[postgresql] / src / backend / access / hash / hashpage.c

/*-------------------------------------------------------------------------
 *
 * hashpage.c
 *        Hash table page management code for the Postgres hash access method
 *
 * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/access/hash/hashpage.c,v 1.68 2007/05/30 20:11:51 tgl Exp $
 *
 * 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/genam.h"
#include "access/hash.h"
#include "miscadmin.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"
#include "utils/lsyscache.h"


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


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


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

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

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

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

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

        buf = ReadBuffer(rel, blkno);

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

        /* ref count and lock type are correct */

        _hash_checkpage(rel, buf, flags);

        return buf;
}

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

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

        buf = ReadOrZeroBuffer(rel, blkno);

        LockBuffer(buf, HASH_WRITE);

        /* ref count and lock type are correct */

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

        return buf;
}

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

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

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

        LockBuffer(buf, HASH_WRITE);

        /* ref count and lock type are correct */

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

        return buf;
}

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

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

        buf = ReadBufferWithStrategy(rel, blkno, bstrategy);

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

        /* ref count and lock type are correct */

        _hash_checkpage(rel, buf, flags);

        return buf;
}

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

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

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

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


/*
 *      _hash_metapinit() -- Initialize the metadata page of a hash index,
 *                              the two buckets that we begin with and the initial
 *                              bitmap page.
 *
 * 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.
 */
void
_hash_metapinit(Relation rel)
{
        HashMetaPage metap;
        HashPageOpaque pageopaque;
        Buffer          metabuf;
        Buffer          buf;
        Page            pg;
        int32           data_width;
        int32           item_width;
        int32           ffactor;
        uint16          i;

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

        /*
         * Determine the target fill factor (in tuples per bucket) for this index.
         * The idea is to make the fill factor correspond to pages about as full
         * as the user-settable fillfactor parameter says.      We can compute it
         * exactly if the index datatype is fixed-width, but for var-width there's
         * some guessing involved.
         */
        data_width = get_typavgwidth(RelationGetDescr(rel)->attrs[0]->atttypid,
                                                                 RelationGetDescr(rel)->attrs[0]->atttypmod);
        item_width = MAXALIGN(sizeof(IndexTupleData)) + MAXALIGN(data_width) +
                sizeof(ItemIdData);             /* include the line pointer */
        ffactor = RelationGetTargetPageUsage(rel, HASH_DEFAULT_FILLFACTOR) / item_width;
        /* keep to a sane range */
        if (ffactor < 10)
                ffactor = 10;

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

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

        metap = (HashMetaPage) pg;

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

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

        /*
         * We initialize the index with two buckets, 0 and 1, occupying physical
         * blocks 1 and 2.      The first freespace bitmap page is in block 3.
         */
        metap->hashm_maxbucket = metap->hashm_lowmask = 1;      /* nbuckets - 1 */
        metap->hashm_highmask = 3;      /* (nbuckets << 1) - 1 */

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

        metap->hashm_spares[1] = 1; /* the first bitmap page is only spare */
        metap->hashm_ovflpoint = 1;
        metap->hashm_firstfree = 0;

        /*
         * Initialize the first two buckets
         */
        for (i = 0; i <= 1; i++)
        {
                buf = _hash_getnewbuf(rel, BUCKET_TO_BLKNO(metap, i));
                pg = BufferGetPage(buf);
                pageopaque = (HashPageOpaque) PageGetSpecialPointer(pg);
                pageopaque->hasho_prevblkno = InvalidBlockNumber;
                pageopaque->hasho_nextblkno = InvalidBlockNumber;
                pageopaque->hasho_bucket = i;
                pageopaque->hasho_flag = LH_BUCKET_PAGE;
                pageopaque->hasho_page_id = HASHO_PAGE_ID;
                _hash_wrtbuf(rel, buf);
        }

        /*
         * Initialize first bitmap page
         */
        _hash_initbitmap(rel, metap, 3);

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

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

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

        /*
         * Obtain the page-zero lock to assert the right to begin a split (see
         * README).
         *
         * Note: deadlock should be impossible here. Our own backend could only be
         * holding bucket sharelocks due to stopped indexscans; those will not
         * block other holders of the page-zero lock, who are only interested in
         * acquiring bucket sharelocks themselves.      Exclusive bucket locks are
         * only taken here and in hashbulkdelete, and neither of these operations
         * needs any additional locks to complete.      (If, due to some flaw in this
         * reasoning, we manage to deadlock anyway, it's okay to error out; the
         * index will be left in a consistent state.)
         */
        _hash_getlock(rel, 0, HASH_EXCLUSIVE);

