Insert CHECK_FOR_INTERRUPTS() calls into btree and hash index scans at the

[PostgreSQL.git] / src / backend / access / nbtree / nbtree.c

/*-------------------------------------------------------------------------
 *
 * nbtree.c
 *        Implementation of Lehman and Yao's btree management algorithm for
 *        Postgres.
 *
 * NOTES
 *        This file contains only the public interface routines.
 *
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *        $PostgreSQL$
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/nbtree.h"
#include "access/relscan.h"
#include "catalog/index.h"
#include "catalog/storage.h"
#include "commands/vacuum.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/indexfsm.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "utils/memutils.h"


/* Working state for btbuild and its callback */
typedef struct
{
        bool            isUnique;
        bool            haveDead;
        Relation        heapRel;
        BTSpool    *spool;

        /*
         * spool2 is needed only when the index is an unique index. Dead tuples
         * are put into spool2 instead of spool in order to avoid uniqueness
         * check.
         */
        BTSpool    *spool2;
        double          indtuples;
} BTBuildState;

/* Working state needed by btvacuumpage */
typedef struct
{
        IndexVacuumInfo *info;
        IndexBulkDeleteResult *stats;
        IndexBulkDeleteCallback callback;
        void       *callback_state;
        BTCycleId       cycleid;
        BlockNumber lastUsedPage;
        BlockNumber totFreePages;       /* true total # of free pages */
        MemoryContext pagedelcontext;
} BTVacState;


static void btbuildCallback(Relation index,
                                HeapTuple htup,
                                Datum *values,
                                bool *isnull,
                                bool tupleIsAlive,
                                void *state);
static void btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
                         IndexBulkDeleteCallback callback, void *callback_state,
                         BTCycleId cycleid);
static void btvacuumpage(BTVacState *vstate, BlockNumber blkno,
                         BlockNumber orig_blkno);


/*
 *      btbuild() -- build a new btree index.
 */
Datum
btbuild(PG_FUNCTION_ARGS)
{
        Relation        heap = (Relation) PG_GETARG_POINTER(0);
        Relation        index = (Relation) PG_GETARG_POINTER(1);
        IndexInfo  *indexInfo = (IndexInfo *) PG_GETARG_POINTER(2);
        IndexBuildResult *result;
        double          reltuples;
        BTBuildState buildstate;

        buildstate.isUnique = indexInfo->ii_Unique;
        buildstate.haveDead = false;
        buildstate.heapRel = heap;
        buildstate.spool = NULL;
        buildstate.spool2 = NULL;
        buildstate.indtuples = 0;

#ifdef BTREE_BUILD_STATS
        if (log_btree_build_stats)
                ResetUsage();
#endif   /* BTREE_BUILD_STATS */

        /*
         * We expect to be called exactly once for any index relation. If that's
         * not the case, big trouble's what we have.
         */
        if (RelationGetNumberOfBlocks(index) != 0)
                elog(ERROR, "index \"%s\" already contains data",
                         RelationGetRelationName(index));

        buildstate.spool = _bt_spoolinit(index, indexInfo->ii_Unique, false);

        /*
         * If building a unique index, put dead tuples in a second spool to keep
         * them out of the uniqueness check.
         */
        if (indexInfo->ii_Unique)
                buildstate.spool2 = _bt_spoolinit(index, false, true);

        /* do the heap scan */
        reltuples = IndexBuildHeapScan(heap, index, indexInfo, true,
                                                                   btbuildCallback, (void *) &buildstate);

        /* okay, all heap tuples are indexed */
        if (buildstate.spool2 && !buildstate.haveDead)
        {
                /* spool2 turns out to be unnecessary */
                _bt_spooldestroy(buildstate.spool2);
                buildstate.spool2 = NULL;
        }

        /*
         * Finish the build by (1) completing the sort of the spool file, (2)
         * inserting the sorted tuples into btree pages and (3) building the upper
         * levels.
         */
        _bt_leafbuild(buildstate.spool, buildstate.spool2);
        _bt_spooldestroy(buildstate.spool);
        if (buildstate.spool2)
                _bt_spooldestroy(buildstate.spool2);

