/src/postgres/src/backend/access/hash/hashpage.c

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/*-------------------------------------------------------------------------
 *
 * hashpage.c
 *    Hash table page management code for the Postgres hash access method
 *
 * Portions Copyright (c) 1996-2025, 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(&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(&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, &metap->hashm_lowmask, sizeof(uint32));
      XLogRegisterBufData(2, &metap->hashm_highmask, sizeof(uint32));
    }

    if (metap_update_splitpoint)
    {
      xlrec.flags |= XLH_SPLIT_META_UPDATE_SPLITPOINT;
      XLogRegisterBufData(2, &metap->hashm_ovflpoint,
                sizeof(uint32));
      XLogRegisterBufData(2,
                &metap->hashm_spares[metap->hashm_ovflpoint],
                sizeof(uint32));
    }

    XLogRegisterData(&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(&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;
}

Coverage Report

Created: 2025-06-15 06:31