gistvacuum.c

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/*-------------------------------------------------------------------------
 *
 * gistvacuum.c
 *    vacuuming routines for the postgres GiST index access method.
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/gist/gistvacuum.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/gist_private.h"
#include "access/transam.h"
#include "commands/vacuum.h"
#include "lib/integerset.h"
#include "miscadmin.h"
#include "storage/indexfsm.h"
#include "storage/lmgr.h"
#include "utils/memutils.h"

/* Working state needed by gistbulkdelete */
typedef struct
{
    IndexVacuumInfo *info;
    IndexBulkDeleteResult *stats;
    IndexBulkDeleteCallback callback;
    void       *callback_state;
    GistNSN     startNSN;

    /*
     * These are used to memorize all internal and empty leaf pages.  They are
     * used for deleting all the empty pages.
     */
    IntegerSet *internal_page_set;
    IntegerSet *empty_leaf_set;
    MemoryContext page_set_context;
} GistVacState;

static void gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
                           IndexBulkDeleteCallback callback, void *callback_state);
static void gistvacuumpage(GistVacState *vstate, BlockNumber blkno,
                           BlockNumber orig_blkno);
static void gistvacuum_delete_empty_pages(IndexVacuumInfo *info,
                                          GistVacState *vstate);
static bool gistdeletepage(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
                           Buffer buffer, OffsetNumber downlink,
                           Buffer leafBuffer);

/*
 * VACUUM bulkdelete stage: remove index entries.
 */
IndexBulkDeleteResult *
gistbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
               IndexBulkDeleteCallback callback, void *callback_state)
{
    /* allocate stats if first time through, else re-use existing struct */
    if (stats == NULL)
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));

    gistvacuumscan(info, stats, callback, callback_state);

    return stats;
}

/*
 * VACUUM cleanup stage: delete empty pages, and update index statistics.
 */
IndexBulkDeleteResult *
gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
{
    /* No-op in ANALYZE ONLY mode */
    if (info->analyze_only)
        return stats;

    /*
     * If gistbulkdelete was called, we need not do anything, just return the
     * stats from the latest gistbulkdelete call.  If it wasn't called, we
     * still need to do a pass over the index, to obtain index statistics.
     */
    if (stats == NULL)
    {
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
        gistvacuumscan(info, stats, NULL, NULL);
    }

    /*
     * It's quite possible for us to be fooled by concurrent page splits into
     * double-counting some index tuples, so disbelieve any total that exceeds
     * the underlying heap's count ... if we know that accurately.  Otherwise
     * this might just make matters worse.
     */
    if (!info->estimated_count)
    {
        if (stats->num_index_tuples > info->num_heap_tuples)
            stats->num_index_tuples = info->num_heap_tuples;
    }

    return stats;
}

/*
 * gistvacuumscan --- scan the index for VACUUMing purposes
 *
 * This scans the index for leaf tuples that are deletable according to the
 * vacuum callback, and updates the stats.  Both btbulkdelete and
 * btvacuumcleanup invoke this (the latter only if no btbulkdelete call
 * occurred).
 *
 * This also makes note of any empty leaf pages, as well as all internal
 * pages while looping over all index pages.  After scanning all the pages, we
 * remove the empty pages so that they can be reused.  Any deleted pages are
 * added directly to the free space map.  (They should've been added there
 * when they were originally deleted, already, but it's possible that the FSM
 * was lost at a crash, for example.)
 *
 * The caller is responsible for initially allocating/zeroing a stats struct.
 */
static void
gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
               IndexBulkDeleteCallback callback, void *callback_state)
{
    Relation    rel = info->index;
    GistVacState vstate;
    BlockNumber num_pages;
    bool        needLock;
    BlockNumber blkno;
    MemoryContext oldctx;

    /*
     * Reset counts that will be incremented during the scan; needed in case
     * of multiple scans during a single VACUUM command.
     */
    stats->estimated_count = false;
    stats->num_index_tuples = 0;
    stats->pages_deleted = 0;
    stats->pages_free = 0;

    /*
     * Create the integer sets to remember all the internal and the empty leaf
     * pages in page_set_context.  Internally, the integer set will remember
     * this context so that the subsequent allocations for these integer sets
     * will be done from the same context.
     */
    vstate.page_set_context = GenerationContextCreate(CurrentMemoryContext,
                                                      "GiST VACUUM page set context",
                                                      16 * 1024);
    oldctx = MemoryContextSwitchTo(vstate.page_set_context);
    vstate.internal_page_set = intset_create();
    vstate.empty_leaf_set = intset_create();
    MemoryContextSwitchTo(oldctx);

