucket, bucket_obuf,
                          BufferGetBlockNumber(bucket_obuf), NULL,
                          maxbucket, highmask, lowmask, NULL, NULL, true,
                          NULL, NULL);
    }
    else
    {
        LockBuffer(bucket_nbuf, BUFFER_LOCK_UNLOCK);
        LockBuffer(bucket_obuf, BUFFER_LOCK_UNLOCK);
    }
}
 
/*
 *  _hash_finish_split() -- Finish the previously interrupted split operation
 *
 * To complete the split operation, we form the hash table of TIDs in new
 * bucket which is then used by split operation to skip tuples that are
 * already moved before the split operation was previously interrupted.
 *
 * The caller must hold a pin, but no lock, on the metapage and old bucket's
 * primary page buffer.  The buffers are returned in the same state.  (The
 * metapage is only touched if it becomes necessary to add or remove overflow
 * pages.)
 */
void
_hash_finish_split(Relation rel, Buffer metabuf, Buffer obuf, Bucket obucket,
                   uint32 maxbucket, uint32 highmask, uint32 lowmask)
{
    HASHCTL     hash_ctl;
    HTAB       *tidhtab;
    Buffer      bucket_nbuf = InvalidBuffer;
    Buffer      nbuf;
    Page        npage;
    BlockNumber nblkno;
    BlockNumber bucket_nblkno;
    HashPageOpaque npageopaque;
    Bucket      nbucket;
    bool        found;
 
    /* Initialize hash tables used to track TIDs */
    hash_ctl.keysize = sizeof(ItemPointerData);
    hash_ctl.entrysize = sizeof(ItemPointerData);
    hash_ctl.hcxt = CurrentMemoryContext;
 
    tidhtab =
        hash_create("bucket ctids",
                    256,        /* arbitrary initial size */
                    &hash_ctl,
                    HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
 
    bucket_nblkno = nblkno = _hash_get_newblock_from_oldbucket(rel, obucket);
 
    /*
     * Scan the new bucket and build hash table of TIDs
     */
    for (;;)
    {
        OffsetNumber noffnum;
        OffsetNumber nmaxoffnum;
 
        nbuf = _hash_getbuf(rel, nblkno, HASH_READ,
                            LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 
        /* remember the primary bucket buffer to acquire cleanup lock on it. */
        if (nblkno == bucket_nblkno)
            bucket_nbuf = nbuf;
 
        npage = BufferGetPage(nbuf);
        npageopaque = HashPageGetOpaque(npage);
 
        /* Scan each tuple in new page */
        nmaxoffnum = PageGetMaxOffsetNumber(npage);
        for (noffnum = FirstOffsetNumber;
             noffnum <= nmaxoffnum;
             noffnum = OffsetNumberNext(noffnum))
        {
            IndexTuple  itup;
 
            /* Fetch the item's TID and insert it in hash table. */
            itup = (IndexTuple) PageGetItem(npage,
                                            PageGetItemId(npage, noffnum));
 
            (void) hash_search(tidhtab, &itup->t_tid, HASH_ENTER, &found);
 
            Assert(!found);
        }
 
        nblkno = npageopaque->hasho_nextblkno;
 
        /*
         * release our write lock without modifying buffer and ensure to
         * retain the pin on primary bucket.
         */
        if (nbuf == bucket_nbuf)
            LockBuffer(nbuf, BUFFER_LOCK_UNLOCK);
        else
            _hash_relbuf(rel, nbuf);
 
        /* Exit loop if no more overflow pages in new bucket */
        if (!BlockNumberIsValid(nblkno))
            break;
    }
 
    /*
     * Conditionally get the cleanup lock on old and new buckets to perform
     * the split operation.  If we don't get the cleanup locks, silently give
     * up and next insertion on old bucket will try again to complete the
     * split.
     */
    if (!ConditionalLockBufferForCleanup(obuf))
    {
        hash_destroy(tidhtab);
        return;
    }
    if (!ConditionalLockBufferForCleanup(bucket_nbuf))
    {
        LockBuffer(obuf, BUFFER_LOCK_UNLOCK);
        hash_destroy(tidhtab);
        return;
    }
 
    npage = BufferGetPage(bucket_nbuf);
    npageopaque = HashPageGetOpaque(npage);
    nbucket = npageopaque->hasho_bucket;
 
    _hash_splitbucket(rel, metabuf, obucket,
                      nbucket, obuf, bucket_nbuf, tidhtab,
                      maxbucket, highmask, lowmask);
 
    _hash_dropbuf(rel, bucket_nbuf);
    hash_destroy(tidhtab);
}
 
/*
 *  log_split_page() -- Log the split operation
 *
 *  We log the split operation when the new page in new bucket gets full,
 *  so we log the entire page.
 *
 *  'buf' must be locked by the caller which is also responsible for unlocking
 *  it.
 */
static void
log_split_page(Relation rel, Buffer buf)
{
    if (RelationNeedsWAL(rel))
    {
        XLogRecPtr  recptr;
 
