hashsearch.c

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/*-------------------------------------------------------------------------
 *
 * hashsearch.c
 *    search code for postgres hash tables
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/hash/hashsearch.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/hash.h"
#include "access/relscan.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/predicate.h"
#include "utils/rel.h"
 
static bool _hash_readpage(IndexScanDesc scan, Buffer *bufP,
                           ScanDirection dir);
static int  _hash_load_qualified_items(IndexScanDesc scan, Page page,
                                       OffsetNumber offnum, ScanDirection dir);
static inline void _hash_saveitem(HashScanOpaque so, int itemIndex,
                                  OffsetNumber offnum, IndexTuple itup);
static void _hash_readnext(IndexScanDesc scan, Buffer *bufp,
                           Page *pagep, HashPageOpaque *opaquep);
 
/*
 *  _hash_next() -- Get the next item in a scan.
 *
 *      On entry, so->currPos describes the current page, which may
 *      be pinned but not locked, and so->currPos.itemIndex identifies
 *      which item was previously returned.
 *
 *      On successful exit, scan->xs_ctup.t_self is set to the TID
 *      of the next heap tuple. so->currPos is updated as needed.
 *
 *      On failure exit (no more tuples), we return false with pin
 *      held on bucket page but no pins or locks held on overflow
 *      page.
 */
bool
_hash_next(IndexScanDesc scan, ScanDirection dir)
{
    Relation    rel = scan->indexRelation;
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    HashScanPosItem *currItem;
    BlockNumber blkno;
    Buffer      buf;
    bool        end_of_scan = false;
 
    /*
     * Advance to the next tuple on the current page; or if done, try to read
     * data from the next or previous page based on the scan direction. Before
     * moving to the next or previous page make sure that we deal with all the
     * killed items.
     */
    if (ScanDirectionIsForward(dir))
    {
        if (++so->currPos.itemIndex > so->currPos.lastItem)
        {
            if (so->numKilled > 0)
                _hash_kill_items(scan);
 
            blkno = so->currPos.nextPage;
            if (BlockNumberIsValid(blkno))
            {
                buf = _hash_getbuf(rel, blkno, HASH_READ, LH_OVERFLOW_PAGE);
                if (!_hash_readpage(scan, &buf, dir))
                    end_of_scan = true;
            }
            else
                end_of_scan = true;
        }
    }
    else
    {
        if (--so->currPos.itemIndex < so->currPos.firstItem)
        {
            if (so->numKilled > 0)
                _hash_kill_items(scan);
 
            blkno = so->currPos.prevPage;
            if (BlockNumberIsValid(blkno))
            {
                buf = _hash_getbuf(rel, blkno, HASH_READ,
                                   LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 
                /*
                 * We always maintain the pin on bucket page for whole scan
                 * operation, so releasing the additional pin we have acquired
                 * here.
                 */
                if (buf == so->hashso_bucket_buf ||
                    buf == so->hashso_split_bucket_buf)
                    _hash_dropbuf(rel, buf);
 
                if (!_hash_readpage(scan, &buf, dir))
                    end_of_scan = true;
            }
            else
                end_of_scan = true;
        }
    }
 
    if (end_of_scan)
    {
        _hash_dropscanbuf(rel, so);
        HashScanPosInvalidate(so->currPos);
        return false;
    }
 
    /* OK, itemIndex says what to return */
    currItem = &so->currPos.items[so->currPos.itemIndex];
    scan->xs_heaptid = currItem->heapTid;
 
    return true;
}
 
/*
 * Advance to next page in a bucket, if any.  If we are scanning the bucket
 * being populated during split operation then this function advances to the
 * bucket being split after the last bucket page of bucket being populated.
 */
static void
_hash_readnext(IndexScanDesc scan,
               Buffer *bufp, Page *pagep, HashPageOpaque *opaquep)
{
    BlockNumber blkno;
    Relation    rel = scan->indexRelation;
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    bool        block_found = false;
 
    blkno = (*opaquep)->hasho_nextblkno;
 
    /*
     * Retain the pin on primary bucket page till the end of scan.  Refer the
     * comments in _hash_first to know the reason of retaining pin.
     */
    if (*bufp == so->hashso_bucket_buf || *bufp == so->hashso_split_bucket_buf)
        LockBuffer(*bufp, BUFFER_LOCK_UNLOCK);
    else
        _hash_relbuf(rel, *bufp);
 
