Re-run pgindent, fixing a problem where comment lines after a blank

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

/*-------------------------------------------------------------------------
 *
 * hashpage.c
 *        Hash table page management code for the Postgres hash access method
 *
 * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        $PostgreSQL: pgsql/src/backend/access/hash/hashpage.c,v 1.54 2005/11/22 18:17:05 momjian 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 "utils/lsyscache.h"


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.
 *
 *              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").
 *
 *              XXX P_NEW is not used because, unlike the tree structures, we
 *              need the bucket blocks to be at certain block numbers.
 *
 *              All call sites should call either _hash_pageinit or _hash_checkpage
 *              on the returned page, depending on whether the block is expected
 *              to be new or not.
 */
Buffer
_hash_getbuf(Relation rel, BlockNumber blkno, int access)
{
        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 */
        return buf;
}

/*
 *      _hash_relbuf() -- release a locked buffer.
 *
 * Lock and pin (refcount) are both dropped.  Note that either read or
 * write lock can be dropped this way, but if we modified the buffer,
 * this is NOT the right way to release a write lock.
 */
void
_hash_relbuf(Relation rel, Buffer buf)
{
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        ReleaseBuffer(buf);
}

/*
 *      _hash_dropbuf() -- release an unlocked buffer.
 *
 * This is used to unpin a buffer on which we hold no lock.  It is assumed
 * that the buffer is not dirty.
 */
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: actually, the buffer manager just marks the shared buffer page
 * dirty here; the real I/O happens later.      This is okay since we are not
 * relying on write ordering anyway.  The WAL mechanism is responsible for
 * guaranteeing correctness after a crash.
 */
void
_hash_wrtbuf(Relation rel, Buffer buf)
{
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        WriteBuffer(buf);
}

/*
 *      _hash_wrtnorelbuf() -- write a hash page to disk, but do not release
 *                                               our reference or lock.
 *
 *              It is an error to call _hash_wrtnorelbuf() without a write lock
 *              and a pin on the buffer.
 *
 * See above NOTE.
 */
void
_hash_wrtnorelbuf(Relation rel, Buffer buf)
{
        WriteNoReleaseBuffer(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_NOLOCK)
                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        if (from_access == HASH_WRITE)
                WriteNoReleaseBuffer(buf);

        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 (tuples per bucket) for this index.
         * The idea is to make the fill factor correspond to pages about 3/4ths
         * full.  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(HashItemData)) + MAXALIGN(data_width) +
                sizeof(ItemIdData);             /* include the line pointer */
        ffactor = (BLCKSZ * 3 / 4) / item_width;
        /* keep to a sane range */
        if (ffactor < 10)
                ffactor = 10;

        metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_WRITE);
        pg = BufferGetPage(metabuf);
        _hash_pageinit(pg, BufferGetPageSize(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_filler = HASHO_FILL;

        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));

        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_getbuf(rel, BUCKET_TO_BLKNO(metap, i), HASH_WRITE);
                pg = BufferGetPage(buf);
                _hash_pageinit(pg, BufferGetPageSize(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_filler = HASHO_FILL;
                _hash_wrtbuf(rel, buf);
        }

        /*
         * Initialize first bitmap page.  Can't do this until we create the first
         * two buckets, else smgr will complain.
         */
        _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;

        /*
         * 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.
         *
         * Ideally we would lock the new bucket too before proceeding, but if we
         * are about to cross a splitpoint then the BUCKET_TO_BLKNO mapping isn't
         * correct yet.  For simplicity we update the metapage first and then
         * lock.  This should be okay because no one else should be trying to lock
         * the new bucket yet...
         */
        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;

        /*
         * 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.
         *
         * XXX should initialize new bucket pages to prevent out-of-order page
         * creation?  Don't wanna do it right here though.
         */
        spare_ndx = _hash_log2(metap->hashm_maxbucket + 1);
        if (spare_ndx > metap->hashm_ovflpoint)
        {
                Assert(spare_ndx == metap->hashm_ovflpoint + 1);
                metap->hashm_spares[spare_ndx] = metap->hashm_spares[metap->hashm_ovflpoint];
                metap->hashm_ovflpoint = spare_ndx;
        }

        /* now we can compute the new bucket's primary block number */
        start_nblkno = BUCKET_TO_BLKNO(metap, new_bucket);

        Assert(!_hash_has_active_scan(rel, new_bucket));

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

        /* 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_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;
        HashItem        hitem;
        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);
        _hash_checkpage(rel, obuf, LH_BUCKET_PAGE);
        opage = BufferGetPage(obuf);
        oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);

        nblkno = start_nblkno;
        nbuf = _hash_getbuf(rel, nblkno, HASH_WRITE);
        npage = BufferGetPage(nbuf);

        /* initialize the new bucket's primary page */
        _hash_pageinit(npage, BufferGetPageSize(nbuf));
        nopaque = (HashPageOpaque) PageGetSpecialPointer(npage);
        nopaque->hasho_prevblkno = InvalidBlockNumber;
        nopaque->hasho_nextblkno = InvalidBlockNumber;
        nopaque->hasho_bucket = nbucket;
        nopaque->hasho_flag = LH_BUCKET_PAGE;
        nopaque->hasho_filler = HASHO_FILL;

        /*
         * 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);
                        _hash_checkpage(rel, obuf, 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.
                 */
                hitem = (HashItem) PageGetItem(opage, PageGetItemId(opage, ooffnum));
                itup = &(hitem->hash_itup);
                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(hitem->hash_itup)
                                + (sizeof(HashItemData) - sizeof(IndexTupleData));

                        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);
                                _hash_checkpage(rel, nbuf, LH_OVERFLOW_PAGE);
                                npage = BufferGetPage(nbuf);
                                /* we don't need nopaque within the loop */
                        }

                        noffnum = OffsetNumberNext(PageGetMaxOffsetNumber(npage));
                        if (PageAddItem(npage, (Item) hitem, 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);
}