    /* Set up info to pass down to gistvacuumpage */
    vstate.info = info;
    vstate.stats = stats;
    vstate.callback = callback;
    vstate.callback_state = callback_state;
    if (RelationNeedsWAL(rel))
        vstate.startNSN = GetInsertRecPtr();
    else
        vstate.startNSN = gistGetFakeLSN(rel);

    /*
     * The outer loop iterates over all index pages, in physical order (we
     * hope the kernel will cooperate in providing read-ahead for speed).  It
     * is critical that we visit all leaf pages, including ones added after we
     * start the scan, else we might fail to delete some deletable tuples.
     * Hence, we must repeatedly check the relation length.  We must acquire
     * the relation-extension lock while doing so to avoid a race condition:
     * if someone else is extending the relation, there is a window where
     * bufmgr/smgr have created a new all-zero page but it hasn't yet been
     * write-locked by gistNewBuffer().  If we manage to scan such a page
     * here, we'll improperly assume it can be recycled.  Taking the lock
     * synchronizes things enough to prevent a problem: either num_pages won't
     * include the new page, or gistNewBuffer already has write lock on the
     * buffer and it will be fully initialized before we can examine it.  (See
     * also vacuumlazy.c, which has the same issue.)  Also, we need not worry
     * if a page is added immediately after we look; the page splitting code
     * already has write-lock on the left page before it adds a right page, so
     * we must already have processed any tuples due to be moved into such a
     * page.
     *
     * We can skip locking for new or temp relations, however, since no one
     * else could be accessing them.
     */
    needLock = !RELATION_IS_LOCAL(rel);

    blkno = GIST_ROOT_BLKNO;
    for (;;)
    {
        /* Get the current relation length */
        if (needLock)
            LockRelationForExtension(rel, ExclusiveLock);
        num_pages = RelationGetNumberOfBlocks(rel);
        if (needLock)
            UnlockRelationForExtension(rel, ExclusiveLock);

        /* Quit if we've scanned the whole relation */
        if (blkno >= num_pages)
            break;
        /* Iterate over pages, then loop back to recheck length */
        for (; blkno < num_pages; blkno++)
            gistvacuumpage(&vstate, blkno, blkno);
    }

    /*
     * If we found any recyclable pages (and recorded them in the FSM), then
     * forcibly update the upper-level FSM pages to ensure that searchers can
     * find them.  It's possible that the pages were also found during
     * previous scans and so this is a waste of time, but it's cheap enough
     * relative to scanning the index that it shouldn't matter much, and
     * making sure that free pages are available sooner not later seems
     * worthwhile.
     *
     * Note that if no recyclable pages exist, we don't bother vacuuming the
     * FSM at all.
     */
    if (stats->pages_free > 0)
        IndexFreeSpaceMapVacuum(rel);

    /* update statistics */
    stats->num_pages = num_pages;

    /*
     * If we saw any empty pages, try to unlink them from the tree so that
     * they can be reused.
     */
    gistvacuum_delete_empty_pages(info, &vstate);

    /* we don't need the internal and empty page sets anymore */
    MemoryContextDelete(vstate.page_set_context);
    vstate.page_set_context = NULL;
    vstate.internal_page_set = NULL;
    vstate.empty_leaf_set = NULL;
}

/*
 * gistvacuumpage --- VACUUM one page
 *
 * This processes a single page for gistbulkdelete().  In some cases we
 * must go back and re-examine previously-scanned pages; this routine
 * recurses when necessary to handle that case.
 *
 * blkno is the page to process.  orig_blkno is the highest block number
 * reached by the outer gistvacuumscan loop (the same as blkno, unless we
 * are recursing to re-examine a previous page).
 */
static void
gistvacuumpage(GistVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
{
    IndexVacuumInfo *info = vstate->info;
    IndexBulkDeleteCallback callback = vstate->callback;
    void       *callback_state = vstate->callback_state;
    Relation    rel = info->index;
    Buffer      buffer;
    Page        page;
    BlockNumber recurse_to;

restart:
    recurse_to = InvalidBlockNumber;