    *bufp = InvalidBuffer;
    /* check for interrupts while we're not holding any buffer lock */
    CHECK_FOR_INTERRUPTS();
    if (BlockNumberIsValid(blkno))
    {
        *bufp = _hash_getbuf(rel, blkno, HASH_READ, LH_OVERFLOW_PAGE);
        block_found = true;
    }
    else if (so->hashso_buc_populated && !so->hashso_buc_split)
    {
        /*
         * end of bucket, scan bucket being split if there was a split in
         * progress at the start of scan.
         */
        *bufp = so->hashso_split_bucket_buf;
 
        /*
         * buffer for bucket being split must be valid as we acquire the pin
         * on it before the start of scan and retain it till end of scan.
         */
        Assert(BufferIsValid(*bufp));
 
        LockBuffer(*bufp, BUFFER_LOCK_SHARE);
        PredicateLockPage(rel, BufferGetBlockNumber(*bufp), scan->xs_snapshot);
 
        /*
         * setting hashso_buc_split to true indicates that we are scanning
         * bucket being split.
         */
        so->hashso_buc_split = true;
 
        block_found = true;
    }
 
    if (block_found)
    {
        *pagep = BufferGetPage(*bufp);
        *opaquep = HashPageGetOpaque(*pagep);
    }
}
 
/*
 * Advance to previous page in a bucket, if any.  If the current scan has
 * started during split operation then this function advances to bucket
 * being populated after the first bucket page of bucket being split.
 */
static void
_hash_readprev(IndexScanDesc scan,
               Buffer *bufp, Page *pagep, HashPageOpaque *opaquep)
{
    BlockNumber blkno;
    Relation    rel = scan->indexRelation;
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    bool        haveprevblk;
 
    blkno = (*opaquep)->hasho_prevblkno;
 
    /*
     * Retain the pin on primary bucket page till the end of scan.  Refer the
     * comments in _hash_first to know the reason of retaining pin.
     */
    if (*bufp == so->hashso_bucket_buf || *bufp == so->hashso_split_bucket_buf)
    {
        LockBuffer(*bufp, BUFFER_LOCK_UNLOCK);
        haveprevblk = false;
    }
    else
    {
        _hash_relbuf(rel, *bufp);
        haveprevblk = true;
    }
 
    *bufp = InvalidBuffer;
    /* check for interrupts while we're not holding any buffer lock */
    CHECK_FOR_INTERRUPTS();
 
    if (haveprevblk)
    {
        Assert(BlockNumberIsValid(blkno));
        *bufp = _hash_getbuf(rel, blkno, HASH_READ,
                             LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
        *pagep = BufferGetPage(*bufp);
        *opaquep = HashPageGetOpaque(*pagep);
 
        /*
         * We always maintain the pin on bucket page for whole scan operation,
         * so releasing the additional pin we have acquired here.
         */
        if (*bufp == so->hashso_bucket_buf || *bufp == so->hashso_split_bucket_buf)
            _hash_dropbuf(rel, *bufp);
    }
    else if (so->hashso_buc_populated && so->hashso_buc_split)
    {
        /*
         * end of bucket, scan bucket being populated if there was a split in
         * progress at the start of scan.
         */
        *bufp = so->hashso_bucket_buf;
 
        /*
         * buffer for bucket being populated must be valid as we acquire the
         * pin on it before the start of scan and retain it till end of scan.
         */
        Assert(BufferIsValid(*bufp));
 
        LockBuffer(*bufp, BUFFER_LOCK_SHARE);
        *pagep = BufferGetPage(*bufp);
        *opaquep = HashPageGetOpaque(*pagep);
 
        /* move to the end of bucket chain */
        while (BlockNumberIsValid((*opaquep)->hasho_nextblkno))
            _hash_readnext(scan, bufp, pagep, opaquep);
 
        /*
         * setting hashso_buc_split to false indicates that we are scanning
         * bucket being populated.
         */
        so->hashso_buc_split = false;
    }
}
 
/*
 *  _hash_first() -- Find the first item in a scan.
 *
 *      We find the first item (or, if backward scan, the last item) in the
 *      index that satisfies the qualification associated with the scan
 *      descriptor.
 *
 *      On successful exit, if the page containing current index tuple is an
 *      overflow page, both pin and lock are released whereas if it is a bucket
 *      page then it is pinned but not locked and data about the matching
 *      tuple(s) on the page has been loaded into so->currPos,
 *      scan->xs_ctup.t_self is set to the heap TID of the current tuple.
 *
 *      On failure exit (no more tuples), we return false, with pin held on
 *      bucket page but no pins or locks held on overflow page.
 */
bool
_hash_first(IndexScanDesc scan, ScanDirection dir)
{
    Relation    rel = scan->indexRelation;
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    ScanKey     cur;
    uint32      hashkey;
    Bucket      bucket;
    Buffer      buf;
    Page        page;
    HashPageOpaque opaque;
    HashScanPosItem *currItem;
 
    pgstat_count_index_scan(rel);
 
