ginget.c

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/*-------------------------------------------------------------------------
 *
 * ginget.c
 *    fetch tuples from a GIN scan.
 *
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *          src/backend/access/gin/ginget.c
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/gin_private.h"
#include "access/relscan.h"
#include "common/pg_prng.h"
#include "miscadmin.h"
#include "storage/predicate.h"
#include "utils/datum.h"
#include "utils/memutils.h"
#include "utils/rel.h"
 
/* GUC parameter */
int         GinFuzzySearchLimit = 0;
 
typedef struct pendingPosition
{
    Buffer      pendingBuffer;
    OffsetNumber firstOffset;
    OffsetNumber lastOffset;
    ItemPointerData item;
    bool       *hasMatchKey;
} pendingPosition;
 
 
/*
 * Goes to the next page if current offset is outside of bounds
 */
static bool
moveRightIfItNeeded(GinBtreeData *btree, GinBtreeStack *stack, Snapshot snapshot)
{
    Page        page = BufferGetPage(stack->buffer);
 
    if (stack->off > PageGetMaxOffsetNumber(page))
    {
        /*
         * We scanned the whole page, so we should take right page
         */
        if (GinPageRightMost(page))
            return false;       /* no more pages */
 
        stack->buffer = ginStepRight(stack->buffer, btree->index, GIN_SHARE);
        stack->blkno = BufferGetBlockNumber(stack->buffer);
        stack->off = FirstOffsetNumber;
        PredicateLockPage(btree->index, stack->blkno, snapshot);
    }
 
    return true;
}
 
/*
 * Scan all pages of a posting tree and save all its heap ItemPointers
 * in scanEntry->matchBitmap
 */
static void
scanPostingTree(Relation index, GinScanEntry scanEntry,
                BlockNumber rootPostingTree)
{
    GinBtreeData btree;
    GinBtreeStack *stack;
    Buffer      buffer;
    Page        page;
 
    /* Descend to the leftmost leaf page */
    stack = ginScanBeginPostingTree(&btree, index, rootPostingTree);
    buffer = stack->buffer;
 
    IncrBufferRefCount(buffer); /* prevent unpin in freeGinBtreeStack */
 
    freeGinBtreeStack(stack);
 
    /*
     * Loop iterates through all leaf pages of posting tree
     */
    for (;;)
    {
        page = BufferGetPage(buffer);
        if ((GinPageGetOpaque(page)->flags & GIN_DELETED) == 0)
        {
            int         n = GinDataLeafPageGetItemsToTbm(page, scanEntry->matchBitmap);
 
            scanEntry->predictNumberResult += n;
        }
 
        if (GinPageRightMost(page))
            break;              /* no more pages */
 
        buffer = ginStepRight(buffer, index, GIN_SHARE);
    }
 
    UnlockReleaseBuffer(buffer);
}
 
/*
 * Collects TIDs into scanEntry->matchBitmap for all heap tuples that
 * match the search entry.  This supports three different match modes:
 *
 * 1. Partial-match support: scan from current point until the
 *    comparePartialFn says we're done.
 * 2. SEARCH_MODE_ALL: scan from current point (which should be first
 *    key for the current attnum) until we hit null items or end of attnum
 * 3. SEARCH_MODE_EVERYTHING: scan from current point (which should be first
 *    key for the current attnum) until we hit end of attnum
 *
 * Returns true if done, false if it's necessary to restart scan from scratch
 */
static bool
collectMatchBitmap(GinBtreeData *btree, GinBtreeStack *stack,
                   GinScanEntry scanEntry, Snapshot snapshot)
{
    OffsetNumber attnum;
    Form_pg_attribute attr;
 
    /* Initialize empty bitmap result */
    scanEntry->matchBitmap = tbm_create(work_mem * 1024L, NULL);
 
    /* Null query cannot partial-match anything */
    if (scanEntry->isPartialMatch &&
        scanEntry->queryCategory != GIN_CAT_NORM_KEY)
        return true;
 
    /* Locate tupdesc entry for key column (for attbyval/attlen data) */
    attnum = scanEntry->attnum;
    attr = TupleDescAttr(btree->ginstate->origTupdesc, attnum - 1);
 
    /*
     * Predicate lock entry leaf page, following pages will be locked by
     * moveRightIfItNeeded()
     */
    PredicateLockPage(btree->index,
                      BufferGetBlockNumber(stack->buffer),
                      snapshot);
 
    for (;;)
    {
        Page        page;
        IndexTuple  itup;
        Datum       idatum;
        GinNullCategory icategory;
 
        /*
         * stack->off points to the interested entry, buffer is already locked
         */
        if (moveRightIfItNeeded(btree, stack, snapshot) == false)
            return true;
 
        page = BufferGetPage(stack->buffer);
        itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, stack->off));
 
        /*
         * If tuple stores another attribute then stop scan
         */
        if (gintuple_get_attrnum(btree->ginstate, itup) != attnum)
            return true;
 
        /* Safe to fetch attribute value */
        idatum = gintuple_get_key(btree->ginstate, itup, &icategory);
 
        /*
         * Check for appropriate scan stop conditions
         */
        if (scanEntry->isPartialMatch)
        {
            int32       cmp;
 
            /*
             * In partial match, stop scan at any null (including
             * placeholders); partial matches never match nulls
             */
            if (icategory != GIN_CAT_NORM_KEY)
                return true;
 
            /*----------
             * Check of partial match.
             * case cmp == 0 => match
             * case cmp > 0 => not match and finish scan
             * case cmp < 0 => not match and continue scan
             *----------
             */
            cmp = DatumGetInt32(FunctionCall4Coll(&btree->ginstate->comparePartialFn[attnum - 1],
                                                  btree->ginstate->supportCollation[attnum - 1],
                                                  scanEntry->queryKey,
                                                  idatum,
                                                  UInt16GetDatum(scanEntry->strategy),
                                                  PointerGetDatum(scanEntry->extra_data)));
 
            if (cmp > 0)
                return true;
            else if (cmp < 0)
            {
                stack->off++;
                continue;
            }
        }
        else if (scanEntry->searchMode == GIN_SEARCH_MODE_ALL)
        {
            /*
             * In ALL mode, we are not interested in null items, so we can
             * stop if we get to a null-item placeholder (which will be the
             * last entry for a given attnum).  We do want to include NULL_KEY
             * and EMPTY_ITEM entries, though.
             */
            if (icategory == GIN_CAT_NULL_ITEM)
                return true;
        }
 
        /*
         * OK, we want to return the TIDs listed in this entry.
         */
        if (GinIsPostingTree(itup))
        {
            BlockNumber rootPostingTree = GinGetPostingTree(itup);
 
            /*
             * We should unlock current page (but not unpin) during tree scan
             * to prevent deadlock with vacuum processes.
             *
             * We save current entry value (idatum) to be able to re-find our
             * tuple after re-locking
             */
            if (icategory == GIN_CAT_NORM_KEY)
                idatum = datumCopy(idatum, attr->attbyval, attr->attlen);
 
            LockBuffer(stack->buffer, GIN_UNLOCK);
 
            /*
             * Acquire predicate lock on the posting tree.  We already hold a
             * lock on the entry page, but insertions to the posting tree
             * don't check for conflicts on that level.
             */
            PredicateLockPage(btree->index, rootPostingTree, snapshot);
 
            /* Collect all the TIDs in this entry's posting tree */
            scanPostingTree(btree->index, scanEntry, rootPostingTree);
 