    /* call vacuum_delay_point while not holding any buffer lock */
    vacuum_delay_point();

    buffer = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                                info->strategy);

    /*
     * We are not going to stay here for a long time, aggressively grab an
     * exclusive lock.
     */
    LockBuffer(buffer, GIST_EXCLUSIVE);
    page = (Page) BufferGetPage(buffer);

    if (gistPageRecyclable(page))
    {
        /* Okay to recycle this page */
        RecordFreeIndexPage(rel, blkno);
        vstate->stats->pages_free++;
        vstate->stats->pages_deleted++;
    }
    else if (GistPageIsDeleted(page))
    {
        /* Already deleted, but can't recycle yet */
        vstate->stats->pages_deleted++;
    }
    else if (GistPageIsLeaf(page))
    {
        OffsetNumber todelete[MaxOffsetNumber];
        int         ntodelete = 0;
        int         nremain;
        GISTPageOpaque opaque = GistPageGetOpaque(page);
        OffsetNumber maxoff = PageGetMaxOffsetNumber(page);

        /*
         * Check whether we need to recurse back to earlier pages.  What we
         * are concerned about is a page split that happened since we started
         * the vacuum scan.  If the split moved some tuples to a lower page
         * then we might have missed 'em.  If so, set up for tail recursion.
         *
         * This is similar to the checks we do during searches, when following
         * a downlink, but we don't need to jump to higher-numbered pages,
         * because we will process them later, anyway.
         */
        if ((GistFollowRight(page) ||
             vstate->startNSN < GistPageGetNSN(page)) &&
            (opaque->rightlink != InvalidBlockNumber) &&
            (opaque->rightlink < orig_blkno))
        {
            recurse_to = opaque->rightlink;
        }

        /*
         * Scan over all items to see which ones need to be deleted according
         * to the callback function.
         */
        if (callback)
        {
            OffsetNumber off;

            for (off = FirstOffsetNumber;
                 off <= maxoff;
                 off = OffsetNumberNext(off))
            {
                ItemId      iid = PageGetItemId(page, off);
                IndexTuple  idxtuple = (IndexTuple) PageGetItem(page, iid);

                if (callback(&(idxtuple->t_tid), callback_state))
                    todelete[ntodelete++] = off;
            }
        }

        /*
         * Apply any needed deletes.  We issue just one WAL record per page,
         * so as to minimize WAL traffic.
         */
        if (ntodelete > 0)
        {
            START_CRIT_SECTION();

            MarkBufferDirty(buffer);

            PageIndexMultiDelete(page, todelete, ntodelete);
            GistMarkTuplesDeleted(page);

            if (RelationNeedsWAL(rel))
            {
                XLogRecPtr  recptr;

                recptr = gistXLogUpdate(buffer,
                                        todelete, ntodelete,
                                        NULL, 0, InvalidBuffer);
                PageSetLSN(page, recptr);
            }
            else
                PageSetLSN(page, gistGetFakeLSN(rel));

            END_CRIT_SECTION();

            vstate->stats->tuples_removed += ntodelete;
            /* must recompute maxoff */
            maxoff = PageGetMaxOffsetNumber(page);
        }

        nremain = maxoff - FirstOffsetNumber + 1;
        if (nremain == 0)
        {
            /*
             * The page is now completely empty.  Remember its block number,
             * so that we will try to delete the page in the second stage.
             *
             * Skip this when recursing, because IntegerSet requires that the
             * values are added in ascending order.  The next VACUUM will pick
             * it up.
             */
            if (blkno == orig_blkno)
                intset_add_member(vstate->empty_leaf_set, blkno);
        }
        else
            vstate->stats->num_index_tuples += nremain;
    }
    else
    {
        /*
         * On an internal page, check for "invalid tuples", left behind by an
         * incomplete page split on PostgreSQL 9.0 or below.  These are not
         * created by newer PostgreSQL versions, but unfortunately, there is
         * no version number anywhere in a GiST index, so we don't know
         * whether this index might still contain invalid tuples or not.
         */
        OffsetNumber maxoff = PageGetMaxOffsetNumber(page);
        OffsetNumber off;

        for (off = FirstOffsetNumber;
             off <= maxoff;
             off = OffsetNumberNext(off))
        {
            ItemId      iid = PageGetItemId(page, off);
            IndexTuple  idxtuple = (IndexTuple) PageGetItem(page, iid);

            if (GistTupleIsInvalid(idxtuple))
                ereport(LOG,
                        (errmsg("index \"%s\" contains an inner tuple marked as invalid",
                                RelationGetRelationName(rel)),
                         errdetail("This is caused by an incomplete page split at crash recovery before upgrading to PostgreSQL 9.1."),
                         errhint("Please REINDEX it.")));
        }