    /*
     * We do not support hash scans with no index qualification, because we
     * would have to read the whole index rather than just one bucket. That
     * creates a whole raft of problems, since we haven't got a practical way
     * to lock all the buckets against splits or compactions.
     */
    if (scan->numberOfKeys < 1)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("hash indexes do not support whole-index scans")));
 
    /* There may be more than one index qual, but we hash only the first */
    cur = &scan->keyData[0];
 
    /* We support only single-column hash indexes */
    Assert(cur->sk_attno == 1);
    /* And there's only one operator strategy, too */
    Assert(cur->sk_strategy == HTEqualStrategyNumber);
 
    /*
     * If the constant in the index qual is NULL, assume it cannot match any
     * items in the index.
     */
    if (cur->sk_flags & SK_ISNULL)
        return false;
 
    /*
     * Okay to compute the hash key.  We want to do this before acquiring any
     * locks, in case a user-defined hash function happens to be slow.
     *
     * If scankey operator is not a cross-type comparison, we can use the
     * cached hash function; otherwise gotta look it up in the catalogs.
     *
     * We support the convention that sk_subtype == InvalidOid means the
     * opclass input type; this is a hack to simplify life for ScanKeyInit().
     */
    if (cur->sk_subtype == rel->rd_opcintype[0] ||
        cur->sk_subtype == InvalidOid)
        hashkey = _hash_datum2hashkey(rel, cur->sk_argument);
    else
        hashkey = _hash_datum2hashkey_type(rel, cur->sk_argument,
                                           cur->sk_subtype);
 
    so->hashso_sk_hash = hashkey;
 
    buf = _hash_getbucketbuf_from_hashkey(rel, hashkey, HASH_READ, NULL);
    PredicateLockPage(rel, BufferGetBlockNumber(buf), scan->xs_snapshot);
    page = BufferGetPage(buf);
    opaque = HashPageGetOpaque(page);
    bucket = opaque->hasho_bucket;
 
    so->hashso_bucket_buf = buf;
 
    /*
     * If a bucket split is in progress, then while scanning the bucket being
     * populated, we need to skip tuples that were copied from bucket being
     * split.  We also need to maintain a pin on the bucket being split to
     * ensure that split-cleanup work done by vacuum doesn't remove tuples
     * from it till this scan is done.  We need to maintain a pin on the
     * bucket being populated to ensure that vacuum doesn't squeeze that
     * bucket till this scan is complete; otherwise, the ordering of tuples
     * can't be maintained during forward and backward scans.  Here, we have
     * to be cautious about locking order: first, acquire the lock on bucket
     * being split; then, release the lock on it but not the pin; then,
     * acquire a lock on bucket being populated and again re-verify whether
     * the bucket split is still in progress.  Acquiring the lock on bucket
     * being split first ensures that the vacuum waits for this scan to
     * finish.
     */
    if (H_BUCKET_BEING_POPULATED(opaque))
    {
        BlockNumber old_blkno;
        Buffer      old_buf;
 
        old_blkno = _hash_get_oldblock_from_newbucket(rel, bucket);
 
        /*
         * release the lock on new bucket and re-acquire it after acquiring
         * the lock on old bucket.
         */
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
 
        old_buf = _hash_getbuf(rel, old_blkno, HASH_READ, LH_BUCKET_PAGE);
 
        /*
         * remember the split bucket buffer so as to use it later for
         * scanning.
         */
        so->hashso_split_bucket_buf = old_buf;
        LockBuffer(old_buf, BUFFER_LOCK_UNLOCK);
 
        LockBuffer(buf, BUFFER_LOCK_SHARE);
        page = BufferGetPage(buf);
        opaque = HashPageGetOpaque(page);
        Assert(opaque->hasho_bucket == bucket);
 
        if (H_BUCKET_BEING_POPULATED(opaque))
            so->hashso_buc_populated = true;
        else
        {
            _hash_dropbuf(rel, so->hashso_split_bucket_buf);
            so->hashso_split_bucket_buf = InvalidBuffer;
        }
    }
 
    /* If a backwards scan is requested, move to the end of the chain */
    if (ScanDirectionIsBackward(dir))
    {
        /*
         * Backward scans that start during split needs to start from end of
         * bucket being split.
         */
        while (BlockNumberIsValid(opaque->hasho_nextblkno) ||
               (so->hashso_buc_populated && !so->hashso_buc_split))
            _hash_readnext(scan, &buf, &page, &opaque);
    }
 