            /*
             * We lock again the entry page and while it was unlocked insert
             * might have occurred, so we need to re-find our position.
             */
            LockBuffer(stack->buffer, GIN_SHARE);
            page = BufferGetPage(stack->buffer);
            if (!GinPageIsLeaf(page))
            {
                /*
                 * Root page becomes non-leaf while we unlock it. We will
                 * start again, this situation doesn't occur often - root can
                 * became a non-leaf only once per life of index.
                 */
                return false;
            }
 
            /* Search forward to re-find idatum */
            for (;;)
            {
                if (moveRightIfItNeeded(btree, stack, snapshot) == false)
                    ereport(ERROR,
                            (errcode(ERRCODE_INTERNAL_ERROR),
                             errmsg("failed to re-find tuple within index \"%s\"",
                                    RelationGetRelationName(btree->index))));
 
                page = BufferGetPage(stack->buffer);
                itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, stack->off));
 
                if (gintuple_get_attrnum(btree->ginstate, itup) == attnum)
                {
                    Datum       newDatum;
                    GinNullCategory newCategory;
 
                    newDatum = gintuple_get_key(btree->ginstate, itup,
                                                &newCategory);
 
                    if (ginCompareEntries(btree->ginstate, attnum,
                                          newDatum, newCategory,
                                          idatum, icategory) == 0)
                        break;  /* Found! */
                }
 
                stack->off++;
            }
 
            if (icategory == GIN_CAT_NORM_KEY && !attr->attbyval)
                pfree(DatumGetPointer(idatum));
        }
        else
        {
            ItemPointer ipd;
            int         nipd;
 
            ipd = ginReadTuple(btree->ginstate, scanEntry->attnum, itup, &nipd);
            tbm_add_tuples(scanEntry->matchBitmap, ipd, nipd, false);
            scanEntry->predictNumberResult += GinGetNPosting(itup);
            pfree(ipd);
        }
 
        /*
         * Done with this entry, go to the next
         */
        stack->off++;
    }
}
 
/*
 * Start* functions setup beginning state of searches: finds correct buffer and pins it.
 */
static void
startScanEntry(GinState *ginstate, GinScanEntry entry, Snapshot snapshot)
{
    GinBtreeData btreeEntry;
    GinBtreeStack *stackEntry;
    Page        page;
    bool        needUnlock;
 
restartScanEntry:
    entry->buffer = InvalidBuffer;
    ItemPointerSetMin(&entry->curItem);
    entry->offset = InvalidOffsetNumber;
    if (entry->list)
        pfree(entry->list);
    entry->list = NULL;
    entry->nlist = 0;
    entry->matchBitmap = NULL;
    entry->matchResult = NULL;
    entry->reduceResult = false;
    entry->predictNumberResult = 0;
 
    /*
     * we should find entry, and begin scan of posting tree or just store
     * posting list in memory
     */
    ginPrepareEntryScan(&btreeEntry, entry->attnum,
                        entry->queryKey, entry->queryCategory,
                        ginstate);
    stackEntry = ginFindLeafPage(&btreeEntry, true, false);
    page = BufferGetPage(stackEntry->buffer);
 
    /* ginFindLeafPage() will have already checked snapshot age. */
    needUnlock = true;
 
    entry->isFinished = true;
 
    if (entry->isPartialMatch ||
        entry->queryCategory == GIN_CAT_EMPTY_QUERY)
    {
        /*
         * btreeEntry.findItem locates the first item >= given search key.
         * (For GIN_CAT_EMPTY_QUERY, it will find the leftmost index item
         * because of the way the GIN_CAT_EMPTY_QUERY category code is
         * assigned.)  We scan forward from there and collect all TIDs needed
         * for the entry type.
         */
        btreeEntry.findItem(&btreeEntry, stackEntry);
        if (collectMatchBitmap(&btreeEntry, stackEntry, entry, snapshot)
            == false)
        {
            /*
             * GIN tree was seriously restructured, so we will cleanup all
             * found data and rescan. See comments near 'return false' in
             * collectMatchBitmap()
             */
            if (entry->matchBitmap)
            {
                if (entry->matchIterator)
                    tbm_end_iterate(entry->matchIterator);
                entry->matchIterator = NULL;
                tbm_free(entry->matchBitmap);
                entry->matchBitmap = NULL;
            }
            LockBuffer(stackEntry->buffer, GIN_UNLOCK);
            freeGinBtreeStack(stackEntry);
            goto restartScanEntry;
        }
 
        if (entry->matchBitmap && !tbm_is_empty(entry->matchBitmap))
        {
            entry->matchIterator = tbm_begin_iterate(entry->matchBitmap);
            entry->isFinished = false;
        }
    }
    else if (btreeEntry.findItem(&btreeEntry, stackEntry))
    {
        IndexTuple  itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, stackEntry->off));
 
        if (GinIsPostingTree(itup))
        {
            BlockNumber rootPostingTree = GinGetPostingTree(itup);
            GinBtreeStack *stack;
            Page        entrypage;
            ItemPointerData minItem;
 
            /*
             * This is an equality scan, so lock the root of the posting tree.
             * It represents a lock on the exact key value, and covers all the
             * items in the posting tree.
             */
            PredicateLockPage(ginstate->index, rootPostingTree, snapshot);
 
            /*
             * We should unlock entry page before touching posting tree to
             * prevent deadlocks with vacuum processes. Because entry is never
             * deleted from page and posting tree is never reduced to the
             * posting list, we can unlock page after getting BlockNumber of
             * root of posting tree.
             */
            LockBuffer(stackEntry->buffer, GIN_UNLOCK);
            needUnlock = false;
 
            stack = ginScanBeginPostingTree(&entry->btree, ginstate->index,
                                            rootPostingTree);
            entry->buffer = stack->buffer;
 
            /*
             * We keep buffer pinned because we need to prevent deletion of
             * page during scan. See GIN's vacuum implementation. RefCount is
             * increased to keep buffer pinned after freeGinBtreeStack() call.
             */
            IncrBufferRefCount(entry->buffer);
 
            entrypage = BufferGetPage(entry->buffer);
 
            /*
             * Load the first page into memory.
             */
            ItemPointerSetMin(&minItem);
            entry->list = GinDataLeafPageGetItems(entrypage, &entry->nlist, minItem);
 
            entry->predictNumberResult = stack->predictNumber * entry->nlist;
 
            LockBuffer(entry->buffer, GIN_UNLOCK);
            freeGinBtreeStack(stack);
            entry->isFinished = false;
        }
        else
        {
            /*
             * Lock the entry leaf page.  This is more coarse-grained than
             * necessary, because it will conflict with any insertions that
             * land on the same leaf page, not only the exact key we searched
             * for.  But locking an individual tuple would require updating
             * that lock whenever it moves because of insertions or vacuums,
             * which seems too complicated.
             */
            PredicateLockPage(ginstate->index,
                              BufferGetBlockNumber(stackEntry->buffer),
                              snapshot);
            if (GinGetNPosting(itup) > 0)
            {
                entry->list = ginReadTuple(ginstate, entry->attnum, itup,
                                           &entry->nlist);
                entry->predictNumberResult = entry->nlist;
 
                entry->isFinished = false;
            }
        }
    }
    else
    {
        /*
         * No entry found.  Predicate lock the leaf page, to lock the place
         * where the entry would've been, had there been one.
         */
        PredicateLockPage(ginstate->index,
                          BufferGetBlockNumber(stackEntry->buffer), snapshot);
    }
 
    if (needUnlock)
        LockBuffer(stackEntry->buffer, GIN_UNLOCK);
    freeGinBtreeStack(stackEntry);
}
 
