/src/postgres/src/backend/access/nbtree/nbtreadpage.c

Source
/*-------------------------------------------------------------------------
 *
 * nbtreadpage.c
 *    Leaf page reading for btree index scans.
 *
 * NOTES
 *    This file contains code to return items that satisfy the scan's
 *    search-type scan keys within caller-supplied btree leaf page.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/nbtree/nbtreadpage.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/nbtree.h"
#include "access/relscan.h"
#include "storage/predicate.h"
#include "utils/datum.h"
#include "utils/rel.h"


/*
 * _bt_readpage state used across _bt_checkkeys calls for a page
 */
typedef struct BTReadPageState
{
  /* Input parameters, set by _bt_readpage for _bt_checkkeys */
  ScanDirection dir;      /* current scan direction */
  OffsetNumber minoff;    /* Lowest non-pivot tuple's offset */
  OffsetNumber maxoff;    /* Highest non-pivot tuple's offset */
  IndexTuple  finaltup;   /* Needed by scans with array keys */
  Page    page;     /* Page being read */
  bool    firstpage;    /* page is first for primitive scan? */
  bool    forcenonrequired; /* treat all keys as nonrequired? */
  int     startikey;    /* start comparisons from this scan key */

  /* Per-tuple input parameters, set by _bt_readpage for _bt_checkkeys */
  OffsetNumber offnum;    /* current tuple's page offset number */

  /* Output parameters, set by _bt_checkkeys for _bt_readpage */
  OffsetNumber skip;      /* Array keys "look ahead" skip offnum */
  bool    continuescan; /* Terminate ongoing (primitive) index scan? */

  /*
   * Private _bt_checkkeys state used to manage "look ahead" optimization
   * and primscan scheduling (only used during scans with array keys)
   */
  int16   rechecks;
  int16   targetdistance;
  int16   nskipadvances;

} BTReadPageState;


static void _bt_set_startikey(IndexScanDesc scan, BTReadPageState *pstate);
static bool _bt_scanbehind_checkkeys(IndexScanDesc scan, ScanDirection dir,
                   IndexTuple finaltup);
static bool _bt_oppodir_checkkeys(IndexScanDesc scan, ScanDirection dir,
                  IndexTuple finaltup);
static void _bt_saveitem(BTScanOpaque so, int itemIndex,
             OffsetNumber offnum, IndexTuple itup);
static int  _bt_setuppostingitems(BTScanOpaque so, int itemIndex,
                  OffsetNumber offnum, const ItemPointerData *heapTid,
                  IndexTuple itup);
static inline void _bt_savepostingitem(BTScanOpaque so, int itemIndex,
                     OffsetNumber offnum,
                     ItemPointer heapTid, int tupleOffset);
static bool _bt_checkkeys(IndexScanDesc scan, BTReadPageState *pstate, bool arrayKeys,
              IndexTuple tuple, int tupnatts);
static bool _bt_check_compare(IndexScanDesc scan, ScanDirection dir,
                IndexTuple tuple, int tupnatts, TupleDesc tupdesc,
                bool advancenonrequired, bool forcenonrequired,
                bool *continuescan, int *ikey);
static bool _bt_check_rowcompare(ScanKey header,
                 IndexTuple tuple, int tupnatts, TupleDesc tupdesc,
                 ScanDirection dir, bool forcenonrequired, bool *continuescan);
static bool _bt_rowcompare_cmpresult(ScanKey subkey, int cmpresult);
static bool _bt_tuple_before_array_skeys(IndexScanDesc scan, ScanDirection dir,
                     IndexTuple tuple, TupleDesc tupdesc, int tupnatts,
                     bool readpagetup, int sktrig, bool *scanBehind);
static void _bt_checkkeys_look_ahead(IndexScanDesc scan, BTReadPageState *pstate,
                   int tupnatts, TupleDesc tupdesc);
static bool _bt_advance_array_keys(IndexScanDesc scan, BTReadPageState *pstate,
                   IndexTuple tuple, int tupnatts, TupleDesc tupdesc,
                   int sktrig, bool sktrig_required);
static bool _bt_advance_array_keys_increment(IndexScanDesc scan, ScanDirection dir,
                       bool *skip_array_set);
static bool _bt_array_increment(Relation rel, ScanKey skey, BTArrayKeyInfo *array);
static bool _bt_array_decrement(Relation rel, ScanKey skey, BTArrayKeyInfo *array);
static void _bt_array_set_low_or_high(Relation rel, ScanKey skey,
                    BTArrayKeyInfo *array, bool low_not_high);
static void _bt_skiparray_set_element(Relation rel, ScanKey skey, BTArrayKeyInfo *array,
                    int32 set_elem_result, Datum tupdatum, bool tupnull);
static void _bt_skiparray_set_isnull(Relation rel, ScanKey skey, BTArrayKeyInfo *array);
static inline int32 _bt_compare_array_skey(FmgrInfo *orderproc,
                       Datum tupdatum, bool tupnull,
                       Datum arrdatum, ScanKey cur);
static void _bt_binsrch_skiparray_skey(bool cur_elem_trig, ScanDirection dir,
                     Datum tupdatum, bool tupnull,
                     BTArrayKeyInfo *array, ScanKey cur,
                     int32 *set_elem_result);
#ifdef USE_ASSERT_CHECKING
static bool _bt_verify_keys_with_arraykeys(IndexScanDesc scan);
#endif


/*
 *  _bt_readpage() -- Load data from current index page into so->currPos
 *
 * Caller must have pinned and read-locked so->currPos.buf; the buffer's state
 * is not changed here.  Also, currPos.moreLeft and moreRight must be valid;
 * they are updated as appropriate.  All other fields of so->currPos are
 * initialized from scratch here.
 *
 * We scan the current page starting at offnum and moving in the indicated
 * direction.  All items matching the scan keys are loaded into currPos.items.
 * moreLeft or moreRight (as appropriate) is cleared if _bt_checkkeys reports
 * that there can be no more matching tuples in the current scan direction
 * (could just be for the current primitive index scan when scan has arrays).
 *
 * In the case of a parallel scan, caller must have called _bt_parallel_seize
 * prior to calling this function; this function will invoke
 * _bt_parallel_release before returning.
 *
 * Returns true if any matching items found on the page, false if none.
 */
bool
_bt_readpage(IndexScanDesc scan, ScanDirection dir, OffsetNumber offnum,
       bool firstpage)
{
  Relation  rel = scan->indexRelation;
  BTScanOpaque so = (BTScanOpaque) scan->opaque;
  Page    page;
  BTPageOpaque opaque;
  OffsetNumber minoff;
  OffsetNumber maxoff;
  BTReadPageState pstate;
  bool    arrayKeys,
        ignore_killed_tuples = scan->ignore_killed_tuples;
  int     itemIndex,
        indnatts;

  /* save the page/buffer block number, along with its sibling links */
  page = BufferGetPage(so->currPos.buf);
  opaque = BTPageGetOpaque(page);
  so->currPos.currPage = BufferGetBlockNumber(so->currPos.buf);
  so->currPos.prevPage = opaque->btpo_prev;
  so->currPos.nextPage = opaque->btpo_next;
  /* delay setting so->currPos.lsn until _bt_drop_lock_and_maybe_pin */
  pstate.dir = so->currPos.dir = dir;
  so->currPos.nextTupleOffset = 0;

  /* either moreRight or moreLeft should be set now (may be unset later) */
  Assert(ScanDirectionIsForward(dir) ? so->currPos.moreRight :
       so->currPos.moreLeft);
  Assert(!P_IGNORE(opaque));
  Assert(BTScanPosIsPinned(so->currPos));
  Assert(!so->needPrimScan);