        XLogBeginInsert();
 
        XLogRegisterBuffer(0, buf, REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
 
        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SPLIT_PAGE);
 
        PageSetLSN(BufferGetPage(buf), recptr);
    }
}
 
/*
 *  _hash_getcachedmetap() -- Returns cached metapage data.
 *
 *  If metabuf is not InvalidBuffer, caller must hold a pin, but no lock, on
 *  the metapage.  If not set, we'll set it before returning if we have to
 *  refresh the cache, and return with a pin but no lock on it; caller is
 *  responsible for releasing the pin.
 *
 *  We refresh the cache if it's not initialized yet or force_refresh is true.
 */
HashMetaPage
_hash_getcachedmetap(Relation rel, Buffer *metabuf, bool force_refresh)
{
    Page        page;
 
    Assert(metabuf);
    if (force_refresh || rel->rd_amcache == NULL)
    {
        char       *cache = NULL;
 
        /*
         * It's important that we don't set rd_amcache to an invalid value.
         * Either MemoryContextAlloc or _hash_getbuf could fail, so don't
         * install a pointer to the newly-allocated storage in the actual
         * relcache entry until both have succeeded.
         */
        if (rel->rd_amcache == NULL)
            cache = MemoryContextAlloc(rel->rd_indexcxt,
                                       sizeof(HashMetaPageData));
 
        /* Read the metapage. */
        if (BufferIsValid(*metabuf))
            LockBuffer(*metabuf, BUFFER_LOCK_SHARE);
        else
            *metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ,
                                    LH_META_PAGE);
        page = BufferGetPage(*metabuf);
 
        /* Populate the cache. */
        if (rel->rd_amcache == NULL)
            rel->rd_amcache = cache;
        memcpy(rel->rd_amcache, HashPageGetMeta(page),
               sizeof(HashMetaPageData));
 
        /* Release metapage lock, but keep the pin. */
        LockBuffer(*metabuf, BUFFER_LOCK_UNLOCK);
    }
 
    return (HashMetaPage) rel->rd_amcache;
}
 
/*
 *  _hash_getbucketbuf_from_hashkey() -- Get the bucket's buffer for the given
 *                                       hashkey.
 *
 *  Bucket pages do not move or get removed once they are allocated. This give
 *  us an opportunity to use the previously saved metapage contents to reach
 *  the target bucket buffer, instead of reading from the metapage every time.
 *  This saves one buffer access every time we want to reach the target bucket
 *  buffer, which is very helpful savings in bufmgr traffic and contention.
 *
 *  The access type parameter (HASH_READ or HASH_WRITE) indicates whether the
 *  bucket buffer has to be locked for reading or writing.
 *
 *  The out parameter cachedmetap is set with metapage contents used for
 *  hashkey to bucket buffer mapping. Some callers need this info to reach the
 *  old bucket in case of bucket split, see _hash_doinsert().
 */
Buffer
_hash_getbucketbuf_from_hashkey(Relation rel, uint32 hashkey, int access,
                                HashMetaPage *cachedmetap)
{
    HashMetaPage metap;
    Buffer      buf;
    Buffer      metabuf = InvalidBuffer;
    Page        page;
    Bucket      bucket;
    BlockNumber blkno;
    HashPageOpaque opaque;
 
    /* We read from target bucket buffer, hence locking is must. */
    Assert(access == HASH_READ || access == HASH_WRITE);
 
    metap = _hash_getcachedmetap(rel, &metabuf, false);
    Assert(metap != NULL);
 
    /*
     * Loop until we get a lock on the correct target bucket.
     */
    for (;;)
    {
        /*
         * Compute the target bucket number, and convert to block number.
         */
        bucket = _hash_hashkey2bucket(hashkey,
                                      metap->hashm_maxbucket,
                                      metap->hashm_highmask,
                                      metap->hashm_lowmask);
 
        blkno = BUCKET_TO_BLKNO(metap, bucket);
 
        /* Fetch the primary bucket page for the bucket */
        buf = _hash_getbuf(rel, blkno, access, LH_BUCKET_PAGE);
        page = BufferGetPage(buf);
        opaque = HashPageGetOpaque(page);
        Assert(opaque->hasho_bucket == bucket);
        Assert(opaque->hasho_prevblkno != InvalidBlockNumber);
 
        /*
         * If this bucket hasn't been split, we're done.
         */
        if (opaque->hasho_prevblkno <= metap->hashm_maxbucket)
            break;
 
        /* Drop lock on this buffer, update cached metapage, and retry. */
        _hash_relbuf(rel, buf);
        metap = _hash_getcachedmetap(rel, &metabuf, true);
        Assert(metap != NULL);
    }
 
    if (BufferIsValid(metabuf))
        _hash_dropbuf(rel, metabuf);
 
    if (cachedmetap)
        *cachedmetap = metap;
 
    return buf;
}