/*
 * Comparison function for scan entry indexes. Sorts by predictNumberResult,
 * least frequent items first.
 */
static int
entryIndexByFrequencyCmp(const void *a1, const void *a2, void *arg)
{
    const GinScanKey key = (const GinScanKey) arg;
    int         i1 = *(const int *) a1;
    int         i2 = *(const int *) a2;
    uint32      n1 = key->scanEntry[i1]->predictNumberResult;
    uint32      n2 = key->scanEntry[i2]->predictNumberResult;
 
    if (n1 < n2)
        return -1;
    else if (n1 == n2)
        return 0;
    else
        return 1;
}
 
static void
startScanKey(GinState *ginstate, GinScanOpaque so, GinScanKey key)
{
    MemoryContext oldCtx = CurrentMemoryContext;
    int         i;
    int         j;
    int        *entryIndexes;
 
    ItemPointerSetMin(&key->curItem);
    key->curItemMatches = false;
    key->recheckCurItem = false;
    key->isFinished = false;
 
    /*
     * Divide the entries into two distinct sets: required and additional.
     * Additional entries are not enough for a match alone, without any items
     * from the required set, but are needed by the consistent function to
     * decide if an item matches. When scanning, we can skip over items from
     * additional entries that have no corresponding matches in any of the
     * required entries. That speeds up queries like "frequent & rare"
     * considerably, if the frequent term can be put in the additional set.
     *
     * There can be many legal ways to divide them entries into these two
     * sets. A conservative division is to just put everything in the required
     * set, but the more you can put in the additional set, the more you can
     * skip during the scan. To maximize skipping, we try to put as many
     * frequent items as possible into additional, and less frequent ones into
     * required. To do that, sort the entries by frequency
     * (predictNumberResult), and put entries into the required set in that
     * order, until the consistent function says that none of the remaining
     * entries can form a match, without any items from the required set. The
     * rest go to the additional set.
     *
     * Exclude-only scan keys are known to have no required entries.
     */
    if (key->excludeOnly)
    {
        MemoryContextSwitchTo(so->keyCtx);
 
        key->nrequired = 0;
        key->nadditional = key->nentries;
        key->additionalEntries = palloc(key->nadditional * sizeof(GinScanEntry));
        for (i = 0; i < key->nadditional; i++)
            key->additionalEntries[i] = key->scanEntry[i];
    }
    else if (key->nentries > 1)
    {
        MemoryContextSwitchTo(so->tempCtx);
 
        entryIndexes = (int *) palloc(sizeof(int) * key->nentries);
        for (i = 0; i < key->nentries; i++)
            entryIndexes[i] = i;
        qsort_arg(entryIndexes, key->nentries, sizeof(int),
                  entryIndexByFrequencyCmp, key);
 
        for (i = 0; i < key->nentries - 1; i++)
        {
            /* Pass all entries <= i as FALSE, and the rest as MAYBE */
            for (j = 0; j <= i; j++)
                key->entryRes[entryIndexes[j]] = GIN_FALSE;
            for (j = i + 1; j < key->nentries; j++)
                key->entryRes[entryIndexes[j]] = GIN_MAYBE;
 
            if (key->triConsistentFn(key) == GIN_FALSE)
                break;
        }
        /* i is now the last required entry. */
 
        MemoryContextSwitchTo(so->keyCtx);
 
        key->nrequired = i + 1;
        key->nadditional = key->nentries - key->nrequired;
        key->requiredEntries = palloc(key->nrequired * sizeof(GinScanEntry));
        key->additionalEntries = palloc(key->nadditional * sizeof(GinScanEntry));
 
        j = 0;
        for (i = 0; i < key->nrequired; i++)
            key->requiredEntries[i] = key->scanEntry[entryIndexes[j++]];
        for (i = 0; i < key->nadditional; i++)
            key->additionalEntries[i] = key->scanEntry[entryIndexes[j++]];
 
        /* clean up after consistentFn calls (also frees entryIndexes) */
        MemoryContextReset(so->tempCtx);
    }
    else
    {
        MemoryContextSwitchTo(so->keyCtx);
 
        key->nrequired = 1;
        key->nadditional = 0;
        key->requiredEntries = palloc(1 * sizeof(GinScanEntry));
        key->requiredEntries[0] = key->scanEntry[0];
    }
    MemoryContextSwitchTo(oldCtx);
}
 
static void
startScan(IndexScanDesc scan)
{
    GinScanOpaque so = (GinScanOpaque) scan->opaque;
    GinState   *ginstate = &so->ginstate;
    uint32      i;
 
    for (i = 0; i < so->totalentries; i++)
        startScanEntry(ginstate, so->entries[i], scan->xs_snapshot);
 
    if (GinFuzzySearchLimit > 0)
    {
        /*
         * If all of keys more than threshold we will try to reduce result, we
         * hope (and only hope, for intersection operation of array our
         * supposition isn't true), that total result will not more than
         * minimal predictNumberResult.
         */
        bool        reduce = true;
 
        for (i = 0; i < so->totalentries; i++)
        {
            if (so->entries[i]->predictNumberResult <= so->totalentries * GinFuzzySearchLimit)
            {
                reduce = false;
                break;
            }
        }
        if (reduce)
        {
            for (i = 0; i < so->totalentries; i++)
            {
                so->entries[i]->predictNumberResult /= so->totalentries;
                so->entries[i]->reduceResult = true;
            }
        }
    }
 
    /*
     * Now that we have the estimates for the entry frequencies, finish
     * initializing the scan keys.
     */
    for (i = 0; i < so->nkeys; i++)
        startScanKey(ginstate, so, so->keys + i);
}
 
/*
 * Load the next batch of item pointers from a posting tree.
 *
 * Note that we copy the page into GinScanEntry->list array and unlock it, but
 * keep it pinned to prevent interference with vacuum.
 */
static void
entryLoadMoreItems(GinState *ginstate, GinScanEntry entry,
                   ItemPointerData advancePast)
{
    Page        page;
    int         i;
    bool        stepright;
 
    if (!BufferIsValid(entry->buffer))
    {
        entry->isFinished = true;
        return;
    }
 
    /*
     * We have two strategies for finding the correct page: step right from
     * the current page, or descend the tree again from the root. If
     * advancePast equals the current item, the next matching item should be
     * on the next page, so we step right. Otherwise, descend from root.
     */
    if (ginCompareItemPointers(&entry->curItem, &advancePast) == 0)
    {
        stepright = true;
        LockBuffer(entry->buffer, GIN_SHARE);
    }
    else
    {
        GinBtreeStack *stack;
 
        ReleaseBuffer(entry->buffer);
 
        /*
         * Set the search key, and find the correct leaf page.
         */
        if (ItemPointerIsLossyPage(&advancePast))
        {
            ItemPointerSet(&entry->btree.itemptr,
                           GinItemPointerGetBlockNumber(&advancePast) + 1,
                           FirstOffsetNumber);
        }
        else
        {
            ItemPointerSet(&entry->btree.itemptr,
                           GinItemPointerGetBlockNumber(&advancePast),
                           OffsetNumberNext(GinItemPointerGetOffsetNumber(&advancePast)));
        }
        entry->btree.fullScan = false;
        stack = ginFindLeafPage(&entry->btree, true, false);
 
        /* we don't need the stack, just the buffer. */
        entry->buffer = stack->buffer;
        IncrBufferRefCount(entry->buffer);
        freeGinBtreeStack(stack);
        stepright = false;
    }
 
    elog(DEBUG2, "entryLoadMoreItems, %u/%u, skip: %d",
         GinItemPointerGetBlockNumber(&advancePast),
         GinItemPointerGetOffsetNumber(&advancePast),
         !stepright);
 
    page = BufferGetPage(entry->buffer);
    for (;;)
    {
        entry->offset = InvalidOffsetNumber;
        if (entry->list)
        {
            pfree(entry->list);
            entry->list = NULL;
            entry->nlist = 0;
        }
 
        if (stepright)
        {
            /*
             * We've processed all the entries on this page. If it was the
             * last page in the tree, we're done.
             */
            if (GinPageRightMost(page))
            {
                UnlockReleaseBuffer(entry->buffer);
                entry->buffer = InvalidBuffer;
                entry->isFinished = true;
                return;
            }
 