#ifdef BTREE_BUILD_STATS
        if (log_btree_build_stats)
        {
                ShowUsage("BTREE BUILD STATS");
                ResetUsage();
        }
#endif   /* BTREE_BUILD_STATS */

        /*
         * If we are reindexing a pre-existing index, it is critical to send out a
         * relcache invalidation SI message to ensure all backends re-read the
         * index metapage.      We expect that the caller will ensure that happens
         * (typically as a side effect of updating index stats, but it must happen
         * even if the stats don't change!)
         */

        /*
         * Return statistics
         */
        result = (IndexBuildResult *) palloc(sizeof(IndexBuildResult));

        result->heap_tuples = reltuples;
        result->index_tuples = buildstate.indtuples;

        PG_RETURN_POINTER(result);
}

/*
 * Per-tuple callback from IndexBuildHeapScan
 */
static void
btbuildCallback(Relation index,
                                HeapTuple htup,
                                Datum *values,
                                bool *isnull,
                                bool tupleIsAlive,
                                void *state)
{
        BTBuildState *buildstate = (BTBuildState *) state;
        IndexTuple      itup;

        /* form an index tuple and point it at the heap tuple */
        itup = index_form_tuple(RelationGetDescr(index), values, isnull);
        itup->t_tid = htup->t_self;

        /*
         * insert the index tuple into the appropriate spool file for subsequent
         * processing
         */
        if (tupleIsAlive || buildstate->spool2 == NULL)
                _bt_spool(itup, buildstate->spool);
        else
        {
                /* dead tuples are put into spool2 */
                buildstate->haveDead = true;
                _bt_spool(itup, buildstate->spool2);
        }

        buildstate->indtuples += 1;

        pfree(itup);
}

/*
 *      btinsert() -- insert an index tuple into a btree.
 *
 *              Descend the tree recursively, find the appropriate location for our
 *              new tuple, and put it there.
 */
Datum
btinsert(PG_FUNCTION_ARGS)
{
        Relation        rel = (Relation) PG_GETARG_POINTER(0);
        Datum      *values = (Datum *) PG_GETARG_POINTER(1);
        bool       *isnull = (bool *) PG_GETARG_POINTER(2);
        ItemPointer ht_ctid = (ItemPointer) PG_GETARG_POINTER(3);
        Relation        heapRel = (Relation) PG_GETARG_POINTER(4);
        bool            checkUnique = PG_GETARG_BOOL(5);
        IndexTuple      itup;

        /* generate an index tuple */
        itup = index_form_tuple(RelationGetDescr(rel), values, isnull);
        itup->t_tid = *ht_ctid;

        _bt_doinsert(rel, itup, checkUnique, heapRel);

        pfree(itup);

        PG_RETURN_BOOL(true);
}

/*
 *      btgettuple() -- Get the next tuple in the scan.
 */
Datum
btgettuple(PG_FUNCTION_ARGS)
{
        IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
        ScanDirection dir = (ScanDirection) PG_GETARG_INT32(1);
        BTScanOpaque so = (BTScanOpaque) scan->opaque;
        bool            res;

        /* btree indexes are never lossy */
        scan->xs_recheck = false;

        /*
         * If we've already initialized this scan, we can just advance it in the
         * appropriate direction.  If we haven't done so yet, we call a routine to
         * get the first item in the scan.
         */
        if (BTScanPosIsValid(so->currPos))
        {
                /*
                 * Check to see if we should kill the previously-fetched tuple.
                 */
                if (scan->kill_prior_tuple)
                {
                        /*
                         * Yes, remember it for later.  (We'll deal with all such tuples
                         * at once right before leaving the index page.)  The test for
                         * numKilled overrun is not just paranoia: if the caller reverses
                         * direction in the indexscan then the same item might get entered
                         * multiple times.      It's not worth trying to optimize that, so we
                         * don't detect it, but instead just forget any excess entries.
                         */
                        if (so->killedItems == NULL)
                                so->killedItems = (int *)
                                        palloc(MaxIndexTuplesPerPage * sizeof(int));
                        if (so->numKilled < MaxIndexTuplesPerPage)
                                so->killedItems[so->numKilled++] = so->currPos.itemIndex;
                }