  /* initialize local variables */
  indnatts = IndexRelationGetNumberOfAttributes(rel);
  arrayKeys = so->numArrayKeys != 0;
  minoff = P_FIRSTDATAKEY(opaque);
  maxoff = PageGetMaxOffsetNumber(page);

  /* initialize page-level state that we'll pass to _bt_checkkeys */
  pstate.minoff = minoff;
  pstate.maxoff = maxoff;
  pstate.finaltup = NULL;
  pstate.page = page;
  pstate.firstpage = firstpage;
  pstate.forcenonrequired = false;
  pstate.startikey = 0;
  pstate.offnum = InvalidOffsetNumber;
  pstate.skip = InvalidOffsetNumber;
  pstate.continuescan = true; /* default assumption */
  pstate.rechecks = 0;
  pstate.targetdistance = 0;
  pstate.nskipadvances = 0;

  if (scan->parallel_scan)
  {
    /* allow next/prev page to be read by other worker without delay */
    if (ScanDirectionIsForward(dir))
      _bt_parallel_release(scan, so->currPos.nextPage,
                 so->currPos.currPage);
    else
      _bt_parallel_release(scan, so->currPos.prevPage,
                 so->currPos.currPage);
  }

  PredicateLockPage(rel, so->currPos.currPage, scan->xs_snapshot);

  if (ScanDirectionIsForward(dir))
  {
    /* SK_SEARCHARRAY forward scans must provide high key up front */
    if (arrayKeys)
    {
      if (!P_RIGHTMOST(opaque))
      {
        ItemId    iid = PageGetItemId(page, P_HIKEY);

        pstate.finaltup = (IndexTuple) PageGetItem(page, iid);

        if (unlikely(so->scanBehind) &&
          !_bt_scanbehind_checkkeys(scan, dir, pstate.finaltup))
        {
          /* Schedule another primitive index scan after all */
          so->currPos.moreRight = false;
          so->needPrimScan = true;
          if (scan->parallel_scan)
            _bt_parallel_primscan_schedule(scan,
                             so->currPos.currPage);
          return false;
        }
      }

      so->scanBehind = so->oppositeDirCheck = false;  /* reset */
    }

    /*
     * Consider pstate.startikey optimization once the ongoing primitive
     * index scan has already read at least one page
     */
    if (!pstate.firstpage && minoff < maxoff)
      _bt_set_startikey(scan, &pstate);

    /* load items[] in ascending order */
    itemIndex = 0;

    offnum = Max(offnum, minoff);

    while (offnum <= maxoff)
    {
      ItemId    iid = PageGetItemId(page, offnum);
      IndexTuple  itup;
      bool    passes_quals;

      /*
       * If the scan specifies not to return killed tuples, then we
       * treat a killed tuple as not passing the qual
       */
      if (ignore_killed_tuples && ItemIdIsDead(iid))
      {
        offnum = OffsetNumberNext(offnum);
        continue;
      }

      itup = (IndexTuple) PageGetItem(page, iid);
      Assert(!BTreeTupleIsPivot(itup));

      pstate.offnum = offnum;
      passes_quals = _bt_checkkeys(scan, &pstate, arrayKeys,
                     itup, indnatts);

      /*
       * Check if we need to skip ahead to a later tuple (only possible
       * when the scan uses array keys)
       */
      if (arrayKeys && OffsetNumberIsValid(pstate.skip))
      {
        Assert(!passes_quals && pstate.continuescan);
        Assert(offnum < pstate.skip);
        Assert(!pstate.forcenonrequired);

        offnum = pstate.skip;
        pstate.skip = InvalidOffsetNumber;
        continue;
      }

      if (passes_quals)
      {
        /* tuple passes all scan key conditions */
        if (!BTreeTupleIsPosting(itup))
        {
          /* Remember it */
          _bt_saveitem(so, itemIndex, offnum, itup);
          itemIndex++;
        }
        else
        {
          int     tupleOffset;

          /* Set up posting list state (and remember first TID) */
          tupleOffset =
            _bt_setuppostingitems(so, itemIndex, offnum,
                        BTreeTupleGetPostingN(itup, 0),
                        itup);
          itemIndex++;

          /* Remember all later TIDs (must be at least one) */
          for (int i = 1; i < BTreeTupleGetNPosting(itup); i++)
          {
            _bt_savepostingitem(so, itemIndex, offnum,
                      BTreeTupleGetPostingN(itup, i),
                      tupleOffset);
            itemIndex++;
          }
        }
      }
      /* When !continuescan, there can't be any more matches, so stop */
      if (!pstate.continuescan)
        break;

      offnum = OffsetNumberNext(offnum);
    }

    /*
     * We don't need to visit page to the right when the high key
     * indicates that no more matches will be found there.
     *
     * Checking the high key like this works out more often than you might
     * think.  Leaf page splits pick a split point between the two most
     * dissimilar tuples (this is weighed against the need to evenly share
     * free space).  Leaf pages with high key attribute values that can
     * only appear on non-pivot tuples on the right sibling page are
     * common.
     */
    if (pstate.continuescan && !so->scanBehind && !P_RIGHTMOST(opaque))
    {
      ItemId    iid = PageGetItemId(page, P_HIKEY);
      IndexTuple  itup = (IndexTuple) PageGetItem(page, iid);
      int     truncatt;

      /* Reset arrays, per _bt_set_startikey contract */
      if (pstate.forcenonrequired)
        _bt_start_array_keys(scan, dir);
      pstate.forcenonrequired = false;
      pstate.startikey = 0; /* _bt_set_startikey ignores P_HIKEY */

      truncatt = BTreeTupleGetNAtts(itup, rel);
      _bt_checkkeys(scan, &pstate, arrayKeys, itup, truncatt);
    }

    if (!pstate.continuescan)
      so->currPos.moreRight = false;

    Assert(itemIndex <= MaxTIDsPerBTreePage);
    so->currPos.firstItem = 0;
    so->currPos.lastItem = itemIndex - 1;
    so->currPos.itemIndex = 0;
  }
  else
  {
    /* SK_SEARCHARRAY backward scans must provide final tuple up front */
    if (arrayKeys)
    {
      if (minoff <= maxoff && !P_LEFTMOST(opaque))
      {
        ItemId    iid = PageGetItemId(page, minoff);

        pstate.finaltup = (IndexTuple) PageGetItem(page, iid);

        if (unlikely(so->scanBehind) &&
          !_bt_scanbehind_checkkeys(scan, dir, pstate.finaltup))
        {
          /* Schedule another primitive index scan after all */
          so->currPos.moreLeft = false;
          so->needPrimScan = true;
          if (scan->parallel_scan)
            _bt_parallel_primscan_schedule(scan,
                             so->currPos.currPage);
          return false;
        }
      }

      so->scanBehind = so->oppositeDirCheck = false;  /* reset */
    }

    /*
     * Consider pstate.startikey optimization once the ongoing primitive
     * index scan has already read at least one page
     */
    if (!pstate.firstpage && minoff < maxoff)
      _bt_set_startikey(scan, &pstate);

    /* load items[] in descending order */
    itemIndex = MaxTIDsPerBTreePage;

    offnum = Min(offnum, maxoff);

    while (offnum >= minoff)
    {
      ItemId    iid = PageGetItemId(page, offnum);
      IndexTuple  itup;
      bool    tuple_alive;
      bool    passes_quals;

