nbtree.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * 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) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/nbtree/nbtree.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/nbtree.h"
#include "access/relscan.h"
#include "access/stratnum.h"
#include "commands/progress.h"
#include "commands/vacuum.h"
#include "nodes/execnodes.h"
#include "pgstat.h"
#include "storage/bulk_write.h"
#include "storage/condition_variable.h"
#include "storage/indexfsm.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/lwlock.h"
#include "storage/read_stream.h"
#include "utils/datum.h"
#include "utils/fmgrprotos.h"
#include "utils/index_selfuncs.h"
#include "utils/memutils.h"
#include "utils/wait_event.h"
 
 
/*
 * BTPARALLEL_NOT_INITIALIZED indicates that the scan has not started.
 *
 * BTPARALLEL_NEED_PRIMSCAN indicates that some process must now seize the
 * scan to advance it via another call to _bt_first.
 *
 * BTPARALLEL_ADVANCING indicates that some process is advancing the scan to
 * a new page; others must wait.
 *
 * BTPARALLEL_IDLE indicates that no backend is currently advancing the scan
 * to a new page; some process can start doing that.
 *
 * BTPARALLEL_DONE indicates that the scan is complete (including error exit).
 */
typedef enum
{
    BTPARALLEL_NOT_INITIALIZED,
    BTPARALLEL_NEED_PRIMSCAN,
    BTPARALLEL_ADVANCING,
    BTPARALLEL_IDLE,
    BTPARALLEL_DONE,
} BTPS_State;
 
/*
 * BTParallelScanDescData contains btree specific shared information required
 * for parallel scan.
 */
typedef struct BTParallelScanDescData
{
    BlockNumber btps_nextScanPage;  /* next page to be scanned */
    BlockNumber btps_lastCurrPage;  /* page whose sibling link was copied into
                                     * btps_nextScanPage */
    BTPS_State  btps_pageStatus;    /* indicates whether next page is
                                     * available for scan. see above for
                                     * possible states of parallel scan. */
    LWLock      btps_lock;      /* protects shared parallel state */
    ConditionVariable btps_cv;  /* used to synchronize parallel scan */
 
    /*
     * btps_arrElems is used when scans need to schedule another primitive
     * index scan with one or more SAOP arrays.  Holds BTArrayKeyInfo.cur_elem
     * offsets for each = scan key associated with a ScalarArrayOp array.
     */
    int         btps_arrElems[FLEXIBLE_ARRAY_MEMBER];
 
    /*
     * Additional space (at the end of the struct) is used when scans need to
     * schedule another primitive index scan with one or more skip arrays.
     * Holds a flattened datum representation for each = scan key associated
     * with a skip array.
     */
}           BTParallelScanDescData;
 
typedef struct BTParallelScanDescData *BTParallelScanDesc;
 
 
static bool _bt_start_prim_scan(IndexScanDesc scan);
static void _bt_parallel_serialize_arrays(Relation rel, BTParallelScanDesc btscan,
                                          BTScanOpaque so);
static void _bt_parallel_restore_arrays(Relation rel, BTParallelScanDesc btscan,
                                        BTScanOpaque so);
static void btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
                         IndexBulkDeleteCallback callback, void *callback_state,
                         BTCycleId cycleid);
static BlockNumber btvacuumpage(BTVacState *vstate, Buffer buf);
static BTVacuumPosting btreevacuumposting(BTVacState *vstate,
                                          IndexTuple posting,
                                          OffsetNumber updatedoffset,
                                          int *nremaining);
 
 
/*
 * Btree handler function: return IndexAmRoutine with access method parameters
 * and callbacks.
 */
Datum
bthandler(PG_FUNCTION_ARGS)
{
    static const IndexAmRoutine amroutine = {
        .type = T_IndexAmRoutine,
        .amstrategies = BTMaxStrategyNumber,
        .amsupport = BTNProcs,
        .amoptsprocnum = BTOPTIONS_PROC,
        .amcanorder = true,
        .amcanorderbyop = false,
        .amcanhash = false,
        .amconsistentequality = true,
        .amconsistentordering = true,
        .amcanbackward = true,
        .amcanunique = true,
        .amcanmulticol = true,
        .amoptionalkey = true,
        .amsearcharray = true,
        .amsearchnulls = true,
        .amstorage = false,
        .amclusterable = true,
        .ampredlocks = true,
        .amcanparallel = true,
        .amcanbuildparallel = true,
        .amcaninclude = true,
        .amusemaintenanceworkmem = false,
        .amsummarizing = false,
        .amparallelvacuumoptions =
        VACUUM_OPTION_PARALLEL_BULKDEL | VACUUM_OPTION_PARALLEL_COND_CLEANUP,
        .amkeytype = InvalidOid,
 
        .ambuild = btbuild,
        .ambuildempty = btbuildempty,
        .aminsert = btinsert,
        .aminsertcleanup = NULL,
        .ambulkdelete = btbulkdelete,
        .amvacuumcleanup = btvacuumcleanup,
        .amcanreturn = btcanreturn,
        .amcostestimate = btcostestimate,
        .amgettreeheight = btgettreeheight,
        .amoptions = btoptions,
        .amproperty = btproperty,
        .ambuildphasename = btbuildphasename,
        .amvalidate = btvalidate,
        .amadjustmembers = btadjustmembers,
        .ambeginscan = btbeginscan,
        .amrescan = btrescan,
        .amgettuple = btgettuple,
        .amgetbitmap = btgetbitmap,
        .amendscan = btendscan,
        .ammarkpos = btmarkpos,
        .amrestrpos = btrestrpos,
        .amestimateparallelscan = btestimateparallelscan,
        .aminitparallelscan = btinitparallelscan,
        .amparallelrescan = btparallelrescan,
        .amtranslatestrategy = bttranslatestrategy,
        .amtranslatecmptype = bttranslatecmptype,
    };
 
    PG_RETURN_POINTER(&amroutine);
}
 
/*
 *  btbuildempty() -- build an empty btree index in the initialization fork
 */
void
btbuildempty(Relation index)
{
    bool        allequalimage = _bt_allequalimage(index, false);
    BulkWriteState *bulkstate;
    BulkWriteBuffer metabuf;
 
    bulkstate = smgr_bulk_start_rel(index, INIT_FORKNUM);
 
    /* Construct metapage. */
    metabuf = smgr_bulk_get_buf(bulkstate);
    _bt_initmetapage((Page) metabuf, P_NONE, 0, allequalimage);
    smgr_bulk_write(bulkstate, BTREE_METAPAGE, metabuf, true);
 
    smgr_bulk_finish(bulkstate);
}
 
/*
 *  btinsert() -- insert an index tuple into a btree.
 *
 *      Descend the tree recursively, find the appropriate location for our
 *      new tuple, and put it there.
 */
bool
btinsert(Relation rel, Datum *values, bool *isnull,
         ItemPointer ht_ctid, Relation heapRel,
         IndexUniqueCheck checkUnique,
         bool indexUnchanged,
         IndexInfo *indexInfo)
{
    bool        result;
    IndexTuple  itup;
 