        /*
         * Remember the block number of this page, so that we can revisit it
         * later in gistvacuum_delete_empty_pages(), when we search for
         * parents of empty leaf pages.
         */
        if (blkno == orig_blkno)
            intset_add_member(vstate->internal_page_set, blkno);
    }

    UnlockReleaseBuffer(buffer);

    /*
     * This is really tail recursion, but if the compiler is too stupid to
     * optimize it as such, we'd eat an uncomfortably large amount of stack
     * space per recursion level (due to the deletable[] array).  A failure is
     * improbable since the number of levels isn't likely to be large ... but
     * just in case, let's hand-optimize into a loop.
     */
    if (recurse_to != InvalidBlockNumber)
    {
        blkno = recurse_to;
        goto restart;
    }
}

/*
 * Scan all internal pages, and try to delete their empty child pages.
 */
static void
gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistVacState *vstate)
{
    Relation    rel = info->index;
    BlockNumber empty_pages_remaining;
    uint64      blkno;

    /*
     * Rescan all inner pages to find those that have empty child pages.
     */
    empty_pages_remaining = intset_num_entries(vstate->empty_leaf_set);
    intset_begin_iterate(vstate->internal_page_set);
    while (empty_pages_remaining > 0 &&
           intset_iterate_next(vstate->internal_page_set, &blkno))
    {
        Buffer      buffer;
        Page        page;
        OffsetNumber off,
                    maxoff;
        OffsetNumber todelete[MaxOffsetNumber];
        BlockNumber leafs_to_delete[MaxOffsetNumber];
        int         ntodelete;
        int         deleted;

        buffer = ReadBufferExtended(rel, MAIN_FORKNUM, (BlockNumber) blkno,
                                    RBM_NORMAL, info->strategy);

        LockBuffer(buffer, GIST_SHARE);
        page = (Page) BufferGetPage(buffer);

        if (PageIsNew(page) || GistPageIsDeleted(page) || GistPageIsLeaf(page))
        {
            /*
             * This page was an internal page earlier, but now it's something
             * else. Shouldn't happen...
             */
            Assert(false);
            UnlockReleaseBuffer(buffer);
            continue;
        }

        /*
         * Scan all the downlinks, and see if any of them point to empty leaf
         * pages.
         */
        maxoff = PageGetMaxOffsetNumber(page);
        ntodelete = 0;
        for (off = FirstOffsetNumber;
             off <= maxoff && ntodelete < maxoff - 1;
             off = OffsetNumberNext(off))
        {
            ItemId      iid = PageGetItemId(page, off);
            IndexTuple  idxtuple = (IndexTuple) PageGetItem(page, iid);
            BlockNumber leafblk;

            leafblk = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
            if (intset_is_member(vstate->empty_leaf_set, leafblk))
            {
                leafs_to_delete[ntodelete] = leafblk;
                todelete[ntodelete++] = off;
            }
        }

        /*
         * In order to avoid deadlock, child page must be locked before
         * parent, so we must release the lock on the parent, lock the child,
         * and then re-acquire the lock the parent.  (And we wouldn't want to
         * do I/O, while holding a lock, anyway.)
         *
         * At the instant that we're not holding a lock on the parent, the
         * downlink might get moved by a concurrent insert, so we must
         * re-check that it still points to the same child page after we have
         * acquired both locks.  Also, another backend might have inserted a
         * tuple to the page, so that it is no longer empty.  gistdeletepage()
         * re-checks all these conditions.
         */
        LockBuffer(buffer, GIST_UNLOCK);

        deleted = 0;
        for (int i = 0; i < ntodelete; i++)
        {
            Buffer      leafbuf;