            /*
             * Step to next page, following the right link. then find the
             * first ItemPointer greater than advancePast.
             */
            entry->buffer = ginStepRight(entry->buffer,
                                         ginstate->index,
                                         GIN_SHARE);
            page = BufferGetPage(entry->buffer);
        }
        stepright = true;
 
        if (GinPageGetOpaque(page)->flags & GIN_DELETED)
            continue;           /* page was deleted by concurrent vacuum */
 
        /*
         * The first item > advancePast might not be on this page, but
         * somewhere to the right, if the page was split, or a non-match from
         * another key in the query allowed us to skip some items from this
         * entry. Keep following the right-links until we re-find the correct
         * page.
         */
        if (!GinPageRightMost(page) &&
            ginCompareItemPointers(&advancePast, GinDataPageGetRightBound(page)) >= 0)
        {
            /*
             * the item we're looking is > the right bound of the page, so it
             * can't be on this page.
             */
            continue;
        }
 
        entry->list = GinDataLeafPageGetItems(page, &entry->nlist, advancePast);
 
        for (i = 0; i < entry->nlist; i++)
        {
            if (ginCompareItemPointers(&advancePast, &entry->list[i]) < 0)
            {
                entry->offset = i;
 
                if (GinPageRightMost(page))
                {
                    /* after processing the copied items, we're done. */
                    UnlockReleaseBuffer(entry->buffer);
                    entry->buffer = InvalidBuffer;
                }
                else
                    LockBuffer(entry->buffer, GIN_UNLOCK);
                return;
            }
        }
    }
}
 
#define gin_rand() pg_prng_double(&pg_global_prng_state)
#define dropItem(e) ( gin_rand() > ((double)GinFuzzySearchLimit)/((double)((e)->predictNumberResult)) )
 
/*
 * Sets entry->curItem to next heap item pointer > advancePast, for one entry
 * of one scan key, or sets entry->isFinished to true if there are no more.
 *
 * Item pointers are returned in ascending order.
 *
 * Note: this can return a "lossy page" item pointer, indicating that the
 * entry potentially matches all items on that heap page.  However, it is
 * not allowed to return both a lossy page pointer and exact (regular)
 * item pointers for the same page.  (Doing so would break the key-combination
 * logic in keyGetItem and scanGetItem; see comment in scanGetItem.)  In the
 * current implementation this is guaranteed by the behavior of tidbitmaps.
 */
static void
entryGetItem(GinState *ginstate, GinScanEntry entry,
             ItemPointerData advancePast)
{
    Assert(!entry->isFinished);
 
    Assert(!ItemPointerIsValid(&entry->curItem) ||
           ginCompareItemPointers(&entry->curItem, &advancePast) <= 0);
 
    if (entry->matchBitmap)
    {
        /* A bitmap result */
        BlockNumber advancePastBlk = GinItemPointerGetBlockNumber(&advancePast);
        OffsetNumber advancePastOff = GinItemPointerGetOffsetNumber(&advancePast);
 
        for (;;)
        {
            /*
             * If we've exhausted all items on this block, move to next block
             * in the bitmap.
             */
            while (entry->matchResult == NULL ||
                   (entry->matchResult->ntuples >= 0 &&
                    entry->offset >= entry->matchResult->ntuples) ||
                   entry->matchResult->blockno < advancePastBlk ||
                   (ItemPointerIsLossyPage(&advancePast) &&
                    entry->matchResult->blockno == advancePastBlk))
            {
                entry->matchResult = tbm_iterate(entry->matchIterator);
 
                if (entry->matchResult == NULL)
                {
                    ItemPointerSetInvalid(&entry->curItem);
                    tbm_end_iterate(entry->matchIterator);
                    entry->matchIterator = NULL;
                    entry->isFinished = true;
                    break;
                }
 
                /*
                 * Reset counter to the beginning of entry->matchResult. Note:
                 * entry->offset is still greater than matchResult->ntuples if
                 * matchResult is lossy.  So, on next call we will get next
                 * result from TIDBitmap.
                 */
                entry->offset = 0;
            }
            if (entry->isFinished)
                break;
 
            /*
             * We're now on the first page after advancePast which has any
             * items on it. If it's a lossy result, return that.
             */
            if (entry->matchResult->ntuples < 0)
            {
                ItemPointerSetLossyPage(&entry->curItem,
                                        entry->matchResult->blockno);
 
                /*
                 * We might as well fall out of the loop; we could not
                 * estimate number of results on this page to support correct
                 * reducing of result even if it's enabled.
                 */
                break;
            }
 
            /*
             * Not a lossy page. Skip over any offsets <= advancePast, and
             * return that.
             */
            if (entry->matchResult->blockno == advancePastBlk)
            {
                /*
                 * First, do a quick check against the last offset on the
                 * page. If that's > advancePast, so are all the other
                 * offsets, so just go back to the top to get the next page.
                 */
                if (entry->matchResult->offsets[entry->matchResult->ntuples - 1] <= advancePastOff)
                {
                    entry->offset = entry->matchResult->ntuples;
                    continue;
                }
 
                /* Otherwise scan to find the first item > advancePast */
                while (entry->matchResult->offsets[entry->offset] <= advancePastOff)
                    entry->offset++;
            }
 
            ItemPointerSet(&entry->curItem,
                           entry->matchResult->blockno,
                           entry->matchResult->offsets[entry->offset]);
            entry->offset++;
 
            /* Done unless we need to reduce the result */
            if (!entry->reduceResult || !dropItem(entry))
                break;
        }
    }
    else if (!BufferIsValid(entry->buffer))
    {
        /*
         * A posting list from an entry tuple, or the last page of a posting
         * tree.
         */
        for (;;)
        {
            if (entry->offset >= entry->nlist)
            {
                ItemPointerSetInvalid(&entry->curItem);
                entry->isFinished = true;
                break;
            }
 
            entry->curItem = entry->list[entry->offset++];
 
            /* If we're not past advancePast, keep scanning */
            if (ginCompareItemPointers(&entry->curItem, &advancePast) <= 0)
                continue;
 
            /* Done unless we need to reduce the result */
            if (!entry->reduceResult || !dropItem(entry))
                break;
        }
    }
    else
    {
        /* A posting tree */
        for (;;)
        {
            /* If we've processed the current batch, load more items */
            while (entry->offset >= entry->nlist)
            {
                entryLoadMoreItems(ginstate, entry, advancePast);
 
                if (entry->isFinished)
                {
                    ItemPointerSetInvalid(&entry->curItem);
                    return;
                }
            }
 
            entry->curItem = entry->list[entry->offset++];
 
            /* If we're not past advancePast, keep scanning */
            if (ginCompareItemPointers(&entry->curItem, &advancePast) <= 0)
                continue;
 
            /* Done unless we need to reduce the result */
            if (!entry->reduceResult || !dropItem(entry))
                break;
 
            /*
             * Advance advancePast (so that entryLoadMoreItems will load the
             * right data), and keep scanning
             */
            advancePast = entry->curItem;
        }
    }
}
 