    /* generate an index tuple */
    itup = index_form_tuple(RelationGetDescr(rel), values, isnull);
    itup->t_tid = *ht_ctid;
 
    result = _bt_doinsert(rel, itup, checkUnique, indexUnchanged, heapRel);
 
    pfree(itup);
 
    return result;
}
 
/*
 *  btgettuple() -- Get the next tuple in the scan.
 */
bool
btgettuple(IndexScanDesc scan, ScanDirection dir)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    bool        res;
 
    Assert(scan->heapRelation != NULL);
 
    /* btree indexes are never lossy */
    scan->xs_recheck = false;
 
    /* Each loop iteration performs another primitive index scan */
    do
    {
        /*
         * 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
         * _bt_first() to get the first item in the scan.
         */
        if (!BTScanPosIsValid(so->currPos))
            res = _bt_first(scan, dir);
        else
        {
            /*
             * 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 = palloc_array(int, MaxTIDsPerBTreePage);
                if (so->numKilled < MaxTIDsPerBTreePage)
                    so->killedItems[so->numKilled++] = so->currPos.itemIndex;
            }
 
            /*
             * Now continue the scan.
             */
            res = _bt_next(scan, dir);
        }
 
        /* If we have a tuple, return it ... */
        if (res)
            break;
        /* ... otherwise see if we need another primitive index scan */
    } while (so->numArrayKeys && _bt_start_prim_scan(scan));
 
    return res;
}
 
/*
 * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
 */
int64
btgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    int64       ntids = 0;
    ItemPointer heapTid;
 
    Assert(scan->heapRelation == NULL);
 
    /* Each loop iteration performs another primitive index scan */
    do
    {
        /* Fetch the first page & tuple */
        if (_bt_first(scan, ForwardScanDirection))
        {
            /* Save tuple ID, and continue scanning */
            heapTid = &scan->xs_heaptid;
            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++;
            }
        }
        /* Now see if we need another primitive index scan */
    } while (so->numArrayKeys && _bt_start_prim_scan(scan));
 
    return ntids;
}
 
/*
 *  btbeginscan() -- start a scan on a btree index
 */
IndexScanDesc
btbeginscan(Relation rel, int nkeys, int norderbys)
{
    IndexScanDesc scan;
    BTScanOpaque so;
 
    /* no order by operators allowed */
    Assert(norderbys == 0);
 
    /* get the scan */
    scan = RelationGetIndexScan(rel, nkeys, norderbys);
 
    /* allocate private workspace */
    so = palloc_object(BTScanOpaqueData);
    BTScanPosInvalidate(so->currPos);
    BTScanPosInvalidate(so->markPos);
    if (scan->numberOfKeys > 0)
        so->keyData = (ScanKey) palloc(scan->numberOfKeys * sizeof(ScanKeyData));
    else
        so->keyData = NULL;
 
    so->skipScan = false;
    so->needPrimScan = false;
    so->scanBehind = false;
    so->oppositeDirCheck = false;
    so->arrayKeys = NULL;
    so->orderProcs = NULL;
    so->arrayContext = NULL;
 
    so->killedItems = NULL;     /* until needed */
    so->numKilled = 0;
 
    /*
     * We don't know yet whether the scan will be index-only, so we do not
     * allocate the tuple workspace arrays until btrescan.  However, we set up
     * scan->xs_itupdesc whether we'll need it or not, since that's so cheap.
     */
    so->currTuples = so->markTuples = NULL;
 
    scan->xs_itupdesc = RelationGetDescr(rel);
 
    scan->opaque = so;
 
    return scan;
}
 
/*
 *  btrescan() -- rescan an index relation
 */
void
btrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
         ScanKey orderbys, int norderbys)
{
    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);
        BTScanPosUnpinIfPinned(so->currPos);
        BTScanPosInvalidate(so->currPos);
    }
 
    /*
     * We prefer to eagerly drop leaf page pins before btgettuple returns.
     * This avoids making VACUUM wait to acquire a cleanup lock on the page.
     *
     * We cannot safely drop leaf page pins during index-only scans due to a
     * race condition involving VACUUM setting pages all-visible in the VM.
     * It's also unsafe for plain index scans that use a non-MVCC snapshot.
     *
     * Also opt out of dropping leaf page pins eagerly during bitmap scans.
     * Pins cannot be held for more than an instant during bitmap scans either
     * way, so we might as well avoid wasting cycles on acquiring page LSNs.
     *
     * See nbtree/README section on making concurrent TID recycling safe.
     *
     * Note: so->dropPin should never change across rescans.
     */
    so->dropPin = (!scan->xs_want_itup &&
                   IsMVCCLikeSnapshot(scan->xs_snapshot) &&
                   scan->heapRelation != NULL);
 
    so->markItemIndex = -1;
    so->needPrimScan = false;
    so->scanBehind = false;
    so->oppositeDirCheck = false;
    BTScanPosUnpinIfPinned(so->markPos);
    BTScanPosInvalidate(so->markPos);
 
    /*
     * Allocate tuple workspace arrays, if needed for an index-only scan and
     * not already done in a previous rescan call.  To save on palloc
     * overhead, both workspaces are allocated as one palloc block; only this
     * function and btendscan know that.
     */
    if (scan->xs_want_itup && so->currTuples == NULL)
    {
        so->currTuples = (char *) palloc(BLCKSZ * 2);
        so->markTuples = so->currTuples + BLCKSZ;
    }
 
    /*
     * Reset the scan keys
     */
    if (scankey && scan->numberOfKeys > 0)
        memcpy(scan->keyData, scankey, scan->numberOfKeys * sizeof(ScanKeyData));
    so->numberOfKeys = 0;       /* until _bt_preprocess_keys sets it */
    so->numArrayKeys = 0;       /* ditto */
}
 
/*
 *  btendscan() -- close down a scan
 */
void
btendscan(IndexScanDesc scan)
{
    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);
        BTScanPosUnpinIfPinned(so->currPos);
    }
 
    so->markItemIndex = -1;
    BTScanPosUnpinIfPinned(so->markPos);
 
    /* No need to invalidate positions, the RAM is about to be freed. */
 
    /* Release storage */
    if (so->keyData != NULL)
        pfree(so->keyData);
    /* so->arrayKeys and so->orderProcs are in arrayContext */
    if (so->arrayContext != NULL)
        MemoryContextDelete(so->arrayContext);
    if (so->killedItems != NULL)
        pfree(so->killedItems);
    if (so->currTuples != NULL)
        pfree(so->currTuples);
    /* so->markTuples should not be pfree'd, see btrescan */
    pfree(so);
}
 
/*
 *  btmarkpos() -- save current scan position
 */
void
btmarkpos(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
    /* There may be an old mark with a pin (but no lock). */
    BTScanPosUnpinIfPinned(so->markPos);
 
    /*
     * 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
    {
        BTScanPosInvalidate(so->markPos);
        so->markItemIndex = -1;
    }
}
 