            /*
             * Don't remove the last downlink from the parent.  That would
             * confuse the insertion code.
             */
            if (PageGetMaxOffsetNumber(page) == FirstOffsetNumber)
                break;

            leafbuf = ReadBufferExtended(rel, MAIN_FORKNUM, leafs_to_delete[i],
                                         RBM_NORMAL, info->strategy);
            LockBuffer(leafbuf, GIST_EXCLUSIVE);
            gistcheckpage(rel, leafbuf);

            LockBuffer(buffer, GIST_EXCLUSIVE);
            if (gistdeletepage(info, vstate->stats,
                               buffer, todelete[i] - deleted,
                               leafbuf))
                deleted++;
            LockBuffer(buffer, GIST_UNLOCK);

            UnlockReleaseBuffer(leafbuf);
        }

        ReleaseBuffer(buffer);

        /* update stats */
        vstate->stats->pages_removed += deleted;

        /*
         * We can stop the scan as soon as we have seen the downlinks, even if
         * we were not able to remove them all.
         */
        empty_pages_remaining -= ntodelete;
    }
}

/*
 * gistdeletepage takes a leaf page, and its parent, and tries to delete the
 * leaf.  Both pages must be locked.
 *
 * Even if the page was empty when we first saw it, a concurrent inserter might
 * have added a tuple to it since.  Similarly, the downlink might have moved.
 * We re-check all the conditions, to make sure the page is still deletable,
 * before modifying anything.
 *
 * Returns true, if the page was deleted, and false if a concurrent update
 * prevented it.
 */
static bool
gistdeletepage(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
               Buffer parentBuffer, OffsetNumber downlink,
               Buffer leafBuffer)
{
    Page        parentPage = BufferGetPage(parentBuffer);
    Page        leafPage = BufferGetPage(leafBuffer);
    ItemId      iid;
    IndexTuple  idxtuple;
    XLogRecPtr  recptr;
    FullTransactionId txid;

    /*
     * Check that the leaf is still empty and deletable.
     */
    if (!GistPageIsLeaf(leafPage))
    {
        /* a leaf page should never become a non-leaf page */
        Assert(false);
        return false;
    }

    if (GistFollowRight(leafPage))
        return false;           /* don't mess with a concurrent page split */

    if (PageGetMaxOffsetNumber(leafPage) != InvalidOffsetNumber)
        return false;           /* not empty anymore */

    /*
     * Ok, the leaf is deletable.  Is the downlink in the parent page still
     * valid?  It might have been moved by a concurrent insert.  We could try
     * to re-find it by scanning the page again, possibly moving right if the
     * was split.  But for now, let's keep it simple and just give up.  The
     * next VACUUM will pick it up.
     */
    if (PageIsNew(parentPage) || GistPageIsDeleted(parentPage) ||
        GistPageIsLeaf(parentPage))
    {
        /* shouldn't happen, internal pages are never deleted */
        Assert(false);
        return false;
    }

    if (PageGetMaxOffsetNumber(parentPage) < downlink
        || PageGetMaxOffsetNumber(parentPage) <= FirstOffsetNumber)
        return false;

    iid = PageGetItemId(parentPage, downlink);
    idxtuple = (IndexTuple) PageGetItem(parentPage, iid);
    if (BufferGetBlockNumber(leafBuffer) !=
        ItemPointerGetBlockNumber(&(idxtuple->t_tid)))
        return false;

    /*
     * All good, proceed with the deletion.
     *
     * The page cannot be immediately recycled, because in-progress scans that
     * saw the downlink might still visit it.  Mark the page with the current
     * next-XID counter, so that we know when it can be recycled.  Once that
     * XID becomes older than GlobalXmin, we know that all scans that are
     * currently in progress must have ended.  (That's much more conservative
     * than needed, but let's keep it safe and simple.)
     */
    txid = ReadNextFullTransactionId();

    START_CRIT_SECTION();

    /* mark the page as deleted */
    MarkBufferDirty(leafBuffer);
    GistPageSetDeleted(leafPage, txid);
    stats->pages_deleted++;

    /* remove the downlink from the parent */
    MarkBufferDirty(parentBuffer);
    PageIndexTupleDelete(parentPage, downlink);

    if (RelationNeedsWAL(info->index))
        recptr = gistXLogPageDelete(leafBuffer, txid, parentBuffer, downlink);
    else
        recptr = gistGetFakeLSN(info->index);
    PageSetLSN(parentPage, recptr);
    PageSetLSN(leafPage, recptr);

    END_CRIT_SECTION();

    return true;
}