Improve handling of dead tuples in hash indexes.

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

/*-------------------------------------------------------------------------
 *
 * hashpage.c
 *        Hash table page management code for the Postgres hash access method
 *
 * Portions Copyright (c) 1996-2016, 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 "miscadmin.h"
#include "storage/lmgr.h"
#include "storage/smgr.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,
                                  Buffer nbuf,
                                  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 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 = 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_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 use P_NEW to extend the relation */
        if (blkno == nblocks)
        {
                buf = ReadBufferExtended(rel, forkNum, P_NEW, RBM_NORMAL, NULL);
                if (BufferGetBlockNumber(buf) != blkno)
                        elog(ERROR, "unexpected hash relation size: %u, should be %u",
                                 BufferGetBlockNumber(buf), blkno);
                LockBuffer(buf, HASH_WRITE);
        }
        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_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 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_metapinit(Relation rel, double num_tuples, ForkNumber forkNum)
{
        HashMetaPage metap;
        HashPageOpaque pageopaque;
        Buffer          metabuf;
        Buffer          buf;
        Page            pg;
        int32           data_width;
        int32           item_width;
        int32           ffactor;
        double          dnumbuckets;
        uint32          num_buckets;
        uint32          log2_num_buckets;
        uint32          i;

        /* safety check */
        if (RelationGetNumberOfBlocksInFork(rel, forkNum) != 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 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 = RelationGetTargetPageUsage(rel, HASH_DEFAULT_FILLFACTOR) / item_width;
        /* keep to a sane range */
        if (ffactor < 10)
                ffactor = 10;

        /*
         * 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
         * next power of 2, however, and 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 = ((uint32) 1) << _hash_log2((uint32) dnumbuckets);

        log2_num_buckets = _hash_log2(num_buckets);
        Assert(num_buckets == (((uint32) 1) << log2_num_buckets));
        Assert(log2_num_buckets < HASH_MAX_SPLITPOINTS);

        /*
         * 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.
         */
        metabuf = _hash_getnewbuf(rel, HASH_METAPAGE, forkNum);
        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 = HashPageGetMeta(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 = HashGetMaxBitmapSize(pg);
        /* 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)
                        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 N buckets, 0 .. N-1, occupying physical
         * blocks 1 to N.  The first freespace bitmap page is in block N+1. Since
         * N is a power of 2, we can set the masks this way:
         */
        metap->hashm_maxbucket = metap->hashm_lowmask = num_buckets - 1;
        metap->hashm_highmask = (num_buckets << 1) - 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[log2_num_buckets] = 1;
        metap->hashm_ovflpoint = log2_num_buckets;
        metap->hashm_firstfree = 0;

        /*
         * 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.
         */
        _hash_chgbufaccess(rel, metabuf, HASH_WRITE, HASH_NOLOCK);

        /*
         * Initialize the first N buckets
         */
        for (i = 0; i < num_buckets; i++)
        {
                /* Allow interrupts, in case N is huge */
                CHECK_FOR_INTERRUPTS();

                buf = _hash_getnewbuf(rel, BUCKET_TO_BLKNO(metap, i), forkNum);
                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);
        }

        /* Now reacquire buffer lock on metapage */
        _hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_WRITE);

        /*
         * Initialize first bitmap page
         */
        _hash_initbitmap(rel, metap, num_buckets + 1, forkNum);

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

        return num_buckets;
}

/*
 *      _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;
        Buffer          buf_nblkno;
        uint32          maxbucket;
        uint32          highmask;
        uint32          lowmask;

        /*
         * 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.
         */
        _hash_chgbufaccess(rel, metabuf, HASH_NOLOCK, HASH_WRITE);

        _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 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 you change this, see also the maximum initial number of buckets in
         * _hash_metapinit().
         */
        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;
                }
        }

        /*
         * 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.
         */
        buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);

        /*
         * 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);

        /* Relocate records to the new bucket */
        _hash_splitbucket(rel, metabuf,
                                          old_bucket, new_bucket,
                                          start_oblkno, buf_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);
}


/*
 * _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, MAIN_FORKNUM, lastblock, zerobuf, false);

        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.)
 *
 * In addition, the caller must have created the new bucket's base page,
 * which is passed in buffer nbuf, pinned and write-locked.  That lock and
 * pin are released here.  (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,
                                  BlockNumber start_oblkno,
                                  Buffer nbuf,
                                  uint32 maxbucket,
                                  uint32 highmask,
                                  uint32 lowmask)
{
        Buffer          obuf;
        Page            opage;
        Page            npage;
        HashPageOpaque oopaque;
        HashPageOpaque nopaque;

        /*
         * 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.
         */
        obuf = _hash_getbuf(rel, start_oblkno, HASH_WRITE, LH_BUCKET_PAGE);
        opage = BufferGetPage(obuf);
        oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);

        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.  Outer loop iterates
         * once per page in old bucket.
         */
        for (;;)
        {
                BlockNumber oblkno;
                OffsetNumber ooffnum;
                OffsetNumber omaxoffnum;
                OffsetNumber deletable[MaxOffsetNumber];
                int                     ndeletable = 0;

                /* Scan each tuple in old page */
                omaxoffnum = PageGetMaxOffsetNumber(opage);
                for (ooffnum = FirstOffsetNumber;
                         ooffnum <= omaxoffnum;
                         ooffnum = OffsetNumberNext(ooffnum))
                {
                        IndexTuple      itup;
                        Size            itemsz;
                        Bucket          bucket;

                        /* skip dead tuples */
                        if (ItemIdIsDead(PageGetItemId(opage, ooffnum)))
                                continue;

                        /*
                         * 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));
                        bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
                                                                                  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.
                                 *
                                 * XXX we have a problem here if we fail to get space for a
                                 * new overflow page: we'll error out leaving the bucket split
                                 * only partially complete, meaning the index is corrupt,
                                 * since searches may fail to find entries they should find.
                                 */
                                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 */
                                }

                                /*
                                 * Insert tuple on new page, using _hash_pgaddtup to ensure
                                 * correct ordering by hashkey.  This is a tad inefficient
                                 * since we may have to shuffle itempointers repeatedly.
                                 * Possible future improvement: accumulate all the items for
                                 * the new page and qsort them before insertion.
                                 */
                                (void) _hash_pgaddtup(rel, nbuf, itemsz, itup);

                                /*
                                 * Mark tuple for deletion from old page.
                                 */
                                deletable[ndeletable++] = ooffnum;
                        }
                        else
                        {
                                /*
                                 * the tuple stays on this page, so nothing to do.
                                 */
                                Assert(bucket == obucket);
                        }
                }

                oblkno = oopaque->hasho_nextblkno;

                /*
                 * Done scanning this old page.  If we moved any tuples, delete them
                 * from the old page.
                 */
                if (ndeletable > 0)
                {
                        PageIndexMultiDelete(opage, deletable, ndeletable);
                        _hash_wrtbuf(rel, obuf);
                }
                else
                        _hash_relbuf(rel, obuf);

                /* Exit loop if no more overflow pages in old bucket */
                if (!BlockNumberIsValid(oblkno))
                        break;

                /* Else, advance to next old page */
                obuf = _hash_getbuf(rel, oblkno, HASH_WRITE, LH_OVERFLOW_PAGE);
                opage = BufferGetPage(obuf);
                oopaque = (HashPageOpaque) PageGetSpecialPointer(opage);
        }

        /*
         * 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, nbuf);

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