/*
 *  btrestrpos() -- restore scan to last saved position
 */
void
btrestrpos(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
    if (so->markItemIndex >= 0)
    {
        /*
         * The scan has never moved to a new page since the last mark.  Just
         * restore the itemIndex.
         *
         * NB: In this case we can't count on anything in so->markPos to be
         * accurate.
         */
        so->currPos.itemIndex = so->markItemIndex;
    }
    else
    {
        /*
         * The scan moved to a new page after last mark or restore, and we are
         * now restoring to the marked page.  We aren't holding any read
         * locks, but if we're still holding the pin for the current position,
         * we must drop it.
         */
        if (BTScanPosIsValid(so->currPos))
        {
            /* Before leaving current page, deal with any killed items */
            if (so->numKilled > 0)
                _bt_killitems(scan);
            BTScanPosUnpinIfPinned(so->currPos);
        }
 
        if (BTScanPosIsValid(so->markPos))
        {
            /* bump pin on mark buffer for assignment to current buffer */
            if (BTScanPosIsPinned(so->markPos))
                IncrBufferRefCount(so->markPos.buf);
            memcpy(&so->currPos, &so->markPos,
                   offsetof(BTScanPosData, items[1]) +
                   so->markPos.lastItem * sizeof(BTScanPosItem));
            if (so->currTuples)
                memcpy(so->currTuples, so->markTuples,
                       so->markPos.nextTupleOffset);
            /* Reset the scan's array keys (see _bt_steppage for why) */
            if (so->numArrayKeys)
            {
                _bt_start_array_keys(scan, so->currPos.dir);
                so->needPrimScan = false;
            }
        }
        else
            BTScanPosInvalidate(so->currPos);
    }
}
 
/*
 * btestimateparallelscan -- estimate storage for BTParallelScanDescData
 */
Size
btestimateparallelscan(Relation rel, int nkeys, int norderbys)
{
    int16       nkeyatts = IndexRelationGetNumberOfKeyAttributes(rel);
    Size        estnbtreeshared,
                genericattrspace;
 
    /*
     * Pessimistically assume that every input scan key will be output with
     * its own SAOP array
     */
    estnbtreeshared = offsetof(BTParallelScanDescData, btps_arrElems) +
        sizeof(int) * nkeys;
 
    /* Single column indexes cannot possibly use a skip array */
    if (nkeyatts == 1)
        return estnbtreeshared;
 
    /*
     * Pessimistically assume that all attributes prior to the least
     * significant attribute require a skip array (and an associated key)
     */
    genericattrspace = datumEstimateSpace((Datum) 0, false, true,
                                          sizeof(Datum));
    for (int attnum = 1; attnum < nkeyatts; attnum++)
    {
        CompactAttribute *attr;
 
        /*
         * We make the conservative assumption that every index column will
         * also require a skip array.
         *
         * Every skip array must have space to store its scan key's sk_flags.
         * We also need space for each skip array's unused btps_arrElems slot
         * (we need to be able to subscript btps_arrElems using a simple
         * so->arrayKeys[]-wise offset for any subsequent SAOP arrays).
         */
        estnbtreeshared = add_size(estnbtreeshared, sizeof(int) * 2);
 
        /* Consider space required to store a datum of opclass input type */
        attr = TupleDescCompactAttr(rel->rd_att, attnum - 1);
        if (attr->attbyval)
        {
            /* This index attribute stores pass-by-value datums */
            Size        estfixed = datumEstimateSpace((Datum) 0, false,
                                                      true, attr->attlen);
 
            estnbtreeshared = add_size(estnbtreeshared, estfixed);
            continue;
        }
 
        /*
         * This index attribute stores pass-by-reference datums.
         *
         * Assume that serializing this array will use just as much space as a
         * pass-by-value datum, in addition to space for the largest possible
         * whole index tuple (this is not just a per-datum portion of the
         * largest possible tuple because that'd be almost as large anyway).
         *
         * This is quite conservative, but it's not clear how we could do much
         * better.  The executor requires an up-front storage request size
         * that reliably covers the scan's high watermark memory usage.  We
         * can't be sure of the real high watermark until the scan is over.
         */
        estnbtreeshared = add_size(estnbtreeshared, genericattrspace);
        estnbtreeshared = add_size(estnbtreeshared, BTMaxItemSize);
    }
 
    return estnbtreeshared;
}
 
/*
 * _bt_start_prim_scan() -- start scheduled primitive index scan?
 *
 * Returns true if _bt_checkkeys scheduled another primitive index scan, just
 * as the last one ended.  Otherwise returns false, indicating that the array
 * keys are now fully exhausted.
 *
 * Only call here during scans with one or more equality type array scan keys,
 * after _bt_first or _bt_next return false.
 */
static bool
_bt_start_prim_scan(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
    Assert(so->numArrayKeys);
 
    so->scanBehind = so->oppositeDirCheck = false;  /* reset */
 
    /*
     * Array keys are advanced within _bt_checkkeys when the scan reaches the
     * leaf level (more precisely, they're advanced when the scan reaches the
     * end of each distinct set of array elements).  This process avoids
     * repeat access to leaf pages (across multiple primitive index scans) by
     * advancing the scan's array keys when it allows the primitive index scan
     * to find nearby matching tuples (or when it eliminates ranges of array
     * key space that can't possibly be satisfied by any index tuple).
     *
     * _bt_checkkeys sets a simple flag variable to schedule another primitive
     * index scan.  The flag tells us what to do.
     *
     * We cannot rely on _bt_first always reaching _bt_checkkeys.  There are
     * various cases where that won't happen.  For example, if the index is
     * completely empty, then _bt_first won't call _bt_readpage/_bt_checkkeys.
     * We also don't expect a call to _bt_checkkeys during searches for a
     * non-existent value that happens to be lower/higher than any existing
     * value in the index.
     *
     * We don't require special handling for these cases -- we don't need to
     * be explicitly instructed to _not_ perform another primitive index scan.
     * It's up to code under the control of _bt_first to always set the flag
     * when another primitive index scan will be required.
     *
     * This works correctly, even with the tricky cases listed above, which
     * all involve access to leaf pages "near the boundaries of the key space"
     * (whether it's from a leftmost/rightmost page, or an imaginary empty
     * leaf root page).  If _bt_checkkeys cannot be reached by a primitive
     * index scan for one set of array keys, then it also won't be reached for
     * any later set ("later" in terms of the direction that we scan the index
     * and advance the arrays).  The array keys won't have advanced in these
     * cases, but that's the correct behavior (even _bt_advance_array_keys
     * won't always advance the arrays at the point they become "exhausted").
     */
    if (so->needPrimScan)
    {
        /*
         * Flag was set -- must call _bt_first again, which will reset the
         * scan's needPrimScan flag
         */
        return true;
    }
 