      /*
       * If the scan specifies not to return killed tuples, then we
       * treat a killed tuple as not passing the qual.  Most of the
       * time, it's a win to not bother examining the tuple's index
       * keys, but just skip to the next tuple (previous, actually,
       * since we're scanning backwards).  However, if this is the first
       * tuple on the page, we do check the index keys, to prevent
       * uselessly advancing to the page to the left.  This is similar
       * to the high key optimization used by forward scans.
       */
      if (ignore_killed_tuples && ItemIdIsDead(iid))
      {
        if (offnum > minoff)
        {
          offnum = OffsetNumberPrev(offnum);
          continue;
        }

        tuple_alive = false;
      }
      else
        tuple_alive = true;

      itup = (IndexTuple) PageGetItem(page, iid);
      Assert(!BTreeTupleIsPivot(itup));

      pstate.offnum = offnum;
      if (arrayKeys && offnum == minoff && pstate.forcenonrequired)
      {
        /* Reset arrays, per _bt_set_startikey contract */
        pstate.forcenonrequired = false;
        pstate.startikey = 0;
        _bt_start_array_keys(scan, dir);
      }
      passes_quals = _bt_checkkeys(scan, &pstate, arrayKeys,
                     itup, indnatts);

      if (arrayKeys && so->scanBehind)
      {
        /*
         * Done scanning this page, but not done with the current
         * primscan.
         *
         * Note: Forward scans don't check this explicitly, since they
         * prefer to reuse pstate.skip for this instead.
         */
        Assert(!passes_quals && pstate.continuescan);
        Assert(!pstate.forcenonrequired);

        break;
      }

      /*
       * Check if we need to skip ahead to a later tuple (only possible
       * when the scan uses array keys)
       */
      if (arrayKeys && OffsetNumberIsValid(pstate.skip))
      {
        Assert(!passes_quals && pstate.continuescan);
        Assert(offnum > pstate.skip);
        Assert(!pstate.forcenonrequired);

        offnum = pstate.skip;
        pstate.skip = InvalidOffsetNumber;
        continue;
      }

      if (passes_quals && tuple_alive)
      {
        /* tuple passes all scan key conditions */
        if (!BTreeTupleIsPosting(itup))
        {
          /* Remember it */
          itemIndex--;
          _bt_saveitem(so, itemIndex, offnum, itup);
        }
        else
        {
          uint16    nitems = BTreeTupleGetNPosting(itup);
          int     tupleOffset;

          /* Set up posting list state (and remember last TID) */
          itemIndex--;
          tupleOffset =
            _bt_setuppostingitems(so, itemIndex, offnum,
                        BTreeTupleGetPostingN(itup, nitems - 1),
                        itup);

          /* Remember all prior TIDs (must be at least one) */
          for (int i = nitems - 2; i >= 0; i--)
          {
            itemIndex--;
            _bt_savepostingitem(so, itemIndex, offnum,
                      BTreeTupleGetPostingN(itup, i),
                      tupleOffset);
          }
        }
      }
      /* When !continuescan, there can't be any more matches, so stop */
      if (!pstate.continuescan)
        break;

      offnum = OffsetNumberPrev(offnum);
    }

    /*
     * We don't need to visit page to the left when no more matches will
     * be found there
     */
    if (!pstate.continuescan)
      so->currPos.moreLeft = false;

    Assert(itemIndex >= 0);
    so->currPos.firstItem = itemIndex;
    so->currPos.lastItem = MaxTIDsPerBTreePage - 1;
    so->currPos.itemIndex = MaxTIDsPerBTreePage - 1;
  }

  /*
   * If _bt_set_startikey told us to temporarily treat the scan's keys as
   * nonrequired (possible only during scans with array keys), there must be
   * no lasting consequences for the scan's array keys.  The scan's arrays
   * should now have exactly the same elements as they would have had if the
   * nonrequired behavior had never been used.  (In general, a scan's arrays
   * are expected to track its progress through the index's key space.)
   *
   * We are required (by _bt_set_startikey) to call _bt_checkkeys against
   * pstate.finaltup with pstate.forcenonrequired=false to allow the scan's
   * arrays to recover.  Assert that that step hasn't been missed.
   */
  Assert(!pstate.forcenonrequired);

  return (so->currPos.firstItem <= so->currPos.lastItem);
}

/*
 * _bt_start_array_keys() -- Initialize array keys at start of a scan
 *
 * Set up the cur_elem counters and fill in the first sk_argument value for
 * each array scankey.
 */
void
_bt_start_array_keys(IndexScanDesc scan, ScanDirection dir)
{
  Relation  rel = scan->indexRelation;
  BTScanOpaque so = (BTScanOpaque) scan->opaque;

  Assert(so->numArrayKeys);
  Assert(so->qual_ok);

  for (int i = 0; i < so->numArrayKeys; i++)
  {
    BTArrayKeyInfo *array = &so->arrayKeys[i];
    ScanKey   skey = &so->keyData[array->scan_key];

    Assert(skey->sk_flags & SK_SEARCHARRAY);
    Assert(skey->sk_strategy == BTEqualStrategyNumber);

    _bt_array_set_low_or_high(rel, skey, array,
                  ScanDirectionIsForward(dir));
  }
  so->scanBehind = so->oppositeDirCheck = false;  /* reset */
}

/*
 * Determines an offset to the first scan key (an so->keyData[]-wise offset)
 * that is _not_ guaranteed to be satisfied by every tuple from pstate.page,
 * which is set in pstate.startikey for _bt_checkkeys calls for the page.
 * This allows caller to save cycles on comparisons of a prefix of keys while
 * reading pstate.page.
 *
 * Also determines if later calls to _bt_checkkeys (for pstate.page) should be
 * forced to treat all required scan keys >= pstate.startikey as nonrequired
 * (that is, if they're to be treated as if any SK_BT_REQFWD/SK_BT_REQBKWD
 * markings that were set by preprocessing were not set at all, for the
 * duration of _bt_checkkeys calls prior to the call for pstate.finaltup).
 * This is indicated to caller by setting pstate.forcenonrequired.
 *
 * Call here at the start of reading a leaf page beyond the first one for the
 * primitive index scan.  We consider all non-pivot tuples, so it doesn't make
 * sense to call here when only a subset of those tuples can ever be read.
 * This is also a good idea on performance grounds; not calling here when on
 * the first page (first for the current primitive scan) avoids wasting cycles
 * during selective point queries.  They typically don't stand to gain as much
 * when we can set pstate.startikey, and are likely to notice the overhead of
 * calling here.  (Also, allowing pstate.forcenonrequired to be set on a
 * primscan's first page would mislead _bt_advance_array_keys, which expects
 * pstate.nskipadvances to be representative of every first page's key space.)
 *
 * Caller must call _bt_start_array_keys and reset startikey/forcenonrequired
 * ahead of the finaltup _bt_checkkeys call when we set forcenonrequired=true.
 * This will give _bt_checkkeys the opportunity to call _bt_advance_array_keys
 * with sktrig_required=true, restoring the invariant that the scan's required
 * arrays always track the scan's progress through the index's key space.
 * Caller won't need to do this on the rightmost/leftmost page in the index
 * (where pstate.finaltup isn't ever set), since forcenonrequired will never
 * be set here in the first place.
 */
static void
_bt_set_startikey(IndexScanDesc scan, BTReadPageState *pstate)
{
  BTScanOpaque so = (BTScanOpaque) scan->opaque;
  Relation  rel = scan->indexRelation;
  TupleDesc tupdesc = RelationGetDescr(rel);
  ItemId    iid;
  IndexTuple  firsttup,
        lasttup;
  int     startikey = 0,
        arrayidx = 0,
        firstchangingattnum;
  bool    start_past_saop_eq = false;