/*
 * Identify the "current" item among the input entry streams for this scan key
 * that is greater than advancePast, and test whether it passes the scan key
 * qual condition.
 *
 * The current item is the smallest curItem among the inputs.  key->curItem
 * is set to that value.  key->curItemMatches is set to indicate whether that
 * TID passes the consistentFn test.  If so, key->recheckCurItem is set true
 * iff recheck is needed for this item pointer (including the case where the
 * item pointer is a lossy page pointer).
 *
 * If all entry streams are exhausted, sets key->isFinished to true.
 *
 * Item pointers must be returned in ascending order.
 *
 * Note: this can return a "lossy page" item pointer, indicating that the
 * key potentially matches all items on that heap page.  However, it is
 * not allowed to return both a lossy page pointer and exact (regular)
 * item pointers for the same page.  (Doing so would break the key-combination
 * logic in scanGetItem.)
 */
static void
keyGetItem(GinState *ginstate, MemoryContext tempCtx, GinScanKey key,
           ItemPointerData advancePast)
{
    ItemPointerData minItem;
    ItemPointerData curPageLossy;
    uint32      i;
    bool        haveLossyEntry;
    GinScanEntry entry;
    GinTernaryValue res;
    MemoryContext oldCtx;
    bool        allFinished;
 
    Assert(!key->isFinished);
 
    /*
     * We might have already tested this item; if so, no need to repeat work.
     * (Note: the ">" case can happen, if advancePast is exact but we
     * previously had to set curItem to a lossy-page pointer.)
     */
    if (ginCompareItemPointers(&key->curItem, &advancePast) > 0)
        return;
 
    /*
     * Find the minimum item > advancePast among the active entry streams.
     *
     * Note: a lossy-page entry is encoded by a ItemPointer with max value for
     * offset (0xffff), so that it will sort after any exact entries for the
     * same page.  So we'll prefer to return exact pointers not lossy
     * pointers, which is good.
     */
    ItemPointerSetMax(&minItem);
    allFinished = true;
    for (i = 0; i < key->nrequired; i++)
    {
        entry = key->requiredEntries[i];
 
        if (entry->isFinished)
            continue;
 
        /*
         * Advance this stream if necessary.
         *
         * In particular, since entry->curItem was initialized with
         * ItemPointerSetMin, this ensures we fetch the first item for each
         * entry on the first call.
         */
        if (ginCompareItemPointers(&entry->curItem, &advancePast) <= 0)
        {
            entryGetItem(ginstate, entry, advancePast);
            if (entry->isFinished)
                continue;
        }
 
        allFinished = false;
        if (ginCompareItemPointers(&entry->curItem, &minItem) < 0)
            minItem = entry->curItem;
    }
 
    if (allFinished && !key->excludeOnly)
    {
        /* all entries are finished */
        key->isFinished = true;
        return;
    }
 
    if (!key->excludeOnly)
    {
        /*
         * For a normal scan key, we now know there are no matches < minItem.
         *
         * If minItem is lossy, it means that there were no exact items on the
         * page among requiredEntries, because lossy pointers sort after exact
         * items. However, there might be exact items for the same page among
         * additionalEntries, so we mustn't advance past them.
         */
        if (ItemPointerIsLossyPage(&minItem))
        {
            if (GinItemPointerGetBlockNumber(&advancePast) <
                GinItemPointerGetBlockNumber(&minItem))
            {
                ItemPointerSet(&advancePast,
                               GinItemPointerGetBlockNumber(&minItem),
                               InvalidOffsetNumber);
            }
        }
        else
        {
            Assert(GinItemPointerGetOffsetNumber(&minItem) > 0);
            ItemPointerSet(&advancePast,
                           GinItemPointerGetBlockNumber(&minItem),
                           OffsetNumberPrev(GinItemPointerGetOffsetNumber(&minItem)));
        }
    }
    else
    {
        /*
         * excludeOnly scan keys don't have any entries that are necessarily
         * present in matching items.  So, we consider the item just after
         * advancePast.
         */
        Assert(key->nrequired == 0);
        ItemPointerSet(&minItem,
                       GinItemPointerGetBlockNumber(&advancePast),
                       OffsetNumberNext(GinItemPointerGetOffsetNumber(&advancePast)));
    }
 
    /*
     * We might not have loaded all the entry streams for this TID yet. We
     * could call the consistent function, passing MAYBE for those entries, to
     * see if it can decide if this TID matches based on the information we
     * have. But if the consistent-function is expensive, and cannot in fact
     * decide with partial information, that could be a big loss. So, load all
     * the additional entries, before calling the consistent function.
     */
    for (i = 0; i < key->nadditional; i++)
    {
        entry = key->additionalEntries[i];
 
        if (entry->isFinished)
            continue;
 
        if (ginCompareItemPointers(&entry->curItem, &advancePast) <= 0)
        {
            entryGetItem(ginstate, entry, advancePast);
            if (entry->isFinished)
                continue;
        }
 
        /*
         * Normally, none of the items in additionalEntries can have a curItem
         * larger than minItem. But if minItem is a lossy page, then there
         * might be exact items on the same page among additionalEntries.
         */
        if (ginCompareItemPointers(&entry->curItem, &minItem) < 0)
        {
            Assert(ItemPointerIsLossyPage(&minItem));
            minItem = entry->curItem;
        }
    }
 
    /*
     * Ok, we've advanced all the entries up to minItem now. Set key->curItem,
     * and perform consistentFn test.
     *
     * Lossy-page entries pose a problem, since we don't know the correct
     * entryRes state to pass to the consistentFn, and we also don't know what
     * its combining logic will be (could be AND, OR, or even NOT). If the
     * logic is OR then the consistentFn might succeed for all items in the
     * lossy page even when none of the other entries match.
     *
     * Our strategy is to call the tri-state consistent function, with the
     * lossy-page entries set to MAYBE, and all the other entries FALSE. If it
     * returns FALSE, none of the lossy items alone are enough for a match, so
     * we don't need to return a lossy-page pointer. Otherwise, return a
     * lossy-page pointer to indicate that the whole heap page must be
     * checked.  (On subsequent calls, we'll do nothing until minItem is past
     * the page altogether, thus ensuring that we never return both regular
     * and lossy pointers for the same page.)
     *
     * An exception is that it doesn't matter what we pass for lossy pointers
     * in "hidden" entries, because the consistentFn's result can't depend on
     * them. We could pass them as MAYBE as well, but if we're using the
     * "shim" implementation of a tri-state consistent function (see
     * ginlogic.c), it's better to pass as few MAYBEs as possible. So pass
     * them as true.
     *
     * Note that only lossy-page entries pointing to the current item's page
     * should trigger this processing; we might have future lossy pages in the
     * entry array, but they aren't relevant yet.
     */
    key->curItem = minItem;
    ItemPointerSetLossyPage(&curPageLossy,
                            GinItemPointerGetBlockNumber(&key->curItem));
    haveLossyEntry = false;
    for (i = 0; i < key->nentries; i++)
    {
        entry = key->scanEntry[i];
        if (entry->isFinished == false &&
            ginCompareItemPointers(&entry->curItem, &curPageLossy) == 0)
        {
            if (i < key->nuserentries)
                key->entryRes[i] = GIN_MAYBE;
            else
                key->entryRes[i] = GIN_TRUE;
            haveLossyEntry = true;
        }
        else
            key->entryRes[i] = GIN_FALSE;
    }
 
    /* prepare for calling consistentFn in temp context */
    oldCtx = MemoryContextSwitchTo(tempCtx);
 
    if (haveLossyEntry)
    {
        /* Have lossy-page entries, so see if whole page matches */
        res = key->triConsistentFn(key);
 
        if (res == GIN_TRUE || res == GIN_MAYBE)
        {
            /* Yes, so clean up ... */
            MemoryContextSwitchTo(oldCtx);
            MemoryContextReset(tempCtx);
 