    /* The top-level index scan ran out of tuples in this scan direction */
    if (scan->parallel_scan != NULL)
        _bt_parallel_done(scan);
 
    return false;
}
 
/*
 * _bt_parallel_serialize_arrays() -- Serialize parallel array state.
 *
 * Caller must have exclusively locked btscan->btps_lock when called.
 */
static void
_bt_parallel_serialize_arrays(Relation rel, BTParallelScanDesc btscan,
                              BTScanOpaque so)
{
    char       *datumshared;
 
    /* Space for serialized datums begins immediately after btps_arrElems[] */
    datumshared = ((char *) &btscan->btps_arrElems[so->numArrayKeys]);
    for (int i = 0; i < so->numArrayKeys; i++)
    {
        BTArrayKeyInfo *array = &so->arrayKeys[i];
        ScanKey     skey = &so->keyData[array->scan_key];
 
        if (array->num_elems != -1)
        {
            /* Save SAOP array's cur_elem (no need to copy key/datum) */
            Assert(!(skey->sk_flags & SK_BT_SKIP));
            btscan->btps_arrElems[i] = array->cur_elem;
            continue;
        }
 
        /* Save all mutable state associated with skip array's key */
        Assert(skey->sk_flags & SK_BT_SKIP);
        memcpy(datumshared, &skey->sk_flags, sizeof(int));
        datumshared += sizeof(int);
 
        if (skey->sk_flags & (SK_BT_MINVAL | SK_BT_MAXVAL))
        {
            /* No sk_argument datum to serialize */
            Assert(skey->sk_argument == 0);
            continue;
        }
 
        datumSerialize(skey->sk_argument, (skey->sk_flags & SK_ISNULL) != 0,
                       array->attbyval, array->attlen, &datumshared);
    }
}
 
/*
 * _bt_parallel_restore_arrays() -- Restore serialized parallel array state.
 *
 * Caller must have exclusively locked btscan->btps_lock when called.
 */
static void
_bt_parallel_restore_arrays(Relation rel, BTParallelScanDesc btscan,
                            BTScanOpaque so)
{
    char       *datumshared;
 
    /* Space for serialized datums begins immediately after btps_arrElems[] */
    datumshared = ((char *) &btscan->btps_arrElems[so->numArrayKeys]);
    for (int i = 0; i < so->numArrayKeys; i++)
    {
        BTArrayKeyInfo *array = &so->arrayKeys[i];
        ScanKey     skey = &so->keyData[array->scan_key];
        bool        isnull;
 
        if (array->num_elems != -1)
        {
            /* Restore SAOP array using its saved cur_elem */
            Assert(!(skey->sk_flags & SK_BT_SKIP));
            array->cur_elem = btscan->btps_arrElems[i];
            skey->sk_argument = array->elem_values[array->cur_elem];
            continue;
        }
 
        /* Restore skip array by restoring its key directly */
        if (!array->attbyval && skey->sk_argument)
            pfree(DatumGetPointer(skey->sk_argument));
        skey->sk_argument = (Datum) 0;
        memcpy(&skey->sk_flags, datumshared, sizeof(int));
        datumshared += sizeof(int);
 
        Assert(skey->sk_flags & SK_BT_SKIP);
 
        if (skey->sk_flags & (SK_BT_MINVAL | SK_BT_MAXVAL))
        {
            /* No sk_argument datum to restore */
            continue;
        }
 
        skey->sk_argument = datumRestore(&datumshared, &isnull);
        if (isnull)
        {
            Assert(skey->sk_argument == 0);
            Assert(skey->sk_flags & SK_SEARCHNULL);
            Assert(skey->sk_flags & SK_ISNULL);
        }
    }
}
 
/*
 * btinitparallelscan -- initialize BTParallelScanDesc for parallel btree scan
 */
void
btinitparallelscan(void *target)
{
    BTParallelScanDesc bt_target = (BTParallelScanDesc) target;
 
    LWLockInitialize(&bt_target->btps_lock,
                     LWTRANCHE_PARALLEL_BTREE_SCAN);
    bt_target->btps_nextScanPage = InvalidBlockNumber;
    bt_target->btps_lastCurrPage = InvalidBlockNumber;
    bt_target->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
    ConditionVariableInit(&bt_target->btps_cv);
}
 
/*
 *  btparallelrescan() -- reset parallel scan
 */
void
btparallelrescan(IndexScanDesc scan)
{
    BTParallelScanDesc btscan;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
 
    Assert(parallel_scan);
 
    btscan = (BTParallelScanDesc) OffsetToPointer(parallel_scan,
                                                  parallel_scan->ps_offset_am);
 
    /*
     * In theory, we don't need to acquire the LWLock here, because there
     * shouldn't be any other workers running at this point, but we do so for
     * consistency.
     */
    LWLockAcquire(&btscan->btps_lock, LW_EXCLUSIVE);
    btscan->btps_nextScanPage = InvalidBlockNumber;
    btscan->btps_lastCurrPage = InvalidBlockNumber;
    btscan->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
    LWLockRelease(&btscan->btps_lock);
}
 
/*
 * _bt_parallel_seize() -- Begin the process of advancing the scan to a new
 *      page.  Other scans must wait until we call _bt_parallel_release()
 *      or _bt_parallel_done().
 *
 * The return value is true if we successfully seized the scan and false
 * if we did not.  The latter case occurs when no pages remain, or when
 * another primitive index scan is scheduled that caller's backend cannot
 * start just yet (only backends that call from _bt_first are capable of
 * starting primitive index scans, which they indicate by passing first=true).
 *
 * If the return value is true, *next_scan_page returns the next page of the
 * scan, and *last_curr_page returns the page that *next_scan_page came from.
 * An invalid *next_scan_page means the scan hasn't yet started, or that
 * caller needs to start the next primitive index scan (if it's the latter
 * case we'll set so.needPrimScan).
 *
 * Callers should ignore the value of *next_scan_page and *last_curr_page if
 * the return value is false.
 */
bool
_bt_parallel_seize(IndexScanDesc scan, BlockNumber *next_scan_page,
                   BlockNumber *last_curr_page, bool first)
{
    Relation    rel = scan->indexRelation;
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    bool        exit_loop = false,
                status = true,
                endscan = false;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
 
    *next_scan_page = InvalidBlockNumber;
    *last_curr_page = InvalidBlockNumber;
 
    /*
     * Reset so->currPos, and initialize moreLeft/moreRight such that the next
     * call to _bt_readnextpage treats this backend similarly to a serial
     * backend that steps from *last_curr_page to *next_scan_page (unless this
     * backend's so->currPos is initialized by _bt_readfirstpage before then).
     */
    BTScanPosInvalidate(so->currPos);
    so->currPos.moreLeft = so->currPos.moreRight = true;
 
    if (first)
    {
        /*
         * Initialize array related state when called from _bt_first, assuming
         * that this will be the first primitive index scan for the scan
         */
        so->needPrimScan = false;
        so->scanBehind = false;
        so->oppositeDirCheck = false;
    }
    else
    {
        /*
         * Don't attempt to seize the scan when it requires another primitive
         * index scan, since caller's backend cannot start it right now
         */
        if (so->needPrimScan)
            return false;
    }
 
    btscan = (BTParallelScanDesc) OffsetToPointer(parallel_scan,
                                                  parallel_scan->ps_offset_am);
 
    while (1)
    {
        LWLockAcquire(&btscan->btps_lock, LW_EXCLUSIVE);
 
        if (btscan->btps_pageStatus == BTPARALLEL_DONE)
        {
            /* We're done with this parallel index scan */
            status = false;
        }
        else if (btscan->btps_pageStatus == BTPARALLEL_IDLE &&
                 btscan->btps_nextScanPage == P_NONE)
        {
            /* End this parallel index scan */
            status = false;
            endscan = true;
        }
        else if (btscan->btps_pageStatus == BTPARALLEL_NEED_PRIMSCAN)
        {
            Assert(so->numArrayKeys);
 
            if (first)
            {
                /* Can start scheduled primitive scan right away, so do so */
                btscan->btps_pageStatus = BTPARALLEL_ADVANCING;
 