                /*
                 * Now continue the scan.
                 */
                res = _bt_next(scan, dir);
        }
        else
                res = _bt_first(scan, dir);

        PG_RETURN_BOOL(res);
}

/*
 * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
 */
Datum
btgetbitmap(PG_FUNCTION_ARGS)
{
        IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
        TIDBitmap *tbm = (TIDBitmap *) PG_GETARG_POINTER(1);
        BTScanOpaque so = (BTScanOpaque) scan->opaque;
        int64           ntids = 0;
        ItemPointer heapTid;

        /* Fetch the first page & tuple. */
        if (!_bt_first(scan, ForwardScanDirection))
        {
                /* empty scan */
                PG_RETURN_INT64(0);
        }
        /* Save tuple ID, and continue scanning */
        heapTid = &scan->xs_ctup.t_self;
        tbm_add_tuples(tbm, heapTid, 1, false);
        ntids++;

        for (;;)
        {
                /*
                 * Advance to next tuple within page.  This is the same as the easy
                 * case in _bt_next().
                 */
                if (++so->currPos.itemIndex > so->currPos.lastItem)
                {
                        /* let _bt_next do the heavy lifting */
                        if (!_bt_next(scan, ForwardScanDirection))
                                break;
                }

                /* Save tuple ID, and continue scanning */
                heapTid = &so->currPos.items[so->currPos.itemIndex].heapTid;
                tbm_add_tuples(tbm, heapTid, 1, false);
                ntids++;
        }

        PG_RETURN_INT64(ntids);
}

/*
 *      btbeginscan() -- start a scan on a btree index
 */
Datum
btbeginscan(PG_FUNCTION_ARGS)
{
        Relation        rel = (Relation) PG_GETARG_POINTER(0);
        int                     keysz = PG_GETARG_INT32(1);
        ScanKey         scankey = (ScanKey) PG_GETARG_POINTER(2);
        IndexScanDesc scan;

        /* get the scan */
        scan = RelationGetIndexScan(rel, keysz, scankey);

        PG_RETURN_POINTER(scan);
}

/*
 *      btrescan() -- rescan an index relation
 */
Datum
btrescan(PG_FUNCTION_ARGS)
{
        IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
        ScanKey         scankey = (ScanKey) PG_GETARG_POINTER(1);
        BTScanOpaque so;

        so = (BTScanOpaque) scan->opaque;

        if (so == NULL)                         /* if called from btbeginscan */
        {
                so = (BTScanOpaque) palloc(sizeof(BTScanOpaqueData));
                so->currPos.buf = so->markPos.buf = InvalidBuffer;
                if (scan->numberOfKeys > 0)
                        so->keyData = (ScanKey) palloc(scan->numberOfKeys * sizeof(ScanKeyData));
                else
                        so->keyData = NULL;
                so->killedItems = NULL; /* until needed */
                so->numKilled = 0;
                scan->opaque = so;
        }

        /* we aren't holding any read locks, but gotta drop the pins */
        if (BTScanPosIsValid(so->currPos))
        {
                /* Before leaving current page, deal with any killed items */
                if (so->numKilled > 0)
                        _bt_killitems(scan, false);
                ReleaseBuffer(so->currPos.buf);
                so->currPos.buf = InvalidBuffer;
        }

        if (BTScanPosIsValid(so->markPos))
        {
                ReleaseBuffer(so->markPos.buf);
                so->markPos.buf = InvalidBuffer;
        }
        so->markItemIndex = -1;