            /* and return lossy pointer for whole page */
            key->curItem = curPageLossy;
            key->curItemMatches = true;
            key->recheckCurItem = true;
            return;
        }
    }
 
    /*
     * At this point we know that we don't need to return a lossy whole-page
     * pointer, but we might have matches for individual exact item pointers,
     * possibly in combination with a lossy pointer. Pass lossy pointers as
     * MAYBE to the ternary consistent function, to let it decide if this
     * tuple satisfies the overall key, even though we don't know if the lossy
     * entries match.
     *
     * Prepare entryRes array to be passed to consistentFn.
     */
    for (i = 0; i < key->nentries; i++)
    {
        entry = key->scanEntry[i];
        if (entry->isFinished)
            key->entryRes[i] = GIN_FALSE;
#if 0
 
        /*
         * This case can't currently happen, because we loaded all the entries
         * for this item earlier.
         */
        else if (ginCompareItemPointers(&entry->curItem, &advancePast) <= 0)
            key->entryRes[i] = GIN_MAYBE;
#endif
        else if (ginCompareItemPointers(&entry->curItem, &curPageLossy) == 0)
            key->entryRes[i] = GIN_MAYBE;
        else if (ginCompareItemPointers(&entry->curItem, &minItem) == 0)
            key->entryRes[i] = GIN_TRUE;
        else
            key->entryRes[i] = GIN_FALSE;
    }
 
    res = key->triConsistentFn(key);
 
    switch (res)
    {
        case GIN_TRUE:
            key->curItemMatches = true;
            /* triConsistentFn set recheckCurItem */
            break;
 
        case GIN_FALSE:
            key->curItemMatches = false;
            break;
 
        case GIN_MAYBE:
            key->curItemMatches = true;
            key->recheckCurItem = true;
            break;
 
        default:
 
            /*
             * the 'default' case shouldn't happen, but if the consistent
             * function returns something bogus, this is the safe result
             */
            key->curItemMatches = true;
            key->recheckCurItem = true;
            break;
    }
 
    /*
     * We have a tuple, and we know if it matches or not. If it's a non-match,
     * we could continue to find the next matching tuple, but let's break out
     * and give scanGetItem a chance to advance the other keys. They might be
     * able to skip past to a much higher TID, allowing us to save work.
     */
 
    /* clean up after consistentFn calls */
    MemoryContextSwitchTo(oldCtx);
    MemoryContextReset(tempCtx);
}
 
/*
 * Get next heap item pointer (after advancePast) from scan.
 * Returns true if anything found.
 * On success, *item and *recheck are set.
 *
 * Note: this is very nearly the same logic as in keyGetItem(), except
 * that we know the keys are to be combined with AND logic, whereas in
 * keyGetItem() the combination logic is known only to the consistentFn.
 */
static bool
scanGetItem(IndexScanDesc scan, ItemPointerData advancePast,
            ItemPointerData *item, bool *recheck)
{
    GinScanOpaque so = (GinScanOpaque) scan->opaque;
    uint32      i;
    bool        match;
 
    /*----------
     * Advance the scan keys in lock-step, until we find an item that matches
     * all the keys. If any key reports isFinished, meaning its subset of the
     * entries is exhausted, we can stop.  Otherwise, set *item to the next
     * matching item.
     *
     * This logic works only if a keyGetItem stream can never contain both
     * exact and lossy pointers for the same page.  Else we could have a
     * case like
     *
     *      stream 1        stream 2
     *      ...             ...
     *      42/6            42/7
     *      50/1            42/0xffff
     *      ...             ...
     *
     * We would conclude that 42/6 is not a match and advance stream 1,
     * thus never detecting the match to the lossy pointer in stream 2.
     * (keyGetItem has a similar problem versus entryGetItem.)
     *----------
     */
    do
    {
        ItemPointerSetMin(item);
        match = true;
        for (i = 0; i < so->nkeys && match; i++)
        {
            GinScanKey  key = so->keys + i;
 
            /*
             * If we're considering a lossy page, skip excludeOnly keys. They
             * can't exclude the whole page anyway.
             */
            if (ItemPointerIsLossyPage(item) && key->excludeOnly)
            {
                /*
                 * ginNewScanKey() should never mark the first key as
                 * excludeOnly.
                 */
                Assert(i > 0);
                continue;
            }
 
            /* Fetch the next item for this key that is > advancePast. */
            keyGetItem(&so->ginstate, so->tempCtx, key, advancePast);
 
            if (key->isFinished)
                return false;
 
            /*
             * If it's not a match, we can immediately conclude that nothing
             * <= this item matches, without checking the rest of the keys.
             */
            if (!key->curItemMatches)
            {
                advancePast = key->curItem;
                match = false;
                break;
            }
 
            /*
             * It's a match. We can conclude that nothing < matches, so the
             * other key streams can skip to this item.
             *
             * Beware of lossy pointers, though; from a lossy pointer, we can
             * only conclude that nothing smaller than this *block* matches.
             */
            if (ItemPointerIsLossyPage(&key->curItem))
            {
                if (GinItemPointerGetBlockNumber(&advancePast) <
                    GinItemPointerGetBlockNumber(&key->curItem))
                {
                    ItemPointerSet(&advancePast,
                                   GinItemPointerGetBlockNumber(&key->curItem),
                                   InvalidOffsetNumber);
                }
            }
            else
            {
                Assert(GinItemPointerGetOffsetNumber(&key->curItem) > 0);
                ItemPointerSet(&advancePast,
                               GinItemPointerGetBlockNumber(&key->curItem),
                               OffsetNumberPrev(GinItemPointerGetOffsetNumber(&key->curItem)));
            }
 
            /*
             * If this is the first key, remember this location as a potential
             * match, and proceed to check the rest of the keys.
             *
             * Otherwise, check if this is the same item that we checked the
             * previous keys for (or a lossy pointer for the same page). If
             * not, loop back to check the previous keys for this item (we
             * will check this key again too, but keyGetItem returns quickly
             * for that)
             */
            if (i == 0)
            {
                *item = key->curItem;
            }
            else
            {
                if (ItemPointerIsLossyPage(&key->curItem) ||
                    ItemPointerIsLossyPage(item))
                {
                    Assert(GinItemPointerGetBlockNumber(&key->curItem) >= GinItemPointerGetBlockNumber(item));
                    match = (GinItemPointerGetBlockNumber(&key->curItem) ==
                             GinItemPointerGetBlockNumber(item));
                }
                else
                {
                    Assert(ginCompareItemPointers(&key->curItem, item) >= 0);
                    match = (ginCompareItemPointers(&key->curItem, item) == 0);
                }
            }
        }
    } while (!match);
 
    Assert(!ItemPointerIsMin(item));
 
    /*
     * Now *item contains the first ItemPointer after previous result that
     * satisfied all the keys for that exact TID, or a lossy reference to the
     * same page.
     *
     * We must return recheck = true if any of the keys are marked recheck.
     */
    *recheck = false;
    for (i = 0; i < so->nkeys; i++)
    {
        GinScanKey  key = so->keys + i;
 
        if (key->recheckCurItem)
        {
            *recheck = true;
            break;
        }
    }
 
    return true;
}
 
 
/*
 * Functions for scanning the pending list
 */
 
 
/*
 * Get ItemPointer of next heap row to be checked from pending list.
 * Returns false if there are no more. On pages with several heap rows
 * it returns each row separately, on page with part of heap row returns
 * per page data.  pos->firstOffset and pos->lastOffset are set to identify
 * the range of pending-list tuples belonging to this heap row.
 *
 * The pendingBuffer is presumed pinned and share-locked on entry, and is
 * pinned and share-locked on success exit.  On failure exit it's released.
 */
static bool
scanGetCandidate(IndexScanDesc scan, pendingPosition *pos)
{
    OffsetNumber maxoff;
    Page        page;
    IndexTuple  itup;
 