                /* Restore scan's array keys from serialized values */
                _bt_parallel_restore_arrays(rel, btscan, so);
                exit_loop = true;
            }
            else
            {
                /*
                 * Don't attempt to seize the scan when it requires another
                 * primitive index scan, since caller's backend cannot start
                 * it right now
                 */
                status = false;
            }
 
            /*
             * Either way, update backend local state to indicate that a
             * pending primitive scan is required
             */
            so->needPrimScan = true;
            so->scanBehind = false;
            so->oppositeDirCheck = false;
        }
        else if (btscan->btps_pageStatus != BTPARALLEL_ADVANCING)
        {
            /*
             * We have successfully seized control of the scan for the purpose
             * of advancing it to a new page!
             */
            btscan->btps_pageStatus = BTPARALLEL_ADVANCING;
            Assert(btscan->btps_nextScanPage != P_NONE);
            *next_scan_page = btscan->btps_nextScanPage;
            *last_curr_page = btscan->btps_lastCurrPage;
            exit_loop = true;
        }
        LWLockRelease(&btscan->btps_lock);
        if (exit_loop || !status)
            break;
        ConditionVariableSleep(&btscan->btps_cv, WAIT_EVENT_BTREE_PAGE);
    }
    ConditionVariableCancelSleep();
 
    /* When the scan has reached the rightmost (or leftmost) page, end it */
    if (endscan)
        _bt_parallel_done(scan);
 
    return status;
}
 
/*
 * _bt_parallel_release() -- Complete the process of advancing the scan to a
 *      new page.  We now have the new value btps_nextScanPage; another backend
 *      can now begin advancing the scan.
 *
 * Callers whose scan uses array keys must save their curr_page argument so
 * that it can be passed to _bt_parallel_primscan_schedule, should caller
 * determine that another primitive index scan is required.
 *
 * If caller's next_scan_page is P_NONE, the scan has reached the index's
 * rightmost/leftmost page.  This is treated as reaching the end of the scan
 * within _bt_parallel_seize.
 *
 * Note: unlike the serial case, parallel scans don't need to remember both
 * sibling links.  next_scan_page is whichever link is next given the scan's
 * direction.  That's all we'll ever need, since the direction of a parallel
 * scan can never change.
 */
void
_bt_parallel_release(IndexScanDesc scan, BlockNumber next_scan_page,
                     BlockNumber curr_page)
{
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
 
    Assert(BlockNumberIsValid(next_scan_page));
 
    btscan = (BTParallelScanDesc) OffsetToPointer(parallel_scan,
                                                  parallel_scan->ps_offset_am);
 
    LWLockAcquire(&btscan->btps_lock, LW_EXCLUSIVE);
    btscan->btps_nextScanPage = next_scan_page;
    btscan->btps_lastCurrPage = curr_page;
    btscan->btps_pageStatus = BTPARALLEL_IDLE;
    LWLockRelease(&btscan->btps_lock);
    ConditionVariableSignal(&btscan->btps_cv);
}
 
/*
 * _bt_parallel_done() -- Mark the parallel scan as complete.
 *
 * When there are no pages left to scan, this function should be called to
 * notify other workers.  Otherwise, they might wait forever for the scan to
 * advance to the next page.
 */
void
_bt_parallel_done(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
    bool        status_changed = false;
 
    Assert(!BTScanPosIsValid(so->currPos));
 
    /* Do nothing, for non-parallel scans */
    if (parallel_scan == NULL)
        return;
 
    /*
     * Should not mark parallel scan done when there's still a pending
     * primitive index scan
     */
    if (so->needPrimScan)
        return;
 
    btscan = (BTParallelScanDesc) OffsetToPointer(parallel_scan,
                                                  parallel_scan->ps_offset_am);
 
    /*
     * Mark the parallel scan as done, unless some other process did so
     * already
     */
    LWLockAcquire(&btscan->btps_lock, LW_EXCLUSIVE);
    Assert(btscan->btps_pageStatus != BTPARALLEL_NEED_PRIMSCAN);
    if (btscan->btps_pageStatus != BTPARALLEL_DONE)
    {
        btscan->btps_pageStatus = BTPARALLEL_DONE;
        status_changed = true;
    }
    LWLockRelease(&btscan->btps_lock);
 
    /* wake up all the workers associated with this parallel scan */
    if (status_changed)
        ConditionVariableBroadcast(&btscan->btps_cv);
}
 
/*
 * _bt_parallel_primscan_schedule() -- Schedule another primitive index scan.
 *
 * Caller passes the curr_page most recently passed to _bt_parallel_release
 * by its backend.  Caller successfully schedules the next primitive index scan
 * if the shared parallel state hasn't been seized since caller's backend last
 * advanced the scan.
 */
void
_bt_parallel_primscan_schedule(IndexScanDesc scan, BlockNumber curr_page)
{
    Relation    rel = scan->indexRelation;
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
 
    Assert(so->numArrayKeys);
 
    btscan = (BTParallelScanDesc) OffsetToPointer(parallel_scan,
                                                  parallel_scan->ps_offset_am);
 
    LWLockAcquire(&btscan->btps_lock, LW_EXCLUSIVE);
    if (btscan->btps_lastCurrPage == curr_page &&
        btscan->btps_pageStatus == BTPARALLEL_IDLE)
    {
        btscan->btps_nextScanPage = InvalidBlockNumber;
        btscan->btps_lastCurrPage = InvalidBlockNumber;
        btscan->btps_pageStatus = BTPARALLEL_NEED_PRIMSCAN;
 
        /* Serialize scan's current array keys */
        _bt_parallel_serialize_arrays(rel, btscan, so);
    }
    LWLockRelease(&btscan->btps_lock);
}
 
/*
 * 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.
 */
IndexBulkDeleteResult *
btbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
             IndexBulkDeleteCallback callback, void *callback_state)
{
    Relation    rel = info->index;
    BTCycleId   cycleid;
 
    /* allocate stats if first time through, else re-use existing struct */
    if (stats == NULL)
        stats = palloc0_object(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);
 
    return stats;
}
 
/*
 * Post-VACUUM cleanup.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
IndexBulkDeleteResult *
btvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
{
    BlockNumber num_delpages;
 