    /* remember which buffer we have pinned, if any */
    Assert(BufferIsInvalid(so->currPos.buf));
    so->currPos.buf = buf;
 
    /* Now find all the tuples satisfying the qualification from a page */
    if (!_hash_readpage(scan, &buf, dir))
        return false;
 
    /* OK, itemIndex says what to return */
    currItem = &so->currPos.items[so->currPos.itemIndex];
    scan->xs_heaptid = currItem->heapTid;
 
    /* if we're here, _hash_readpage found a valid tuples */
    return true;
}
 
/*
 *  _hash_readpage() -- Load data from current index page into so->currPos
 *
 *  We scan all the items in the current index page and save them into
 *  so->currPos if it satisfies the qualification. If no matching items
 *  are found in the current page, we move to the next or previous page
 *  in a bucket chain as indicated by the direction.
 *
 *  Return true if any matching items are found else return false.
 */
static bool
_hash_readpage(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
{
    Relation    rel = scan->indexRelation;
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    Buffer      buf;
    Page        page;
    HashPageOpaque opaque;
    OffsetNumber offnum;
    uint16      itemIndex;
 
    buf = *bufP;
    Assert(BufferIsValid(buf));
    _hash_checkpage(rel, buf, LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
    page = BufferGetPage(buf);
    opaque = HashPageGetOpaque(page);
 
    so->currPos.buf = buf;
    so->currPos.currPage = BufferGetBlockNumber(buf);
 
    if (ScanDirectionIsForward(dir))
    {
        BlockNumber prev_blkno = InvalidBlockNumber;
 
        for (;;)
        {
            /* new page, locate starting position by binary search */
            offnum = _hash_binsearch(page, so->hashso_sk_hash);
 
            itemIndex = _hash_load_qualified_items(scan, page, offnum, dir);
 
            if (itemIndex != 0)
                break;
 
            /*
             * Could not find any matching tuples in the current page, move to
             * the next page. Before leaving the current page, deal with any
             * killed items.
             */
            if (so->numKilled > 0)
                _hash_kill_items(scan);
 
            /*
             * If this is a primary bucket page, hasho_prevblkno is not a real
             * block number.
             */
            if (so->currPos.buf == so->hashso_bucket_buf ||
                so->currPos.buf == so->hashso_split_bucket_buf)
                prev_blkno = InvalidBlockNumber;
            else
                prev_blkno = opaque->hasho_prevblkno;
 
            _hash_readnext(scan, &buf, &page, &opaque);
            if (BufferIsValid(buf))
            {
                so->currPos.buf = buf;
                so->currPos.currPage = BufferGetBlockNumber(buf);
            }
            else
            {
                /*
                 * Remember next and previous block numbers for scrollable
                 * cursors to know the start position and return false
                 * indicating that no more matching tuples were found. Also,
                 * don't reset currPage or lsn, because we expect
                 * _hash_kill_items to be called for the old page after this
                 * function returns.
                 */
                so->currPos.prevPage = prev_blkno;
                so->currPos.nextPage = InvalidBlockNumber;
                so->currPos.buf = buf;
                return false;
            }
        }
 
        so->currPos.firstItem = 0;
        so->currPos.lastItem = itemIndex - 1;
        so->currPos.itemIndex = 0;
    }
    else
    {
        BlockNumber next_blkno = InvalidBlockNumber;
 
        for (;;)
        {
            /* new page, locate starting position by binary search */
            offnum = _hash_binsearch_last(page, so->hashso_sk_hash);
 
            itemIndex = _hash_load_qualified_items(scan, page, offnum, dir);
 
            if (itemIndex != MaxIndexTuplesPerPage)
                break;
 
            /*
             * Could not find any matching tuples in the current page, move to
             * the previous page. Before leaving the current page, deal with
             * any killed items.
             */
            if (so->numKilled > 0)
                _hash_kill_items(scan);
 
            if (so->currPos.buf == so->hashso_bucket_buf ||
                so->currPos.buf == so->hashso_split_bucket_buf)
                next_blkno = opaque->hasho_nextblkno;
 