    ItemPointerSetInvalid(&pos->item);
    for (;;)
    {
        page = BufferGetPage(pos->pendingBuffer);
 
        maxoff = PageGetMaxOffsetNumber(page);
        if (pos->firstOffset > maxoff)
        {
            BlockNumber blkno = GinPageGetOpaque(page)->rightlink;
 
            if (blkno == InvalidBlockNumber)
            {
                UnlockReleaseBuffer(pos->pendingBuffer);
                pos->pendingBuffer = InvalidBuffer;
 
                return false;
            }
            else
            {
                /*
                 * Here we must prevent deletion of next page by insertcleanup
                 * process, which may be trying to obtain exclusive lock on
                 * current page.  So, we lock next page before releasing the
                 * current one
                 */
                Buffer      tmpbuf = ReadBuffer(scan->indexRelation, blkno);
 
                LockBuffer(tmpbuf, GIN_SHARE);
                UnlockReleaseBuffer(pos->pendingBuffer);
 
                pos->pendingBuffer = tmpbuf;
                pos->firstOffset = FirstOffsetNumber;
            }
        }
        else
        {
            itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, pos->firstOffset));
            pos->item = itup->t_tid;
            if (GinPageHasFullRow(page))
            {
                /*
                 * find itempointer to the next row
                 */
                for (pos->lastOffset = pos->firstOffset + 1; pos->lastOffset <= maxoff; pos->lastOffset++)
                {
                    itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, pos->lastOffset));
                    if (!ItemPointerEquals(&pos->item, &itup->t_tid))
                        break;
                }
            }
            else
            {
                /*
                 * All itempointers are the same on this page
                 */
                pos->lastOffset = maxoff + 1;
            }
 
            /*
             * Now pos->firstOffset points to the first tuple of current heap
             * row, pos->lastOffset points to the first tuple of next heap row
             * (or to the end of page)
             */
            break;
        }
    }
 
    return true;
}
 
/*
 * Scan pending-list page from current tuple (off) up till the first of:
 * - match is found (then returns true)
 * - no later match is possible
 * - tuple's attribute number is not equal to entry's attrnum
 * - reach end of page
 *
 * datum[]/category[]/datumExtracted[] arrays are used to cache the results
 * of gintuple_get_key() on the current page.
 */
static bool
matchPartialInPendingList(GinState *ginstate, Page page,
                          OffsetNumber off, OffsetNumber maxoff,
                          GinScanEntry entry,
                          Datum *datum, GinNullCategory *category,
                          bool *datumExtracted)
{
    IndexTuple  itup;
    int32       cmp;
 
    /* Partial match to a null is not possible */
    if (entry->queryCategory != GIN_CAT_NORM_KEY)
        return false;
 
    while (off < maxoff)
    {
        itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, off));
 
        if (gintuple_get_attrnum(ginstate, itup) != entry->attnum)
            return false;
 
        if (datumExtracted[off - 1] == false)
        {
            datum[off - 1] = gintuple_get_key(ginstate, itup,
                                              &category[off - 1]);
            datumExtracted[off - 1] = true;
        }
 
        /* Once we hit nulls, no further match is possible */
        if (category[off - 1] != GIN_CAT_NORM_KEY)
            return false;
 
        /*----------
         * Check partial match.
         * case cmp == 0 => match
         * case cmp > 0 => not match and end scan (no later match possible)
         * case cmp < 0 => not match and continue scan
         *----------
         */
        cmp = DatumGetInt32(FunctionCall4Coll(&ginstate->comparePartialFn[entry->attnum - 1],
                                              ginstate->supportCollation[entry->attnum - 1],
                                              entry->queryKey,
                                              datum[off - 1],
                                              UInt16GetDatum(entry->strategy),
                                              PointerGetDatum(entry->extra_data)));
        if (cmp == 0)
            return true;
        else if (cmp > 0)
            return false;
 
        off++;
    }
 
    return false;
}
 
/*
 * Set up the entryRes array for each key by looking at
 * every entry for current heap row in pending list.
 *
 * Returns true if each scan key has at least one entryRes match.
 * This corresponds to the situations where the normal index search will
 * try to apply the key's consistentFn.  (A tuple not meeting that requirement
 * cannot be returned by the normal search since no entry stream will
 * source its TID.)
 *
 * The pendingBuffer is presumed pinned and share-locked on entry.
 */
static bool
collectMatchesForHeapRow(IndexScanDesc scan, pendingPosition *pos)
{
    GinScanOpaque so = (GinScanOpaque) scan->opaque;
    OffsetNumber attrnum;
    Page        page;
    IndexTuple  itup;
    int         i,
                j;
 
    /*
     * Reset all entryRes and hasMatchKey flags
     */
    for (i = 0; i < so->nkeys; i++)
    {
        GinScanKey  key = so->keys + i;
 
        memset(key->entryRes, GIN_FALSE, key->nentries);
    }
    memset(pos->hasMatchKey, false, so->nkeys);
 
    /*
     * Outer loop iterates over multiple pending-list pages when a single heap
     * row has entries spanning those pages.
     */
    for (;;)
    {
        Datum       datum[BLCKSZ / sizeof(IndexTupleData)];
        GinNullCategory category[BLCKSZ / sizeof(IndexTupleData)];
        bool        datumExtracted[BLCKSZ / sizeof(IndexTupleData)];
 
        Assert(pos->lastOffset > pos->firstOffset);
        memset(datumExtracted + pos->firstOffset - 1, 0,
               sizeof(bool) * (pos->lastOffset - pos->firstOffset));
 
        page = BufferGetPage(pos->pendingBuffer);
 
        for (i = 0; i < so->nkeys; i++)
        {
            GinScanKey  key = so->keys + i;
 
            for (j = 0; j < key->nentries; j++)
            {
                GinScanEntry entry = key->scanEntry[j];
                OffsetNumber StopLow = pos->firstOffset,
                            StopHigh = pos->lastOffset,
                            StopMiddle;
 
                /* If already matched on earlier page, do no extra work */
                if (key->entryRes[j])
                    continue;
 
                /*
                 * Interesting tuples are from pos->firstOffset to
                 * pos->lastOffset and they are ordered by (attnum, Datum) as
                 * it's done in entry tree.  So we can use binary search to
                 * avoid linear scanning.
                 */
                while (StopLow < StopHigh)
                {
                    int         res;
 