    /* No-op in ANALYZE ONLY mode */
    if (info->analyze_only)
        return stats;
 
    /*
     * If btbulkdelete was called, we need not do anything (we just maintain
     * the information used within _bt_vacuum_needs_cleanup() by calling
     * _bt_set_cleanup_info() below).
     *
     * If btbulkdelete was _not_ called, then we have a choice to make: we
     * must decide whether or not a btvacuumscan() call is needed now (i.e.
     * whether the ongoing VACUUM operation can entirely avoid a physical scan
     * of the index).  A call to _bt_vacuum_needs_cleanup() decides it for us
     * now.
     */
    if (stats == NULL)
    {
        /* Check if VACUUM operation can entirely avoid btvacuumscan() call */
        if (!_bt_vacuum_needs_cleanup(info->index))
            return NULL;
 
        /*
         * Since we aren't going to actually delete any leaf items, there's no
         * need to go through all the vacuum-cycle-ID pushups here.
         *
         * Posting list tuples are a source of inaccuracy for cleanup-only
         * scans.  btvacuumscan() will assume that the number of index tuples
         * from each page can be used as num_index_tuples, even though
         * num_index_tuples is supposed to represent the number of TIDs in the
         * index.  This naive approach can underestimate the number of tuples
         * in the index significantly.
         *
         * We handle the problem by making num_index_tuples an estimate in
         * cleanup-only case.
         */
        stats = palloc0_object(IndexBulkDeleteResult);
        btvacuumscan(info, stats, NULL, NULL, 0);
        stats->estimated_count = true;
    }
 
    /*
     * Maintain num_delpages value in metapage for _bt_vacuum_needs_cleanup().
     *
     * num_delpages is the number of deleted pages now in the index that were
     * not safe to place in the FSM to be recycled just yet.  num_delpages is
     * greater than 0 only when _bt_pagedel() actually deleted pages during
     * our call to btvacuumscan().  Even then, _bt_pendingfsm_finalize() must
     * have failed to place any newly deleted pages in the FSM just moments
     * ago.  (Actually, there are edge cases where recycling of the current
     * VACUUM's newly deleted pages does not even become safe by the time the
     * next VACUUM comes around.  See nbtree/README.)
     */
    Assert(stats->pages_deleted >= stats->pages_free);
    num_delpages = stats->pages_deleted - stats->pages_free;
    _bt_set_cleanup_info(info->index, num_delpages);
 
    /*
     * 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;
}
 
/*
 * 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 and _bt_vacuum_needs_cleanup returned true).
 *
 * 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;
    bool        needLock;
    BlockRangeReadStreamPrivate p;
    ReadStream *stream = NULL;
 
    /*
     * Reset fields that track information about the entire index now.  This
     * avoids double-counting in the case where a single VACUUM command
     * requires multiple scans of the index.
     *
     * Avoid resetting the tuples_removed and pages_newly_deleted fields here,
     * since they track information about the VACUUM command, and so must last
     * across each call to btvacuumscan().
     *
     * (Note that pages_free is treated as state about the whole index, not
     * the current VACUUM.  This is appropriate because RecordFreeIndexPage()
     * calls are idempotent, and get repeated for the same deleted pages in
     * some scenarios.  The point for us is to track the number of recyclable
     * pages in the index at the end of the VACUUM command.)
     */
    stats->num_pages = 0;
    stats->num_index_tuples = 0;
    stats->pages_deleted = 0;
    stats->pages_free = 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;
 
    /* Create a temporary memory context to run _bt_pagedel in */
    vstate.pagedelcontext = AllocSetContextCreate(CurrentMemoryContext,
                                                  "_bt_pagedel",
                                                  ALLOCSET_DEFAULT_SIZES);
 
    /* Initialize vstate fields used by _bt_pendingfsm_finalize */
    vstate.bufsize = 0;
    vstate.maxbufsize = 0;
    vstate.pendingpages = NULL;
    vstate.npendingpages = 0;
    /* Consider applying _bt_pendingfsm_finalize optimization */
    _bt_pendingfsm_init(rel, &vstate, (callback == NULL));
 
    /*
     * 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.  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.
     *
     * XXX: Now that new pages are locked with RBM_ZERO_AND_LOCK, I don't
     * think the use of the extension lock is still required.
     *
     * We can skip locking for new or temp relations, however, since no one
     * else could be accessing them.
     */
    needLock = !RELATION_IS_LOCAL(rel);
 
    p.current_blocknum = BTREE_METAPAGE + 1;
 
    /*
     * It is safe to use batchmode as block_range_read_stream_cb takes no
     * locks.
     */
    stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE |
                                        READ_STREAM_FULL |
                                        READ_STREAM_USE_BATCHING,
                                        info->strategy,
                                        rel,
                                        MAIN_FORKNUM,
                                        block_range_read_stream_cb,
                                        &p,
                                        0);
    for (;;)
    {
        /* Get the current relation length */
        if (needLock)
            LockRelationForExtension(rel, ExclusiveLock);
        num_pages = RelationGetNumberOfBlocks(rel);
        if (needLock)
            UnlockRelationForExtension(rel, ExclusiveLock);
 
        if (info->report_progress)
            pgstat_progress_update_param(PROGRESS_SCAN_BLOCKS_TOTAL,
                                         num_pages);
 
        /* Quit if we've scanned the whole relation */
        if (p.current_blocknum >= num_pages)
            break;
 
        p.last_exclusive = num_pages;
 
        /* Iterate over pages, then loop back to recheck relation length */
        while (true)
        {
            BlockNumber current_block;
            Buffer      buf;
 
            /* call vacuum_delay_point while not holding any buffer lock */
            vacuum_delay_point(false);
 
            buf = read_stream_next_buffer(stream, NULL);
 
            if (!BufferIsValid(buf))
                break;
 
            current_block = btvacuumpage(&vstate, buf);
 
            if (info->report_progress)
                pgstat_progress_update_param(PROGRESS_SCAN_BLOCKS_DONE,
                                             current_block);
        }
 
        /*
         * We have to reset the read stream to use it again. After returning
         * InvalidBuffer, the read stream API won't invoke our callback again
         * until the stream has been reset.
         */
        read_stream_reset(stream);
    }
 
    read_stream_end(stream);
 
    /* Set statistics num_pages field to final size of index */
    stats->num_pages = num_pages;
 
    MemoryContextDelete(vstate.pagedelcontext);
 
    /*
     * If there were any calls to _bt_pagedel() during scan of the index then
     * see if any of the resulting pages can be placed in the FSM now.  When
     * it's not safe we'll have to leave it up to a future VACUUM operation.
     *
     * Finally, if we placed any pages in the FSM (either just now or during
     * the scan), forcibly update the upper-level FSM pages to ensure that
     * searchers can find them.
     */
    _bt_pendingfsm_finalize(rel, &vstate);
    if (stats->pages_free > 0)
        IndexFreeSpaceMapVacuum(rel);
}
 