  Assert(!so->scanBehind);
  Assert(pstate->minoff < pstate->maxoff);
  Assert(!pstate->firstpage);
  Assert(pstate->startikey == 0);
  Assert(!so->numArrayKeys || pstate->finaltup ||
       P_RIGHTMOST(BTPageGetOpaque(pstate->page)) ||
       P_LEFTMOST(BTPageGetOpaque(pstate->page)));

  if (so->numberOfKeys == 0)
    return;

  /* minoff is an offset to the lowest non-pivot tuple on the page */
  iid = PageGetItemId(pstate->page, pstate->minoff);
  firsttup = (IndexTuple) PageGetItem(pstate->page, iid);

  /* maxoff is an offset to the highest non-pivot tuple on the page */
  iid = PageGetItemId(pstate->page, pstate->maxoff);
  lasttup = (IndexTuple) PageGetItem(pstate->page, iid);

  /* Determine the first attribute whose values change on caller's page */
  firstchangingattnum = _bt_keep_natts_fast(rel, firsttup, lasttup);

  for (; startikey < so->numberOfKeys; startikey++)
  {
    ScanKey   key = so->keyData + startikey;
    BTArrayKeyInfo *array;
    Datum   firstdatum,
          lastdatum;
    bool    firstnull,
          lastnull;
    int32   result;

    /*
     * Determine if it's safe to set pstate.startikey to an offset to a
     * key that comes after this key, by examining this key
     */
    if (key->sk_flags & SK_ROW_HEADER)
    {
      /* RowCompare inequality (header key) */
      ScanKey   subkey = (ScanKey) DatumGetPointer(key->sk_argument);
      bool    satisfied = false;

      for (;;)
      {
        int     cmpresult;
        bool    firstsatisfies = false;

        if (subkey->sk_attno > firstchangingattnum) /* >, not >= */
          break;   /* unsafe, preceding attr has multiple
                 * distinct values */

        if (subkey->sk_flags & SK_ISNULL)
          break;   /* unsafe, unsatisfiable NULL subkey arg */

        firstdatum = index_getattr(firsttup, subkey->sk_attno,
                       tupdesc, &firstnull);
        lastdatum = index_getattr(lasttup, subkey->sk_attno,
                      tupdesc, &lastnull);

        if (firstnull || lastnull)
          break;   /* unsafe, NULL value won't satisfy subkey */

        /*
         * Compare the first tuple's datum for this row compare member
         */
        cmpresult = DatumGetInt32(FunctionCall2Coll(&subkey->sk_func,
                              subkey->sk_collation,
                              firstdatum,
                              subkey->sk_argument));
        if (subkey->sk_flags & SK_BT_DESC)
          INVERT_COMPARE_RESULT(cmpresult);

        if (cmpresult != 0 || (subkey->sk_flags & SK_ROW_END))
        {
          firstsatisfies = _bt_rowcompare_cmpresult(subkey,
                                cmpresult);
          if (!firstsatisfies)
          {
            /* Unsafe, firstdatum does not satisfy subkey */
            break;
          }
        }

        /*
         * Compare the last tuple's datum for this row compare member
         */
        cmpresult = DatumGetInt32(FunctionCall2Coll(&subkey->sk_func,
                              subkey->sk_collation,
                              lastdatum,
                              subkey->sk_argument));
        if (subkey->sk_flags & SK_BT_DESC)
          INVERT_COMPARE_RESULT(cmpresult);

        if (cmpresult != 0 || (subkey->sk_flags & SK_ROW_END))
        {
          if (!firstsatisfies)
          {
            /*
             * It's only safe to set startikey beyond the row
             * compare header key when both firsttup and lasttup
             * satisfy the key as a whole based on the same
             * deciding subkey/attribute.  That can't happen now.
             */
            break;  /* unsafe */
          }

          satisfied = _bt_rowcompare_cmpresult(subkey, cmpresult);
          break;   /* safe iff 'satisfied' is true */
        }

        /* Move on to next row member/subkey */
        if (subkey->sk_flags & SK_ROW_END)
          break;   /* defensive */
        subkey++;

        /*
         * We deliberately don't check if the next subkey has the same
         * strategy as this iteration's subkey (which happens when
         * subkeys for both ASC and DESC columns are used together),
         * nor if any subkey is marked required.  This is safe because
         * in general all prior index attributes must have only one
         * distinct value (across all of the tuples on the page) in
         * order for us to even consider any subkey's attribute.
         */
      }

      if (satisfied)
      {
        /* Safe, row compare satisfied by every tuple on page */
        continue;
      }

      break;       /* unsafe */
    }
    if (key->sk_strategy != BTEqualStrategyNumber)
    {
      /*
       * Scalar inequality key.
       *
       * It's definitely safe for _bt_checkkeys to avoid assessing this
       * inequality when the page's first and last non-pivot tuples both
       * satisfy the inequality (since the same must also be true of all
       * the tuples in between these two).
       *
       * Unlike the "=" case, it doesn't matter if this attribute has
       * more than one distinct value (though it _is_ necessary for any
       * and all _prior_ attributes to contain no more than one distinct
       * value amongst all of the tuples from pstate.page).
       */
      if (key->sk_attno > firstchangingattnum) /* >, not >= */
        break;     /* unsafe, preceding attr has multiple
                 * distinct values */

      firstdatum = index_getattr(firsttup, key->sk_attno, tupdesc, &firstnull);
      lastdatum = index_getattr(lasttup, key->sk_attno, tupdesc, &lastnull);

      if (key->sk_flags & SK_ISNULL)
      {
        /* IS NOT NULL key */
        Assert(key->sk_flags & SK_SEARCHNOTNULL);

        if (firstnull || lastnull)
          break;   /* unsafe */

        /* Safe, IS NOT NULL key satisfied by every tuple */
        continue;
      }

      /* Test firsttup */
      if (firstnull ||
        !DatumGetBool(FunctionCall2Coll(&key->sk_func,
                        key->sk_collation, firstdatum,
                        key->sk_argument)))
        break;     /* unsafe */

      /* Test lasttup */
      if (lastnull ||
        !DatumGetBool(FunctionCall2Coll(&key->sk_func,
                        key->sk_collation, lastdatum,
                        key->sk_argument)))
        break;     /* unsafe */

      /* Safe, scalar inequality satisfied by every tuple */
      continue;
    }

    /* Some = key (could be a scalar = key, could be an array = key) */
    Assert(key->sk_strategy == BTEqualStrategyNumber);

    if (!(key->sk_flags & SK_SEARCHARRAY))
    {
      /*
       * Scalar = key (possibly an IS NULL key).
       *
       * It is unsafe to set pstate.startikey to an ikey beyond this
       * key, unless the = key is satisfied by every possible tuple on
       * the page (possible only when attribute has just one distinct
       * value among all tuples on the page).
       */
      if (key->sk_attno >= firstchangingattnum)
        break;     /* unsafe, multiple distinct attr values */

      firstdatum = index_getattr(firsttup, key->sk_attno, tupdesc,
                     &firstnull);
      if (key->sk_flags & SK_ISNULL)
      {
        /* IS NULL key */
        Assert(key->sk_flags & SK_SEARCHNULL);

        if (!firstnull)
          break;   /* unsafe */

        /* Safe, IS NULL key satisfied by every tuple */
        continue;
      }
      if (firstnull ||
        !DatumGetBool(FunctionCall2Coll(&key->sk_func,
                        key->sk_collation, firstdatum,
                        key->sk_argument)))
        break;     /* unsafe */