                    StopMiddle = StopLow + ((StopHigh - StopLow) >> 1);
 
                    itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, StopMiddle));
 
                    attrnum = gintuple_get_attrnum(&so->ginstate, itup);
 
                    if (key->attnum < attrnum)
                    {
                        StopHigh = StopMiddle;
                        continue;
                    }
                    if (key->attnum > attrnum)
                    {
                        StopLow = StopMiddle + 1;
                        continue;
                    }
 
                    if (datumExtracted[StopMiddle - 1] == false)
                    {
                        datum[StopMiddle - 1] =
                            gintuple_get_key(&so->ginstate, itup,
                                             &category[StopMiddle - 1]);
                        datumExtracted[StopMiddle - 1] = true;
                    }
 
                    if (entry->queryCategory == GIN_CAT_EMPTY_QUERY)
                    {
                        /* special behavior depending on searchMode */
                        if (entry->searchMode == GIN_SEARCH_MODE_ALL)
                        {
                            /* match anything except NULL_ITEM */
                            if (category[StopMiddle - 1] == GIN_CAT_NULL_ITEM)
                                res = -1;
                            else
                                res = 0;
                        }
                        else
                        {
                            /* match everything */
                            res = 0;
                        }
                    }
                    else
                    {
                        res = ginCompareEntries(&so->ginstate,
                                                entry->attnum,
                                                entry->queryKey,
                                                entry->queryCategory,
                                                datum[StopMiddle - 1],
                                                category[StopMiddle - 1]);
                    }
 
                    if (res == 0)
                    {
                        /*
                         * Found exact match (there can be only one, except in
                         * EMPTY_QUERY mode).
                         *
                         * If doing partial match, scan forward from here to
                         * end of page to check for matches.
                         *
                         * See comment above about tuple's ordering.
                         */
                        if (entry->isPartialMatch)
                            key->entryRes[j] =
                                matchPartialInPendingList(&so->ginstate,
                                                          page,
                                                          StopMiddle,
                                                          pos->lastOffset,
                                                          entry,
                                                          datum,
                                                          category,
                                                          datumExtracted);
                        else
                            key->entryRes[j] = true;
 
                        /* done with binary search */
                        break;
                    }
                    else if (res < 0)
                        StopHigh = StopMiddle;
                    else
                        StopLow = StopMiddle + 1;
                }
 
                if (StopLow >= StopHigh && entry->isPartialMatch)
                {
                    /*
                     * No exact match on this page.  If doing partial match,
                     * scan from the first tuple greater than target value to
                     * end of page.  Note that since we don't remember whether
                     * the comparePartialFn told us to stop early on a
                     * previous page, we will uselessly apply comparePartialFn
                     * to the first tuple on each subsequent page.
                     */
                    key->entryRes[j] =
                        matchPartialInPendingList(&so->ginstate,
                                                  page,
                                                  StopHigh,
                                                  pos->lastOffset,
                                                  entry,
                                                  datum,
                                                  category,
                                                  datumExtracted);
                }
 
                pos->hasMatchKey[i] |= key->entryRes[j];
            }
        }
 
        /* Advance firstOffset over the scanned tuples */
        pos->firstOffset = pos->lastOffset;
 
        if (GinPageHasFullRow(page))
        {
            /*
             * We have examined all pending entries for the current heap row.
             * Break out of loop over pages.
             */
            break;
        }
        else
        {
            /*
             * Advance to next page of pending entries for the current heap
             * row.  Complain if there isn't one.
             */
            ItemPointerData item = pos->item;
 
            if (scanGetCandidate(scan, pos) == false ||
                !ItemPointerEquals(&pos->item, &item))
                elog(ERROR, "could not find additional pending pages for same heap tuple");
        }
    }
 
    /*
     * All scan keys except excludeOnly require at least one entry to match.
     * excludeOnly keys are an exception, because their implied
     * GIN_CAT_EMPTY_QUERY scanEntry always matches.  So return "true" if all
     * non-excludeOnly scan keys have at least one match.
     */
    for (i = 0; i < so->nkeys; i++)
    {
        if (pos->hasMatchKey[i] == false && !so->keys[i].excludeOnly)
            return false;
    }
 
    return true;
}
 
/*
 * Collect all matched rows from pending list into bitmap.
 */
static void
scanPendingInsert(IndexScanDesc scan, TIDBitmap *tbm, int64 *ntids)
{
    GinScanOpaque so = (GinScanOpaque) scan->opaque;
    MemoryContext oldCtx;
    bool        recheck,
                match;
    int         i;
    pendingPosition pos;
    Buffer      metabuffer = ReadBuffer(scan->indexRelation, GIN_METAPAGE_BLKNO);
    Page        page;
    BlockNumber blkno;
 
    *ntids = 0;
 
    /*
     * Acquire predicate lock on the metapage, to conflict with any fastupdate
     * insertions.
     */
    PredicateLockPage(scan->indexRelation, GIN_METAPAGE_BLKNO, scan->xs_snapshot);
 
    LockBuffer(metabuffer, GIN_SHARE);
    page = BufferGetPage(metabuffer);
    blkno = GinPageGetMeta(page)->head;
 
    /*
     * fetch head of list before unlocking metapage. head page must be pinned
     * to prevent deletion by vacuum process
     */
    if (blkno == InvalidBlockNumber)
    {
        /* No pending list, so proceed with normal scan */
        UnlockReleaseBuffer(metabuffer);
        return;
    }
 
    pos.pendingBuffer = ReadBuffer(scan->indexRelation, blkno);
    LockBuffer(pos.pendingBuffer, GIN_SHARE);
    pos.firstOffset = FirstOffsetNumber;
    UnlockReleaseBuffer(metabuffer);
    pos.hasMatchKey = palloc(sizeof(bool) * so->nkeys);
 
    /*
     * loop for each heap row. scanGetCandidate returns full row or row's
     * tuples from first page.
     */
    while (scanGetCandidate(scan, &pos))
    {
        /*
         * Check entries in tuple and set up entryRes array.
         *
         * If pending tuples belonging to the current heap row are spread
         * across several pages, collectMatchesForHeapRow will read all of
         * those pages.
         */
        if (!collectMatchesForHeapRow(scan, &pos))
            continue;
 
        /*
         * Matching of entries of one row is finished, so check row using
         * consistent functions.
         */
        oldCtx = MemoryContextSwitchTo(so->tempCtx);
        recheck = false;
        match = true;
 
        for (i = 0; i < so->nkeys; i++)
        {
            GinScanKey  key = so->keys + i;
 
            if (!key->boolConsistentFn(key))
            {
                match = false;
                break;
            }
            recheck |= key->recheckCurItem;
        }
 
        MemoryContextSwitchTo(oldCtx);
        MemoryContextReset(so->tempCtx);
 
        if (match)
        {
            tbm_add_tuples(tbm, &pos.item, 1, recheck);
            (*ntids)++;
        }
    }
 
    pfree(pos.hasMatchKey);
}
 
 
#define GinIsVoidRes(s)     ( ((GinScanOpaque) scan->opaque)->isVoidRes )
 
int64
gingetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
{
    GinScanOpaque so = (GinScanOpaque) scan->opaque;
    int64       ntids;
    ItemPointerData iptr;
    bool        recheck;
 
    /*
     * Set up the scan keys, and check for unsatisfiable query.
     */
    ginFreeScanKeys(so);        /* there should be no keys yet, but just to be
                                 * sure */
    ginNewScanKey(scan);
 
    if (GinIsVoidRes(scan))
        return 0;
 
    ntids = 0;
 
    /*
     * First, scan the pending list and collect any matching entries into the
     * bitmap.  After we scan a pending item, some other backend could post it
     * into the main index, and so we might visit it a second time during the
     * main scan.  This is okay because we'll just re-set the same bit in the
     * bitmap.  (The possibility of duplicate visits is a major reason why GIN
     * can't support the amgettuple API, however.) Note that it would not do
     * to scan the main index before the pending list, since concurrent
     * cleanup could then make us miss entries entirely.
     */
    scanPendingInsert(scan, tbm, &ntids);
 
    /*
     * Now scan the main index.
     */
    startScan(scan);
 
    ItemPointerSetMin(&iptr);
 
    for (;;)
    {
        CHECK_FOR_INTERRUPTS();
 
        if (!scanGetItem(scan, iptr, &iptr, &recheck))
            break;
 
        if (ItemPointerIsLossyPage(&iptr))
            tbm_add_page(tbm, ItemPointerGetBlockNumber(&iptr));
        else
            tbm_add_tuples(tbm, &iptr, 1, recheck);
        ntids++;
    }
 
    return ntids;
}