        /*
         * Reset the scan keys. Note that keys ordering stuff moved to _bt_first.
         * - vadim 05/05/97
         */
        if (scankey && scan->numberOfKeys > 0)
                memmove(scan->keyData,
                                scankey,
                                scan->numberOfKeys * sizeof(ScanKeyData));
        so->numberOfKeys = 0;           /* until _bt_preprocess_keys sets it */

        PG_RETURN_VOID();
}

/*
 *      btendscan() -- close down a scan
 */
Datum
btendscan(PG_FUNCTION_ARGS)
{
        IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
        BTScanOpaque so = (BTScanOpaque) scan->opaque;

        /* we aren't holding any read locks, but gotta drop the pins */
        if (BTScanPosIsValid(so->currPos))
        {
                /* Before leaving current page, deal with any killed items */
                if (so->numKilled > 0)
                        _bt_killitems(scan, false);
                ReleaseBuffer(so->currPos.buf);
                so->currPos.buf = InvalidBuffer;
        }

        if (BTScanPosIsValid(so->markPos))
        {
                ReleaseBuffer(so->markPos.buf);
                so->markPos.buf = InvalidBuffer;
        }
        so->markItemIndex = -1;

        if (so->killedItems != NULL)
                pfree(so->killedItems);
        if (so->keyData != NULL)
                pfree(so->keyData);
        pfree(so);

        PG_RETURN_VOID();
}

/*
 *      btmarkpos() -- save current scan position
 */
Datum
btmarkpos(PG_FUNCTION_ARGS)
{
        IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
        BTScanOpaque so = (BTScanOpaque) scan->opaque;

        /* we aren't holding any read locks, but gotta drop the pin */
        if (BTScanPosIsValid(so->markPos))
        {
                ReleaseBuffer(so->markPos.buf);
                so->markPos.buf = InvalidBuffer;
        }

        /*
         * Just record the current itemIndex.  If we later step to next page
         * before releasing the marked position, _bt_steppage makes a full copy of
         * the currPos struct in markPos.  If (as often happens) the mark is moved
         * before we leave the page, we don't have to do that work.
         */
        if (BTScanPosIsValid(so->currPos))
                so->markItemIndex = so->currPos.itemIndex;
        else
                so->markItemIndex = -1;

        PG_RETURN_VOID();
}

/*
 *      btrestrpos() -- restore scan to last saved position
 */
Datum
btrestrpos(PG_FUNCTION_ARGS)
{
        IndexScanDesc scan = (IndexScanDesc) PG_GETARG_POINTER(0);
        BTScanOpaque so = (BTScanOpaque) scan->opaque;

        if (so->markItemIndex >= 0)
        {
                /*
                 * The mark position is on the same page we are currently on. Just
                 * restore the itemIndex.
                 */
                so->currPos.itemIndex = so->markItemIndex;
        }
        else
        {
                /* we aren't holding any read locks, but gotta drop the pin */
                if (BTScanPosIsValid(so->currPos))
                {
                        /* Before leaving current page, deal with any killed items */
                        if (so->numKilled > 0 &&
                                so->currPos.buf != so->markPos.buf)
                                _bt_killitems(scan, false);
                        ReleaseBuffer(so->currPos.buf);
                        so->currPos.buf = InvalidBuffer;
                }

                if (BTScanPosIsValid(so->markPos))
                {
                        /* bump pin on mark buffer for assignment to current buffer */
                        IncrBufferRefCount(so->markPos.buf);
                        memcpy(&so->currPos, &so->markPos,
                                   offsetof(BTScanPosData, items[1]) +
                                   so->markPos.lastItem * sizeof(BTScanPosItem));
                }
        }

        PG_RETURN_VOID();
}

/*
 * Bulk deletion of all index entries pointing to a set of heap tuples.
 * The set of target tuples is specified via a callback routine that tells
 * whether any given heap tuple (identified by ItemPointer) is being deleted.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
Datum
btbulkdelete(PG_FUNCTION_ARGS)
{
        IndexVacuumInfo *info = (IndexVacuumInfo *) PG_GETARG_POINTER(0);
        IndexBulkDeleteResult *volatile stats = (IndexBulkDeleteResult *) PG_GETARG_POINTER(1);
        IndexBulkDeleteCallback callback = (IndexBulkDeleteCallback) PG_GETARG_POINTER(2);
        void       *callback_state = (void *) PG_GETARG_POINTER(3);
        Relation        rel = info->index;
        BTCycleId       cycleid;