/*
 * btvacuumpage --- VACUUM one page
 *
 * This processes a single page for btvacuumscan().  In some cases we must
 * backtrack to re-examine and VACUUM pages that were on buf's page during
 * a previous call here.  This is how we handle page splits (that happened
 * after our cycleid was acquired) whose right half page happened to reuse
 * a block that we might have processed at some point before it was
 * recycled (i.e. before the page split).
 *
 * Returns BlockNumber of a scanned page (not backtracked).
 */
static BlockNumber
btvacuumpage(BTVacState *vstate, Buffer buf)
{
    IndexVacuumInfo *info = vstate->info;
    IndexBulkDeleteResult *stats = vstate->stats;
    IndexBulkDeleteCallback callback = vstate->callback;
    void       *callback_state = vstate->callback_state;
    Relation    rel = info->index;
    Relation    heaprel = info->heaprel;
    bool        attempt_pagedel;
    BlockNumber blkno,
                backtrack_to;
    BlockNumber scanblkno = BufferGetBlockNumber(buf);
    Page        page;
    BTPageOpaque opaque;
 
    blkno = scanblkno;
 
backtrack:
 
    attempt_pagedel = false;
    backtrack_to = P_NONE;
 
    _bt_lockbuf(rel, buf, BT_READ);
    page = BufferGetPage(buf);
    opaque = NULL;
    if (!PageIsNew(page))
    {
        _bt_checkpage(rel, buf);
        opaque = BTPageGetOpaque(page);
    }
 
    Assert(blkno <= scanblkno);
    if (blkno != scanblkno)
    {
        /*
         * We're backtracking.
         *
         * We followed a right link to a sibling leaf page (a page that
         * happens to be from a block located before scanblkno).  The only
         * case we want to do anything with is a live leaf page having the
         * current vacuum cycle ID.
         *
         * The page had better be in a state that's consistent with what we
         * expect.  Check for conditions that imply corruption in passing.  It
         * can't be half-dead because only an interrupted VACUUM process can
         * leave pages in that state, so we'd definitely have dealt with it
         * back when the page was the scanblkno page (half-dead pages are
         * always marked fully deleted by _bt_pagedel(), barring corruption).
         */
        if (!opaque || !P_ISLEAF(opaque) || P_ISHALFDEAD(opaque))
        {
            Assert(false);
            ereport(LOG,
                    (errcode(ERRCODE_INDEX_CORRUPTED),
                     errmsg_internal("right sibling %u of scanblkno %u unexpectedly in an inconsistent state in index \"%s\"",
                                     blkno, scanblkno, RelationGetRelationName(rel))));
            _bt_relbuf(rel, buf);
            return scanblkno;
        }
 
        /*
         * We may have already processed the page in an earlier call, when the
         * page was scanblkno.  This happens when the leaf page split occurred
         * after the scan began, but before the right sibling page became the
         * scanblkno.
         *
         * Page may also have been deleted by current btvacuumpage() call,
         * since _bt_pagedel() sometimes deletes the right sibling page of
         * scanblkno in passing (it does so after we decided where to
         * backtrack to).  We don't need to process this page as a deleted
         * page a second time now (in fact, it would be wrong to count it as a
         * deleted page in the bulk delete statistics a second time).
         */
        if (opaque->btpo_cycleid != vstate->cycleid || P_ISDELETED(opaque))
        {
            /* Done with current scanblkno (and all lower split pages) */
            _bt_relbuf(rel, buf);
            return scanblkno;
        }
    }
 
    if (!opaque || BTPageIsRecyclable(page, heaprel))
    {
        /* Okay to recycle this page (which could be leaf or internal) */
        RecordFreeIndexPage(rel, blkno);
        stats->pages_deleted++;
        stats->pages_free++;
    }
    else if (P_ISDELETED(opaque))
    {
        /*
         * Already deleted page (which could be leaf or internal).  Can't
         * recycle yet.
         */
        stats->pages_deleted++;
    }
    else if (P_ISHALFDEAD(opaque))
    {
        /* Half-dead leaf page (from interrupted VACUUM) -- finish deleting */
        attempt_pagedel = true;
 
        /*
         * _bt_pagedel() will increment both pages_newly_deleted and
         * pages_deleted stats in all cases (barring corruption)
         */
    }
    else if (P_ISLEAF(opaque))
    {
        OffsetNumber deletable[MaxIndexTuplesPerPage];
        int         ndeletable;
        BTVacuumPosting updatable[MaxIndexTuplesPerPage];
        int         nupdatable;
        OffsetNumber offnum,
                    minoff,
                    maxoff;
        int         nhtidsdead,
                    nhtidslive;
 
        /*
         * Trade in the initial read lock for a full cleanup 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.
         */
        _bt_upgradelockbufcleanup(rel, buf);
 
        /*
         * Check whether we need to backtrack to earlier pages.  What we are
         * concerned about is a page split that happened since we started the
         * vacuum scan.  If the split moved tuples on the right half of the
         * split (i.e. the tuples that sort high) to a block that we already
         * passed over, then we might have missed the tuples.  We need to
         * backtrack now.  (Must do this before possibly clearing btpo_cycleid
         * or deleting scanblkno page below!)
         */
        if (vstate->cycleid != 0 &&
            opaque->btpo_cycleid == vstate->cycleid &&
            !(opaque->btpo_flags & BTP_SPLIT_END) &&
            !P_RIGHTMOST(opaque) &&
            opaque->btpo_next < scanblkno)
            backtrack_to = opaque->btpo_next;
 
        ndeletable = 0;
        nupdatable = 0;
        minoff = P_FIRSTDATAKEY(opaque);
        maxoff = PageGetMaxOffsetNumber(page);
        nhtidsdead = 0;
        nhtidslive = 0;
        if (callback)
        {
            /* btbulkdelete callback tells us what to delete (or update) */
            for (offnum = minoff;
                 offnum <= maxoff;
                 offnum = OffsetNumberNext(offnum))
            {
                IndexTuple  itup;
 
                itup = (IndexTuple) PageGetItem(page,
                                                PageGetItemId(page, offnum));
 
                Assert(!BTreeTupleIsPivot(itup));
                if (!BTreeTupleIsPosting(itup))
                {
                    /* Regular tuple, standard table TID representation */
                    if (callback(&itup->t_tid, callback_state))
                    {
                        deletable[ndeletable++] = offnum;
                        nhtidsdead++;
                    }
                    else
                        nhtidslive++;
                }
                else
                {
                    BTVacuumPosting vacposting;
                    int         nremaining;
 
                    /* Posting list tuple */
                    vacposting = btreevacuumposting(vstate, itup, offnum,
                                                    &nremaining);
                    if (vacposting == NULL)
                    {
                        /*
                         * All table TIDs from the posting tuple remain, so no
                         * delete or update required
                         */
                        Assert(nremaining == BTreeTupleGetNPosting(itup));
                    }
                    else if (nremaining > 0)
                    {
 