      /* Safe, scalar = key satisfied by every tuple */
      continue;
    }

    /* = array key (could be a SAOP array, could be a skip array) */
    array = &so->arrayKeys[arrayidx++];
    Assert(array->scan_key == startikey);
    if (array->num_elems != -1)
    {
      /*
       * SAOP array = key.
       *
       * Handle this like we handle scalar = keys (though binary search
       * for a matching element, to avoid relying on key's sk_argument).
       */
      if (key->sk_attno >= firstchangingattnum)
        break;     /* unsafe, multiple distinct attr values */

      firstdatum = index_getattr(firsttup, key->sk_attno, tupdesc,
                     &firstnull);
      _bt_binsrch_array_skey(&so->orderProcs[startikey],
                   false, NoMovementScanDirection,
                   firstdatum, firstnull, array, key,
                   &result);
      if (result != 0)
        break;     /* unsafe */

      /* Safe, SAOP = key satisfied by every tuple */
      start_past_saop_eq = true;
      continue;
    }

    /*
     * Skip array = key
     */
    Assert(key->sk_flags & SK_BT_SKIP);
    if (array->null_elem)
    {
      /*
       * Non-range skip array = key.
       *
       * Safe, non-range skip array "satisfied" by every tuple on page
       * (safe even when "key->sk_attno > firstchangingattnum").
       */
      continue;
    }

    /*
     * Range skip array = key.
     *
     * Handle this like we handle scalar inequality keys (but avoid using
     * key's sk_argument directly, as in the SAOP array case).
     */
    if (key->sk_attno > firstchangingattnum) /* >, not >= */
      break;       /* unsafe, preceding attr has multiple
                 * distinct values */

    firstdatum = index_getattr(firsttup, key->sk_attno, tupdesc, &firstnull);
    lastdatum = index_getattr(lasttup, key->sk_attno, tupdesc, &lastnull);

    /* Test firsttup */
    _bt_binsrch_skiparray_skey(false, ForwardScanDirection,
                   firstdatum, firstnull, array, key,
                   &result);
    if (result != 0)
      break;       /* unsafe */

    /* Test lasttup */
    _bt_binsrch_skiparray_skey(false, ForwardScanDirection,
                   lastdatum, lastnull, array, key,
                   &result);
    if (result != 0)
      break;       /* unsafe */

    /* Safe, range skip array satisfied by every tuple on page */
  }

  /*
   * Use of forcenonrequired is typically undesirable, since it'll force
   * _bt_readpage caller to read every tuple on the page -- even though, in
   * general, it might well be possible to end the scan on an earlier tuple.
   * However, caller must use forcenonrequired when start_past_saop_eq=true,
   * since the usual required array behavior might fail to roll over to the
   * SAOP array.
   *
   * We always prefer forcenonrequired=true during scans with skip arrays
   * (except on the first page of each primitive index scan), though -- even
   * when "startikey == 0".  That way, _bt_advance_array_keys's low-order
   * key precheck optimization can always be used (unless on the first page
   * of the scan).  It seems slightly preferable to check more tuples when
   * that allows us to do significantly less skip array maintenance.
   */
  pstate->forcenonrequired = (start_past_saop_eq || so->skipScan);
  pstate->startikey = startikey;

  /*
   * _bt_readpage caller is required to call _bt_checkkeys against page's
   * finaltup with forcenonrequired=false whenever we initially set
   * forcenonrequired=true.  That way the scan's arrays will reliably track
   * its progress through the index's key space.
   *
   * We don't expect this when _bt_readpage caller has no finaltup due to
   * its page being the rightmost (or the leftmost, during backwards scans).
   * When we see that _bt_readpage has no finaltup, back out of everything.
   */
  Assert(!pstate->forcenonrequired || so->numArrayKeys);
  if (pstate->forcenonrequired && !pstate->finaltup)
  {
    pstate->forcenonrequired = false;
    pstate->startikey = 0;
  }
}

/*
 * Test whether caller's finaltup tuple is still before the start of matches
 * for the current array keys.
 *
 * Called at the start of reading a page during a scan with array keys, though
 * only when the so->scanBehind flag was set on the scan's prior page.
 *
 * Returns false if the tuple is still before the start of matches.  When that
 * happens, caller should cut its losses and start a new primitive index scan.
 * Otherwise returns true.
 */
static bool
_bt_scanbehind_checkkeys(IndexScanDesc scan, ScanDirection dir,
             IndexTuple finaltup)
{
  Relation  rel = scan->indexRelation;
  TupleDesc tupdesc = RelationGetDescr(rel);
  BTScanOpaque so = (BTScanOpaque) scan->opaque;
  int     nfinaltupatts = BTreeTupleGetNAtts(finaltup, rel);
  bool    scanBehind;

  Assert(so->numArrayKeys);

  if (_bt_tuple_before_array_skeys(scan, dir, finaltup, tupdesc,
                   nfinaltupatts, false, 0, &scanBehind))
    return false;

  /*
   * If scanBehind was set, all of the untruncated attribute values from
   * finaltup that correspond to an array match the array's current element,
   * but there are other keys associated with truncated suffix attributes.
   * Array advancement must have incremented the scan's arrays on the
   * previous page, resulting in a set of array keys that happen to be an
   * exact match for the current page high key's untruncated prefix values.
   *
   * This page definitely doesn't contain tuples that the scan will need to
   * return.  The next page may or may not contain relevant tuples.  Handle
   * this by cutting our losses and starting a new primscan.
   */
  if (scanBehind)
    return false;

  if (!so->oppositeDirCheck)
    return true;

  return _bt_oppodir_checkkeys(scan, dir, finaltup);
}

/*
 * Test whether an indextuple fails to satisfy an inequality required in the
 * opposite direction only.
 *
 * Caller's finaltup tuple is the page high key (for forwards scans), or the
 * first non-pivot tuple (for backwards scans).  Called during scans with
 * required array keys and required opposite-direction inequalities.
 *
 * Returns false if an inequality scan key required in the opposite direction
 * only isn't satisfied (and any earlier required scan keys are satisfied).
 * Otherwise returns true.
 *
 * An unsatisfied inequality required in the opposite direction only might
 * well enable skipping over many leaf pages, provided another _bt_first call
 * takes place.  This type of unsatisfied inequality won't usually cause
 * _bt_checkkeys to stop the scan to consider array advancement/starting a new
 * primitive index scan.
 */
static bool
_bt_oppodir_checkkeys(IndexScanDesc scan, ScanDirection dir,
            IndexTuple finaltup)
{
  Relation  rel = scan->indexRelation;
  TupleDesc tupdesc = RelationGetDescr(rel);
  BTScanOpaque so = (BTScanOpaque) scan->opaque;
  int     nfinaltupatts = BTreeTupleGetNAtts(finaltup, rel);
  bool    continuescan;
  ScanDirection flipped = -dir;
  int     ikey = 0;

  Assert(so->numArrayKeys);

  _bt_check_compare(scan, flipped, finaltup, nfinaltupatts, tupdesc, false,
            false, &continuescan,
            &ikey);

  if (!continuescan && so->keyData[ikey].sk_strategy != BTEqualStrategyNumber)
    return false;

  return true;
}

/* Save an index item into so->currPos.items[itemIndex] */
static void
_bt_saveitem(BTScanOpaque so, int itemIndex,
       OffsetNumber offnum, IndexTuple itup)
{
  BTScanPosItem *currItem = &so->currPos.items[itemIndex];

  Assert(!BTreeTupleIsPivot(itup) && !BTreeTupleIsPosting(itup));

  currItem->heapTid = itup->t_tid;
  currItem->indexOffset = offnum;
  if (so->currTuples)
  {
    Size    itupsz = IndexTupleSize(itup);

    currItem->tupleOffset = so->currPos.nextTupleOffset;
    memcpy(so->currTuples + so->currPos.nextTupleOffset, itup, itupsz);
    so->currPos.nextTupleOffset += MAXALIGN(itupsz);
  }
}