        /* Write-lock the meta page */
        _hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_WRITE);

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

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

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

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

        old_bucket = (new_bucket & metap->hashm_lowmask);

        start_oblkno = BUCKET_TO_BLKNO(metap, old_bucket);

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

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

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

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

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

        /*
         * If the split point is increasing (hashm_maxbucket's log base 2
         * increases), we need to allocate a new batch of bucket pages.
         */
        spare_ndx = _hash_log2(new_bucket + 1);
        if (spare_ndx > metap->hashm_ovflpoint)
        {
                Assert(spare_ndx == metap->hashm_ovflpoint + 1);
                /*
                 * The number of buckets in the new splitpoint is equal to the
                 * total number already in existence, i.e. new_bucket.  Currently
                 * this maps one-to-one to blocks required, but someday we may need
                 * a more complicated calculation here.
                 */
                if (!_hash_alloc_buckets(rel, start_nblkno, new_bucket))
                {
                        /* can't split due to BlockNumber overflow */
                        _hash_droplock(rel, start_oblkno, HASH_EXCLUSIVE);
                        _hash_droplock(rel, start_nblkno, HASH_EXCLUSIVE);
                        goto fail;
                }
        }

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

        metap->hashm_maxbucket = new_bucket;

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

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

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

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

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

        /* Release split lock; okay for other splits to occur now */
        _hash_droplock(rel, 0, HASH_EXCLUSIVE);

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

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

        return;

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

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

        /* Release split lock */
        _hash_droplock(rel, 0, HASH_EXCLUSIVE);
}


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

        lastblock = firstblock + nblocks - 1;

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

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

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

        return true;
}


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

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

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

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

        /*
         * Partition the tuples in the old bucket between the old bucket and the
         * new bucket, advancing along the old bucket's overflow bucket chain and
         * adding overflow pages to the new bucket as needed.
         */
        ooffnum = FirstOffsetNumber;
        omaxoffnum = PageGetMaxOffsetNumber(opage);
        for (;;)
        {
                /*
                 * at each iteration through this loop, each of these variables should
                 * be up-to-date: obuf opage oopaque ooffnum omaxoffnum
                 */

                /* check if we're at the end of the page */
                if (ooffnum > omaxoffnum)
                {
                        /* at end of page, but check for an(other) overflow page */
                        oblkno = oopaque->hasho_nextblkno;
                        if (!BlockNumberIsValid(oblkno))
                                break;

                        /*
                         * we ran out of tuples on this particular page, but we have more
                         * overflow pages; advance to next page.
                         */
                        _hash_wrtbuf(rel, obuf);

                        obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
                        opage = BufferGetPage(obuf);
                        oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
                        ooffnum = FirstOffsetNumber;
                        omaxoffnum = PageGetMaxOffsetNumber(opage);
                        continue;
                }

                /*
                 * Re-hash the tuple to determine which bucket it now belongs in.
                 *
                 * It is annoying to call the hash function while holding locks, but
                 * releasing and relocking the page for each tuple is unappealing too.
                 */
                itup = (IndexTuple) PageGetItem(opage, PageGetItemId(opage, ooffnum));
                datum = index_getattr(itup, 1, itupdesc, &null);
                Assert(!null);

                bucket = _hash_hashkey2bucket(_hash_datum2hashkey(rel, datum),
                                                                          maxbucket, highmask, lowmask);

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

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

                        noffnum = OffsetNumberNext(PageGetMaxOffsetNumber(npage));
                        if (PageAddItem(npage, (Item) itup, itemsz, noffnum, LP_USED)
                                == InvalidOffsetNumber)
                                elog(ERROR, "failed to add index item to \"%s\"",
                                         RelationGetRelationName(rel));

                        /*
                         * now delete the tuple from the old bucket.  after this section
                         * of code, 'ooffnum' will actually point to the ItemId to which
                         * we would point if we had advanced it before the deletion
                         * (PageIndexTupleDelete repacks the ItemId array).  this also
                         * means that 'omaxoffnum' is exactly one less than it used to be,
                         * so we really can just decrement it instead of calling
                         * PageGetMaxOffsetNumber.
                         */
                        PageIndexTupleDelete(opage, ooffnum);
                        omaxoffnum = OffsetNumberPrev(omaxoffnum);
                }
                else
                {
                        /*
                         * the tuple stays on this page.  we didn't move anything, so we
                         * didn't delete anything and therefore we don't have to change
                         * 'omaxoffnum'.
                         */
                        Assert(bucket == obucket);
                        ooffnum = OffsetNumberNext(ooffnum);
                }
        }

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

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