        /* allocate stats if first time through, else re-use existing struct */
        if (stats == NULL)
                stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));

        /* Establish the vacuum cycle ID to use for this scan */
        /* The ENSURE stuff ensures we clean up shared memory on failure */
        PG_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
        {
                cycleid = _bt_start_vacuum(rel);

                btvacuumscan(info, stats, callback, callback_state, cycleid);
        }
        PG_END_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
        _bt_end_vacuum(rel);

        PG_RETURN_POINTER(stats);
}

/*
 * Post-VACUUM cleanup.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
Datum
btvacuumcleanup(PG_FUNCTION_ARGS)
{
        IndexVacuumInfo *info = (IndexVacuumInfo *) PG_GETARG_POINTER(0);
        IndexBulkDeleteResult *stats = (IndexBulkDeleteResult *) PG_GETARG_POINTER(1);

        /* No-op in ANALYZE ONLY mode */
        if (info->analyze_only)
                PG_RETURN_POINTER(stats);

        /*
         * If btbulkdelete was called, we need not do anything, just return the
         * stats from the latest btbulkdelete call.  If it wasn't called, we must
         * still do a pass over the index, to recycle any newly-recyclable pages
         * and to obtain index statistics.
         *
         * Since we aren't going to actually delete any leaf items, there's no
         * need to go through all the vacuum-cycle-ID pushups.
         */
        if (stats == NULL)
        {
                stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
                btvacuumscan(info, stats, NULL, NULL, 0);
        }

        /* Finally, vacuum the FSM */
        IndexFreeSpaceMapVacuum(info->index);

        /*
         * During a non-FULL vacuum 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. (We
         * can't check this during btbulkdelete.)
         */
        if (!info->vacuum_full)
        {
                if (stats->num_index_tuples > info->num_heap_tuples)
                        stats->num_index_tuples = info->num_heap_tuples;
        }

        PG_RETURN_POINTER(stats);
}

/*
 * btvacuumscan --- scan the index for VACUUMing purposes
 *
 * This combines the functions of looking for leaf tuples that are deletable
 * according to the vacuum callback, looking for empty pages that can be
 * deleted, and looking for old deleted pages that can be recycled.  Both
 * btbulkdelete and btvacuumcleanup invoke this (the latter only if no
 * btbulkdelete call occurred).
 *
 * The caller is responsible for initially allocating/zeroing a stats struct
 * and for obtaining a vacuum cycle ID if necessary.
 */
static void
btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
                         IndexBulkDeleteCallback callback, void *callback_state,
                         BTCycleId cycleid)
{
        Relation        rel = info->index;
        BTVacState      vstate;
        BlockNumber num_pages;
        BlockNumber blkno;
        bool            needLock;

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

        /* Set up info to pass down to btvacuumpage */
        vstate.info = info;
        vstate.stats = stats;
        vstate.callback = callback;
        vstate.callback_state = callback_state;
        vstate.cycleid = cycleid;
        vstate.lastUsedPage = BTREE_METAPAGE;
        vstate.totFreePages = 0;

        /* Create a temporary memory context to run _bt_pagedel in */
        vstate.pagedelcontext = AllocSetContextCreate(CurrentMemoryContext,
                                                                                                  "_bt_pagedel",
                                                                                                  ALLOCSET_DEFAULT_MINSIZE,
                                                                                                  ALLOCSET_DEFAULT_INITSIZE,
                                                                                                  ALLOCSET_DEFAULT_MAXSIZE);