/*
 * Setup state to save TIDs/items from a single posting list tuple.
 *
 * Saves an index item into so->currPos.items[itemIndex] for TID that is
 * returned to scan first.  Second or subsequent TIDs for posting list should
 * be saved by calling _bt_savepostingitem().
 *
 * Returns an offset into tuple storage space that main tuple is stored at if
 * needed.
 */
static int
_bt_setuppostingitems(BTScanOpaque so, int itemIndex, OffsetNumber offnum,
            const ItemPointerData *heapTid, IndexTuple itup)
{
  BTScanPosItem *currItem = &so->currPos.items[itemIndex];

  Assert(BTreeTupleIsPosting(itup));

  currItem->heapTid = *heapTid;
  currItem->indexOffset = offnum;
  if (so->currTuples)
  {
    /* Save base IndexTuple (truncate posting list) */
    IndexTuple  base;
    Size    itupsz = BTreeTupleGetPostingOffset(itup);

    itupsz = MAXALIGN(itupsz);
    currItem->tupleOffset = so->currPos.nextTupleOffset;
    base = (IndexTuple) (so->currTuples + so->currPos.nextTupleOffset);
    memcpy(base, itup, itupsz);
    /* Defensively reduce work area index tuple header size */
    base->t_info &= ~INDEX_SIZE_MASK;
    base->t_info |= itupsz;
    so->currPos.nextTupleOffset += itupsz;

    return currItem->tupleOffset;
  }

  return 0;
}

/*
 * Save an index item into so->currPos.items[itemIndex] for current posting
 * tuple.
 *
 * Assumes that _bt_setuppostingitems() has already been called for current
 * posting list tuple.  Caller passes its return value as tupleOffset.
 */
static inline void
_bt_savepostingitem(BTScanOpaque so, int itemIndex, OffsetNumber offnum,
          ItemPointer heapTid, int tupleOffset)
{
  BTScanPosItem *currItem = &so->currPos.items[itemIndex];

  currItem->heapTid = *heapTid;
  currItem->indexOffset = offnum;

  /*
   * Have index-only scans return the same base IndexTuple for every TID
   * that originates from the same posting list
   */
  if (so->currTuples)
    currItem->tupleOffset = tupleOffset;
}

#define LOOK_AHEAD_REQUIRED_RECHECKS  3
#define LOOK_AHEAD_DEFAULT_DISTANCE   5
#define NSKIPADVANCES_THRESHOLD     3

/*
 * Test whether an indextuple satisfies all the scankey conditions.
 *
 * Returns true if so, false if not.  If not,
 * we also determine whether there's any need to continue the scan beyond
 * this tuple, and set pstate.continuescan accordingly.  See comments for
 * _bt_preprocess_keys() about how this is done.
 *
 * Forward scan callers can pass a high key tuple in the hopes of having
 * us set pstate.continuescan to false, avoiding an unnecessary visit to
 * the page to the right.
 *
 * Advances the scan's array keys when necessary for arrayKeys=true callers.
 * Scans without any array keys must always pass arrayKeys=false.
 *
 * Also stops and starts primitive index scans for arrayKeys=true callers.
 * Scans with array keys are required to set up page state that helps us with
 * this.  The page's finaltup tuple (the page high key for a forward scan, or
 * the page's first non-pivot tuple for a backward scan) must be set in
 * pstate.finaltup ahead of the first call here for the page.  Set it to
 * NULL for rightmost page (or the leftmost page for backwards scans).
 *
 * scan: index scan descriptor (containing a search-type scankey)
 * pstate: page level input and output parameters
 * arrayKeys: should we advance the scan's array keys if necessary?
 * tuple: index tuple to test
 * tupnatts: number of attributes in tupnatts (high key may be truncated)
 */
static bool
_bt_checkkeys(IndexScanDesc scan, BTReadPageState *pstate, bool arrayKeys,
        IndexTuple tuple, int tupnatts)
{
  TupleDesc tupdesc = RelationGetDescr(scan->indexRelation);
  BTScanOpaque so PG_USED_FOR_ASSERTS_ONLY = (BTScanOpaque) scan->opaque;
  ScanDirection dir = pstate->dir;
  int     ikey = pstate->startikey;
  bool    res;

  Assert(BTreeTupleGetNAtts(tuple, scan->indexRelation) == tupnatts);
  Assert(!so->needPrimScan && !so->scanBehind && !so->oppositeDirCheck);
  Assert(arrayKeys || so->numArrayKeys == 0);

  res = _bt_check_compare(scan, dir, tuple, tupnatts, tupdesc, arrayKeys,
              pstate->forcenonrequired, &pstate->continuescan,
              &ikey);

  /*
   * If _bt_check_compare relied on the pstate.startikey optimization, call
   * again (in assert-enabled builds) to verify it didn't affect our answer.
   *
   * Note: we can't do this when !pstate.forcenonrequired, since any arrays
   * before pstate.startikey won't have advanced on this page at all.
   */
  Assert(!pstate->forcenonrequired || arrayKeys);
#ifdef USE_ASSERT_CHECKING
  if (pstate->startikey > 0 && !pstate->forcenonrequired)
  {
    bool    dres,
          dcontinuescan;
    int     dikey = 0;

    /* Pass advancenonrequired=false to avoid array side-effects */
    dres = _bt_check_compare(scan, dir, tuple, tupnatts, tupdesc, false,
                 pstate->forcenonrequired, &dcontinuescan,
                 &dikey);
    Assert(res == dres);
    Assert(pstate->continuescan == dcontinuescan);

    /*
     * Should also get the same ikey result.  We need a slightly weaker
     * assertion during arrayKeys calls, since they might be using an
     * array that couldn't be marked required during preprocessing.
     */
    Assert(arrayKeys || ikey == dikey);
    Assert(ikey <= dikey);
  }
#endif

  /*
   * Only one _bt_check_compare call is required when there are no array
   * scan keys.  Otherwise, we can only accept _bt_check_compare's answer
   * unreservedly when it set pstate.continuescan=true.
   */
  if (!arrayKeys || pstate->continuescan)
    return res;

  /*
   * _bt_check_compare call set continuescan=false in the presence of array
   * keys.  This could mean that the tuple is just past the end of matches
   * for the current array keys.
   *
   * It's also possible that the scan is still _before_ the _start_ of
   * tuples matching the current set of array keys.  Check for that first.
   */
  Assert(!pstate->forcenonrequired);
  if (_bt_tuple_before_array_skeys(scan, dir, tuple, tupdesc, tupnatts, true,
                   ikey, NULL))
  {
    /* Override _bt_check_compare, continue primitive scan */
    pstate->continuescan = true;

    /*
     * We will end up here repeatedly given a group of tuples > the
     * previous array keys and < the now-current keys (for a backwards
     * scan it's just the same, though the operators swap positions).
     *
     * We must avoid allowing this linear search process to scan very many
     * tuples from well before the start of tuples matching the current
     * array keys (or from well before the point where we'll once again
     * have to advance the scan's array keys).
     *
     * We keep the overhead under control by speculatively "looking ahead"
     * to later still-unscanned items from this same leaf page.  We'll
     * only attempt this once the number of tuples that the linear search
     * process has examined starts to get out of hand.
     */
    pstate->rechecks++;
    if (pstate->rechecks >= LOOK_AHEAD_REQUIRED_RECHECKS)
    {
      /* See if we should skip ahead within the current leaf page */
      _bt_checkkeys_look_ahead(scan, pstate, tupnatts, tupdesc);