                        /*
                         * Store metadata about posting list tuple in
                         * updatable array for entire page.  Existing tuple
                         * will be updated during the later call to
                         * _bt_delitems_vacuum().
                         */
                        Assert(nremaining < BTreeTupleGetNPosting(itup));
                        updatable[nupdatable++] = vacposting;
                        nhtidsdead += BTreeTupleGetNPosting(itup) - nremaining;
                    }
                    else
                    {
                        /*
                         * All table TIDs from the posting list must be
                         * deleted.  We'll delete the index tuple completely
                         * (no update required).
                         */
                        Assert(nremaining == 0);
                        deletable[ndeletable++] = offnum;
                        nhtidsdead += BTreeTupleGetNPosting(itup);
                        pfree(vacposting);
                    }
 
                    nhtidslive += nremaining;
                }
            }
        }
 
        /*
         * Apply any needed deletes or updates.  We issue just one
         * _bt_delitems_vacuum() call per page, so as to minimize WAL traffic.
         */
        if (ndeletable > 0 || nupdatable > 0)
        {
            Assert(nhtidsdead >= ndeletable + nupdatable);
            _bt_delitems_vacuum(rel, buf, deletable, ndeletable, updatable,
                                nupdatable);
 
            stats->tuples_removed += nhtidsdead;
            /* must recompute maxoff */
            maxoff = PageGetMaxOffsetNumber(page);
 
            /* can't leak memory here */
            for (int i = 0; i < nupdatable; i++)
                pfree(updatable[i]);
        }
        else
        {
            /*
             * If the leaf 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_vacuum
             * 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.
             */
            Assert(nhtidsdead == 0);
            if (vstate->cycleid != 0 &&
                opaque->btpo_cycleid == vstate->cycleid)
            {
                opaque->btpo_cycleid = 0;
                MarkBufferDirtyHint(buf, true);
            }
        }
 
        /*
         * If the leaf page is now empty, try to delete it; else count the
         * live tuples (live table TIDs in posting lists are counted as
         * separate live tuples).  We don't delete when backtracking, though,
         * since that would require teaching _bt_pagedel() about backtracking
         * (doesn't seem worth adding more complexity to deal with that).
         *
         * We don't count the number of live TIDs during cleanup-only calls to
         * btvacuumscan (i.e. when callback is not set).  We count the number
         * of index tuples directly instead.  This avoids the expense of
         * directly examining all of the tuples on each page.  VACUUM will
         * treat num_index_tuples as an estimate in cleanup-only case, so it
         * doesn't matter that this underestimates num_index_tuples
         * significantly in some cases.
         */
        if (minoff > maxoff)
            attempt_pagedel = (blkno == scanblkno);
        else if (callback)
            stats->num_index_tuples += nhtidslive;
        else
            stats->num_index_tuples += maxoff - minoff + 1;
 
        Assert(!attempt_pagedel || nhtidslive == 0);
    }
 
    if (attempt_pagedel)
    {
        MemoryContext oldcontext;
 
        /* Run pagedel in a temp context to avoid memory leakage */
        MemoryContextReset(vstate->pagedelcontext);
        oldcontext = MemoryContextSwitchTo(vstate->pagedelcontext);
 
        /*
         * _bt_pagedel maintains the bulk delete stats on our behalf;
         * pages_newly_deleted and pages_deleted are likely to be incremented
         * during call
         */
        Assert(blkno == scanblkno);
        _bt_pagedel(rel, buf, vstate);
 
        MemoryContextSwitchTo(oldcontext);
        /* pagedel released buffer, so we shouldn't */
    }
    else
        _bt_relbuf(rel, buf);
 
    if (backtrack_to != P_NONE)
    {
        blkno = backtrack_to;
 
        /* check for vacuum delay while not holding any buffer lock */
        vacuum_delay_point(false);
 
        /*
         * 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);
        goto backtrack;
    }
 
    return scanblkno;
}
 
/*
 * btreevacuumposting --- determine TIDs still needed in posting list
 *
 * Returns metadata describing how to build replacement tuple without the TIDs
 * that VACUUM needs to delete.  Returned value is NULL in the common case
 * where no changes are needed to caller's posting list tuple (we avoid
 * allocating memory here as an optimization).
 *
 * The number of TIDs that should remain in the posting list tuple is set for
 * caller in *nremaining.
 */
static BTVacuumPosting
btreevacuumposting(BTVacState *vstate, IndexTuple posting,
                   OffsetNumber updatedoffset, int *nremaining)
{
    int         live = 0;
    int         nitem = BTreeTupleGetNPosting(posting);
    ItemPointer items = BTreeTupleGetPosting(posting);
    BTVacuumPosting vacposting = NULL;
 
    for (int i = 0; i < nitem; i++)
    {
        if (!vstate->callback(items + i, vstate->callback_state))
        {
            /* Live table TID */
            live++;
        }
        else if (vacposting == NULL)
        {
            /*
             * First dead table TID encountered.
             *
             * It's now clear that we need to delete one or more dead table
             * TIDs, so start maintaining metadata describing how to update
             * existing posting list tuple.
             */
            vacposting = palloc(offsetof(BTVacuumPostingData, deletetids) +
                                nitem * sizeof(uint16));
 
            vacposting->itup = posting;
            vacposting->updatedoffset = updatedoffset;
            vacposting->ndeletedtids = 0;
            vacposting->deletetids[vacposting->ndeletedtids++] = i;
        }
        else
        {
            /* Second or subsequent dead table TID */
            vacposting->deletetids[vacposting->ndeletedtids++] = i;
        }
    }
 
    *nremaining = live;
    return vacposting;
}
 
/*
 *  btcanreturn() -- Check whether btree indexes support index-only scans.
 *
 * btrees always do, so this is trivial.
 */
bool
btcanreturn(Relation index, int attno)
{
    return true;
}
 
/*
 * btgettreeheight() -- Compute tree height for use by btcostestimate().
 */
int
btgettreeheight(Relation rel)
{
    return _bt_getrootheight(rel);
}
 
CompareType
bttranslatestrategy(StrategyNumber strategy, Oid opfamily)
{
    switch (strategy)
    {
        case BTLessStrategyNumber:
            return COMPARE_LT;
        case BTLessEqualStrategyNumber:
            return COMPARE_LE;
        case BTEqualStrategyNumber:
            return COMPARE_EQ;
        case BTGreaterEqualStrategyNumber:
            return COMPARE_GE;
        case BTGreaterStrategyNumber:
            return COMPARE_GT;
        default:
            return COMPARE_INVALID;
    }
}
 
StrategyNumber
bttranslatecmptype(CompareType cmptype, Oid opfamily)
{
    switch (cmptype)
    {
        case COMPARE_LT:
            return BTLessStrategyNumber;
        case COMPARE_LE:
            return BTLessEqualStrategyNumber;
        case COMPARE_EQ:
            return BTEqualStrategyNumber;
        case COMPARE_GE:
            return BTGreaterEqualStrategyNumber;
        case COMPARE_GT:
            return BTGreaterStrategyNumber;
        default:
            return InvalidStrategy;
    }
}