        /*
         * The outer loop iterates over all index pages except the metapage, 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 _bt_getbuf(). 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 _bt_getbuf
         * 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 = BTREE_METAPAGE + 1;
        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++)
                {
                        btvacuumpage(&vstate, blkno, blkno);
                }
        }

        /*
         * During VACUUM FULL, we truncate off any recyclable pages at the end of
         * the index.  In a normal vacuum it'd be unsafe to do this except by
         * acquiring exclusive lock on the index and then rechecking all the
         * pages; doesn't seem worth it.
         */
        if (info->vacuum_full && vstate.lastUsedPage < num_pages - 1)
        {
                BlockNumber new_pages = vstate.lastUsedPage + 1;

                /*
                 * Okay to truncate.
                 */
                RelationTruncate(rel, new_pages);

                /* update statistics */
                stats->pages_removed += num_pages - new_pages;
                vstate.totFreePages -= (num_pages - new_pages);
                num_pages = new_pages;
        }

        MemoryContextDelete(vstate.pagedelcontext);

        /* update statistics */
        stats->num_pages = num_pages;
        stats->pages_free = vstate.totFreePages;
}

/*
 * btvacuumpage --- VACUUM one page
 *
 * This processes a single page for btvacuumscan().  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 btvacuumscan loop (the same as blkno, unless we
 * are recursing to re-examine a previous page).
 */
static void
btvacuumpage(BTVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
{
        IndexVacuumInfo *info = vstate->info;
        IndexBulkDeleteResult *stats = vstate->stats;
        IndexBulkDeleteCallback callback = vstate->callback;
        void       *callback_state = vstate->callback_state;
        Relation        rel = info->index;
        bool            delete_now;
        BlockNumber recurse_to;
        Buffer          buf;
        Page            page;
        BTPageOpaque opaque;

restart:
        delete_now = false;
        recurse_to = P_NONE;

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

        /*
         * We can't use _bt_getbuf() here because it always applies
         * _bt_checkpage(), which will barf on an all-zero page. We want to
         * recycle all-zero pages, not fail.  Also, we want to use a nondefault
         * buffer access strategy.
         */
        buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                                                         info->strategy);
        LockBuffer(buf, BT_READ);
        page = BufferGetPage(buf);
        opaque = (BTPageOpaque) PageGetSpecialPointer(page);
        if (!PageIsNew(page))
                _bt_checkpage(rel, buf);

        /*
         * If we are recursing, the only case we want to do anything with is a
         * live leaf page having the current vacuum cycle ID.  Any other state
         * implies we already saw the page (eg, deleted it as being empty).
         */
        if (blkno != orig_blkno)
        {
                if (_bt_page_recyclable(page) ||
                        P_IGNORE(opaque) ||
                        !P_ISLEAF(opaque) ||
                        opaque->btpo_cycleid != vstate->cycleid)
                {
                        _bt_relbuf(rel, buf);
                        return;
                }
        }

        /* If the page is in use, update lastUsedPage */
        if (!_bt_page_recyclable(page) && vstate->lastUsedPage < blkno)
                vstate->lastUsedPage = blkno;

        /* Page is valid, see what to do with it */
        if (_bt_page_recyclable(page))
        {
                /* Okay to recycle this page */
                RecordFreeIndexPage(rel, blkno);
                vstate->totFreePages++;
                stats->pages_deleted++;
        }
        else if (P_ISDELETED(opaque))
        {
                /* Already deleted, but can't recycle yet */
                stats->pages_deleted++;
        }
        else if (P_ISHALFDEAD(opaque))
        {
                /* Half-dead, try to delete */
                delete_now = true;
        }
        else if (P_ISLEAF(opaque))
        {
                OffsetNumber deletable[MaxOffsetNumber];
                int                     ndeletable;
                OffsetNumber offnum,
                                        minoff,
                                        maxoff;

                /*
                 * Trade in the initial read lock for a super-exclusive write lock on
                 * this page.  We must get such a lock on every leaf page over the
                 * course of the vacuum scan, whether or not it actually contains any
                 * deletable tuples --- see nbtree/README.
                 */
                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
                LockBufferForCleanup(buf);