      /*
       * Might have set pstate.skip to a later page offset.  When that
       * happens then _bt_readpage caller will inexpensively skip ahead
       * to a later tuple from the same page (the one just after the
       * tuple we successfully "looked ahead" to).
       */
    }

    /* This indextuple doesn't match the current qual, in any case */
    return false;
  }

  /*
   * Caller's tuple is >= the current set of array keys and other equality
   * constraint scan keys (or <= if this is a backwards scan).  It's now
   * clear that we _must_ advance any required array keys in lockstep with
   * the scan.
   */
  return _bt_advance_array_keys(scan, pstate, tuple, tupnatts, tupdesc,
                  ikey, true);
}

/*
 * Test whether an indextuple satisfies current scan condition.
 *
 * Return true if so, false if not.  If not, also sets *continuescan to false
 * when it's also not possible for any later tuples to pass the current qual
 * (with the scan's current set of array keys, in the current scan direction),
 * in addition to setting *ikey to the so->keyData[] subscript/offset for the
 * unsatisfied scan key (needed when caller must consider advancing the scan's
 * array keys).
 *
 * This is a subroutine for _bt_checkkeys.  We provisionally assume that
 * reaching the end of the current set of required keys (in particular the
 * current required array keys) ends the ongoing (primitive) index scan.
 * Callers without array keys should just end the scan right away when they
 * find that continuescan has been set to false here by us.  Things are more
 * complicated for callers with array keys.
 *
 * Callers with array keys must first consider advancing the arrays when
 * continuescan has been set to false here by us.  They must then consider if
 * it really does make sense to end the current (primitive) index scan, in
 * light of everything that is known at that point.  (In general when we set
 * continuescan=false for these callers it must be treated as provisional.)
 *
 * We deal with advancing unsatisfied non-required arrays directly, though.
 * This is safe, since by definition non-required keys can't end the scan.
 * This is just how we determine if non-required arrays are just unsatisfied
 * by the current array key, or if they're truly unsatisfied (that is, if
 * they're unsatisfied by every possible array key).
 *
 * Pass advancenonrequired=false to avoid all array related side effects.
 * This allows _bt_advance_array_keys caller to avoid infinite recursion.
 *
 * Pass forcenonrequired=true to instruct us to treat all keys as nonrequired.
 * This is used to make it safe to temporarily stop properly maintaining the
 * scan's required arrays.  _bt_checkkeys caller (_bt_readpage, actually)
 * determines a prefix of keys that must satisfy every possible corresponding
 * index attribute value from its page, which is passed to us via *ikey arg
 * (this is the first key that might be unsatisfied by tuples on the page).
 * Obviously, we won't maintain any array keys from before *ikey, so it's
 * quite possible for such arrays to "fall behind" the index's keyspace.
 * Caller will need to "catch up" by passing forcenonrequired=true (alongside
 * an *ikey=0) once the page's finaltup is reached.
 *
 * Note: it's safe to pass an *ikey > 0 with forcenonrequired=false, but only
 * when caller determines that it won't affect array maintenance.
 */
static bool
_bt_check_compare(IndexScanDesc scan, ScanDirection dir,
          IndexTuple tuple, int tupnatts, TupleDesc tupdesc,
          bool advancenonrequired, bool forcenonrequired,
          bool *continuescan, int *ikey)
{
  BTScanOpaque so = (BTScanOpaque) scan->opaque;

  *continuescan = true;   /* default assumption */

  for (; *ikey < so->numberOfKeys; (*ikey)++)
  {
    ScanKey   key = so->keyData + *ikey;
    Datum   datum;
    bool    isNull;
    bool    requiredSameDir = false,
          requiredOppositeDirOnly = false;

    /*
     * Check if the key is required in the current scan direction, in the
     * opposite scan direction _only_, or in neither direction (except
     * when we're forced to treat all scan keys as nonrequired)
     */
    if (forcenonrequired)
    {
      /* treating scan's keys as non-required */
    }
    else if (((key->sk_flags & SK_BT_REQFWD) && ScanDirectionIsForward(dir)) ||
         ((key->sk_flags & SK_BT_REQBKWD) && ScanDirectionIsBackward(dir)))
      requiredSameDir = true;
    else if (((key->sk_flags & SK_BT_REQFWD) && ScanDirectionIsBackward(dir)) ||
         ((key->sk_flags & SK_BT_REQBKWD) && ScanDirectionIsForward(dir)))
      requiredOppositeDirOnly = true;

    if (key->sk_attno > tupnatts)
    {
      /*
       * This attribute is truncated (must be high key).  The value for
       * this attribute in the first non-pivot tuple on the page to the
       * right could be any possible value.  Assume that truncated
       * attribute passes the qual.
       */
      Assert(BTreeTupleIsPivot(tuple));
      continue;
    }

    /*
     * A skip array scan key uses one of several sentinel values.  We just
     * fall back on _bt_tuple_before_array_skeys when we see such a value.
     */
    if (key->sk_flags & (SK_BT_MINVAL | SK_BT_MAXVAL |
               SK_BT_NEXT | SK_BT_PRIOR))
    {
      Assert(key->sk_flags & SK_SEARCHARRAY);
      Assert(key->sk_flags & SK_BT_SKIP);
      Assert(requiredSameDir || forcenonrequired);

      /*
       * Cannot fall back on _bt_tuple_before_array_skeys when we're
       * treating the scan's keys as nonrequired, though.  Just handle
       * this like any other non-required equality-type array key.
       */
      if (forcenonrequired)
        return _bt_advance_array_keys(scan, NULL, tuple, tupnatts,
                        tupdesc, *ikey, false);

      *continuescan = false;
      return false;
    }

    /* row-comparison keys need special processing */
    if (key->sk_flags & SK_ROW_HEADER)
    {
      if (_bt_check_rowcompare(key, tuple, tupnatts, tupdesc, dir,
                   forcenonrequired, continuescan))
        continue;
      return false;
    }

    datum = index_getattr(tuple,
                key->sk_attno,
                tupdesc,
                &isNull);

    if (key->sk_flags & SK_ISNULL)
    {
      /* Handle IS NULL/NOT NULL tests */
      if (key->sk_flags & SK_SEARCHNULL)
      {
        if (isNull)
          continue; /* tuple satisfies this qual */
      }
      else
      {
        Assert(key->sk_flags & SK_SEARCHNOTNULL);
        Assert(!(key->sk_flags & SK_BT_SKIP));
        if (!isNull)
          continue; /* tuple satisfies this qual */
      }

      /*
       * Tuple fails this qual.  If it's a required qual for the current
       * scan direction, then we can conclude no further tuples will
       * pass, either.
       */
      if (requiredSameDir)
        *continuescan = false;
      else if (unlikely(key->sk_flags & SK_BT_SKIP))
      {
        /*
         * If we're treating scan keys as nonrequired, and encounter a
         * skip array scan key whose current element is NULL, then it
         * must be a non-range skip array.  It must be satisfied, so
         * there's no need to call _bt_advance_array_keys to check.
         */
        Assert(forcenonrequired && *ikey > 0);
        continue;
      }