            _hash_readprev(scan, &buf, &page, &opaque);
            if (BufferIsValid(buf))
            {
                so->currPos.buf = buf;
                so->currPos.currPage = BufferGetBlockNumber(buf);
            }
            else
            {
                /*
                 * Remember next and previous block numbers for scrollable
                 * cursors to know the start position and return false
                 * indicating that no more matching tuples were found. Also,
                 * don't reset currPage or lsn, because we expect
                 * _hash_kill_items to be called for the old page after this
                 * function returns.
                 */
                so->currPos.prevPage = InvalidBlockNumber;
                so->currPos.nextPage = next_blkno;
                so->currPos.buf = buf;
                return false;
            }
        }
 
        so->currPos.firstItem = itemIndex;
        so->currPos.lastItem = MaxIndexTuplesPerPage - 1;
        so->currPos.itemIndex = MaxIndexTuplesPerPage - 1;
    }
 
    if (so->currPos.buf == so->hashso_bucket_buf ||
        so->currPos.buf == so->hashso_split_bucket_buf)
    {
        so->currPos.prevPage = InvalidBlockNumber;
        so->currPos.nextPage = opaque->hasho_nextblkno;
        LockBuffer(so->currPos.buf, BUFFER_LOCK_UNLOCK);
    }
    else
    {
        so->currPos.prevPage = opaque->hasho_prevblkno;
        so->currPos.nextPage = opaque->hasho_nextblkno;
        _hash_relbuf(rel, so->currPos.buf);
        so->currPos.buf = InvalidBuffer;
    }
 
    Assert(so->currPos.firstItem <= so->currPos.lastItem);
    return true;
}
 
/*
 * Load all the qualified items from a current index page
 * into so->currPos. Helper function for _hash_readpage.
 */
static int
_hash_load_qualified_items(IndexScanDesc scan, Page page,
                           OffsetNumber offnum, ScanDirection dir)
{
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    IndexTuple  itup;
    int         itemIndex;
    OffsetNumber maxoff;
 
    maxoff = PageGetMaxOffsetNumber(page);
 
    if (ScanDirectionIsForward(dir))
    {
        /* load items[] in ascending order */
        itemIndex = 0;
 
        while (offnum <= maxoff)
        {
            Assert(offnum >= FirstOffsetNumber);
            itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
 
            /*
             * skip the tuples that are moved by split operation for the scan
             * that has started when split was in progress. Also, skip the
             * tuples that are marked as dead.
             */
            if ((so->hashso_buc_populated && !so->hashso_buc_split &&
                 (itup->t_info & INDEX_MOVED_BY_SPLIT_MASK)) ||
                (scan->ignore_killed_tuples &&
                 (ItemIdIsDead(PageGetItemId(page, offnum)))))
            {
                offnum = OffsetNumberNext(offnum);  /* move forward */
                continue;
            }
 
            if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup) &&
                _hash_checkqual(scan, itup))
            {
                /* tuple is qualified, so remember it */
                _hash_saveitem(so, itemIndex, offnum, itup);
                itemIndex++;
            }
            else
            {
                /*
                 * No more matching tuples exist in this page. so, exit while
                 * loop.
                 */
                break;
            }
 
            offnum = OffsetNumberNext(offnum);
        }
 
        Assert(itemIndex <= MaxIndexTuplesPerPage);
        return itemIndex;
    }
    else
    {
        /* load items[] in descending order */
        itemIndex = MaxIndexTuplesPerPage;
 
        while (offnum >= FirstOffsetNumber)
        {
            Assert(offnum <= maxoff);
            itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
 
            /*
             * skip the tuples that are moved by split operation for the scan
             * that has started when split was in progress. Also, skip the
             * tuples that are marked as dead.
             */
            if ((so->hashso_buc_populated && !so->hashso_buc_split &&
                 (itup->t_info & INDEX_MOVED_BY_SPLIT_MASK)) ||
                (scan->ignore_killed_tuples &&
                 (ItemIdIsDead(PageGetItemId(page, offnum)))))
            {
                offnum = OffsetNumberPrev(offnum);  /* move back */
                continue;
            }
 
            if (so->hashso_sk_hash == _hash_get_indextuple_hashkey(itup) &&
                _hash_checkqual(scan, itup))
            {
                itemIndex--;
                /* tuple is qualified, so remember it */
                _hash_saveitem(so, itemIndex, offnum, itup);
            }
            else
            {
                /*
                 * No more matching tuples exist in this page. so, exit while
                 * loop.
                 */
                break;
            }
 
            offnum = OffsetNumberPrev(offnum);
        }
 
        Assert(itemIndex >= 0);
        return itemIndex;
    }
}
 
/* Save an index item into so->currPos.items[itemIndex] */
static inline void
_hash_saveitem(HashScanOpaque so, int itemIndex,
               OffsetNumber offnum, IndexTuple itup)
{
    HashScanPosItem *currItem = &so->currPos.items[itemIndex];
 
    currItem->heapTid = itup->t_tid;
    currItem->indexOffset = offnum;
}