                /*
                 * 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.
                 * (Must do this before possibly clearing btpo_cycleid below!)
                 */
                if (vstate->cycleid != 0 &&
                        opaque->btpo_cycleid == vstate->cycleid &&
                        !(opaque->btpo_flags & BTP_SPLIT_END) &&
                        !P_RIGHTMOST(opaque) &&
                        opaque->btpo_next < orig_blkno)
                        recurse_to = opaque->btpo_next;

                /*
                 * Scan over all items to see which ones need deleted according to the
                 * callback function.
                 */
                ndeletable = 0;
                minoff = P_FIRSTDATAKEY(opaque);
                maxoff = PageGetMaxOffsetNumber(page);
                if (callback)
                {
                        for (offnum = minoff;
                                 offnum <= maxoff;
                                 offnum = OffsetNumberNext(offnum))
                        {
                                IndexTuple      itup;
                                ItemPointer htup;

                                itup = (IndexTuple) PageGetItem(page,
                                                                                                PageGetItemId(page, offnum));
                                htup = &(itup->t_tid);
                                if (callback(htup, callback_state))
                                        deletable[ndeletable++] = offnum;
                        }
                }

                /*
                 * Apply any needed deletes.  We issue just one _bt_delitems() call
                 * per page, so as to minimize WAL traffic.
                 */
                if (ndeletable > 0)
                {
                        _bt_delitems(rel, buf, deletable, ndeletable);
                        stats->tuples_removed += ndeletable;
                        /* must recompute maxoff */
                        maxoff = PageGetMaxOffsetNumber(page);
                }
                else
                {
                        /*
                         * If the page has been split during this vacuum cycle, it seems
                         * worth expending a write to clear btpo_cycleid even if we don't
                         * have any deletions to do.  (If we do, _bt_delitems takes care
                         * of this.)  This ensures we won't process the page again.
                         *
                         * We treat this like a hint-bit update because there's no need to
                         * WAL-log it.
                         */
                        if (vstate->cycleid != 0 &&
                                opaque->btpo_cycleid == vstate->cycleid)
                        {
                                opaque->btpo_cycleid = 0;
                                SetBufferCommitInfoNeedsSave(buf);
                        }
                }

                /*
                 * If it's now empty, try to delete; else count the live tuples. We
                 * don't delete when recursing, though, to avoid putting entries into
                 * freePages out-of-order (doesn't seem worth any extra code to handle
                 * the case).
                 */
                if (minoff > maxoff)
                        delete_now = (blkno == orig_blkno);
                else
                        stats->num_index_tuples += maxoff - minoff + 1;
        }

        if (delete_now)
        {
                MemoryContext oldcontext;
                int                     ndel;

                /* Run pagedel in a temp context to avoid memory leakage */
                MemoryContextReset(vstate->pagedelcontext);
                oldcontext = MemoryContextSwitchTo(vstate->pagedelcontext);

                ndel = _bt_pagedel(rel, buf, NULL, info->vacuum_full);

                /* count only this page, else may double-count parent */
                if (ndel)
                        stats->pages_deleted++;

                /*
                 * During VACUUM FULL it's okay to recycle deleted pages immediately,
                 * since there can be no other transactions scanning the index.  Note
                 * that we will only recycle the current page and not any parent pages
                 * that _bt_pagedel might have recursed to; this seems reasonable in
                 * the name of simplicity.      (Trying to do otherwise would mean we'd
                 * have to sort the list of recyclable pages we're building.)
                 */
                if (ndel && info->vacuum_full)
                {
                        RecordFreeIndexPage(rel, blkno);
                        vstate->totFreePages++;
                }

                MemoryContextSwitchTo(oldcontext);
                /* pagedel released buffer, so we shouldn't */
        }
        else
                _bt_relbuf(rel, buf);

        /*
         * 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 != P_NONE)
        {
                blkno = recurse_to;
                goto restart;
        }
}