      /*
       * This indextuple doesn't match the qual.
       */
      return false;
    }

    if (isNull)
    {
      /*
       * Scalar scan key isn't satisfied by NULL tuple value.
       *
       * If we're treating scan keys as nonrequired, and key is for a
       * skip array, then we must attempt to advance the array to NULL
       * (if we're successful then the tuple might match the qual).
       */
      if (unlikely(forcenonrequired && key->sk_flags & SK_BT_SKIP))
        return _bt_advance_array_keys(scan, NULL, tuple, tupnatts,
                        tupdesc, *ikey, false);

      if (key->sk_flags & SK_BT_NULLS_FIRST)
      {
        /*
         * Since NULLs are sorted before non-NULLs, we know we have
         * reached the lower limit of the range of values for this
         * index attr.  On a backward scan, we can stop if this qual
         * is one of the "must match" subset.  We can stop regardless
         * of whether the qual is > or <, so long as it's required,
         * because it's not possible for any future tuples to pass. On
         * a forward scan, however, we must keep going, because we may
         * have initially positioned to the start of the index.
         * (_bt_advance_array_keys also relies on this behavior during
         * forward scans.)
         */
        if ((requiredSameDir || requiredOppositeDirOnly) &&
          ScanDirectionIsBackward(dir))
          *continuescan = false;
      }
      else
      {
        /*
         * Since NULLs are sorted after non-NULLs, we know we have
         * reached the upper limit of the range of values for this
         * index attr.  On a forward scan, we can stop if this qual is
         * one of the "must match" subset.  We can stop regardless of
         * whether the qual is > or <, so long as it's required,
         * because it's not possible for any future tuples to pass. On
         * a backward scan, however, we must keep going, because we
         * may have initially positioned to the end of the index.
         * (_bt_advance_array_keys also relies on this behavior during
         * backward scans.)
         */
        if ((requiredSameDir || requiredOppositeDirOnly) &&
          ScanDirectionIsForward(dir))
          *continuescan = false;
      }

      /*
       * This indextuple doesn't match the qual.
       */
      return false;
    }

    if (!DatumGetBool(FunctionCall2Coll(&key->sk_func, key->sk_collation,
                      datum, key->sk_argument)))
    {
      /*
       * Tuple fails this qual.  If it's a required qual for the current
       * scan direction, then we can conclude no further tuples will
       * pass, either.
       */
      if (requiredSameDir)
        *continuescan = false;

      /*
       * If this is a non-required array key, the tuple needs to be
       * checked against every possible array key.  Handle this by
       * "advancing" the scan key's array to a matching value (if we're
       * successful then the tuple might match the qual).
       */
      else if (advancenonrequired && (key->sk_flags & SK_SEARCHARRAY))
        return _bt_advance_array_keys(scan, NULL, tuple, tupnatts,
                        tupdesc, *ikey, false);

      /*
       * This indextuple doesn't match the qual.
       */
      return false;
    }
  }

  /* If we get here, the tuple passes all index quals. */
  return true;
}

/*
 * Test whether an indextuple satisfies a row-comparison scan condition.
 *
 * Return true if so, false if not.  If not, also clear *continuescan if
 * it's not possible for any future tuples in the current scan direction
 * to pass the qual.
 *
 * This is a subroutine for _bt_checkkeys/_bt_check_compare.  Caller passes us
 * a row compare header key taken from so->keyData[].
 *
 * Row value comparisons can be described in terms of logical expansions that
 * use only scalar operators.  Consider the following example row comparison:
 *
 * "(a, b, c) > (7, 'bar', 62)"
 *
 * This can be evaluated as:
 *
 * "(a = 7 AND b = 'bar' AND c > 62) OR (a = 7 AND b > 'bar') OR (a > 7)".
 *
 * Notice that this condition is satisfied by _all_ rows that satisfy "a > 7",
 * and by a subset of all rows that satisfy "a >= 7" (possibly all such rows).
 * It _can't_ be satisfied by other rows (where "a < 7" or where "a IS NULL").
 * A row comparison header key can therefore often be treated as if it was a
 * simple scalar inequality on the row compare's most significant column.
 * (For example, _bt_advance_array_keys and most preprocessing routines treat
 * row compares like any other same-strategy inequality on the same column.)
 *
 * Things get more complicated for our row compare given a row where "a = 7".
 * Note that a row compare isn't necessarily satisfied by _every_ tuple that
 * appears between the first and last satisfied tuple returned by the scan,
 * due to the way that its lower-order subkeys are only conditionally applied.
 * A forwards scan that uses our example qual might initially return a tuple
 * "(a, b, c) = (7, 'zebra', 54)".  But it won't subsequently return a tuple
 * "(a, b, c) = (7, NULL, 1)" located to the right of the first matching tuple
 * (assume that "b" was declared NULLS LAST here).  The scan will only return
 * additional matches upon reaching tuples where "a > 7".  If you rereview our
 * example row comparison's logical expansion, you'll understand why this is.
 * (Here we assume that all subkeys could be marked required, guaranteeing
 * that row comparison order matches index order.  This is the common case.)
 *
 * Note that a row comparison header key behaves _exactly_ the same as a
 * similar scalar inequality key on the row's most significant column once the
 * scan reaches the point where it no longer needs to evaluate lower-order
 * subkeys (or before the point where it starts needing to evaluate them).
 * For example, once a forwards scan that uses our example qual reaches the
 * first tuple "a > 7", we'll behave in just the same way as our caller would
 * behave with a similar scalar inequality "a > 7" for the remainder of the
 * scan (assuming that the scan never changes direction/never goes backwards).
 * We'll even set continuescan=false according to exactly the same rules as
 * the ones our caller applies with simple scalar inequalities, including the
 * rules it applies when NULL tuple values don't satisfy an inequality qual.
 */
static bool
_bt_check_rowcompare(ScanKey header, IndexTuple tuple, int tupnatts,
           TupleDesc tupdesc, ScanDirection dir,
           bool forcenonrequired, bool *continuescan)
{
  ScanKey   subkey = (ScanKey) DatumGetPointer(header->sk_argument);
  int32   cmpresult = 0;
  bool    result;

  /* First subkey should be same as the header says */
  Assert(header->sk_flags & SK_ROW_HEADER);
  Assert(subkey->sk_attno == header->sk_attno);
  Assert(subkey->sk_strategy == header->sk_strategy);

  /* Loop over columns of the row condition */
  for (;;)
  {
    Datum   datum;
    bool    isNull;

    Assert(subkey->sk_flags & SK_ROW_MEMBER);

    /* When a NULL row member is compared, the row never matches */
    if (subkey->sk_flags & SK_ISNULL)
    {
      /*
       * Unlike the simple-scankey case, this isn't a disallowed case
       * (except when it's the first row element that has the NULL arg).
       * But it can never match.  If all the earlier row comparison
       * columns are required for the scan direction, we can stop the
       * scan, because there can't be another tuple that will succeed.
       */
      Assert(subkey != (ScanKey) DatumGetPointer(header->sk_argument));
      subkey--;
      if (forcenonrequired)
      {
        /* treating scan's keys as non-required */
      }
      else if ((subkey->sk_flags & SK_BT_REQFWD) &&
           ScanDirectionIsForward(dir))
        *continuescan = false;
      else if ((subkey->sk_flags & SK_BT_REQBKWD) &&
           ScanDirectionIsBackward(dir))
        *continuescan = false;
      return false;
    }

    if (subkey->sk_attno > tupnatts)
    {
      /*
       * This attribute is truncated (must be high key).  The value for
       * this attribute in the first non-pivot tuple on the page to the
       * right could be any possible value.  Assume that truncated
       * attribute passes the qual.
       */
      Assert(BTreeTupleIsPivot(tuple));
      return true;
    }

    datum = index_getattr(tuple,
                subkey->sk_attno,
                tupdesc,
                &isNull);

    if (isNull)
    {
      int     reqflags;

      if (forcenonrequired)
      {
        /* treating scan's keys as non-required */
      }
      else if (subkey->sk_flags & SK_BT_NULLS_FIRST)
      {
        /*
         * Since NULLs are sorted before non-NULLs, we know we have
         * reached the lower limit of the range of values for this
         * index attr.  On a backward scan, we can stop if this qual
         * is one of the "must match" subset.  However, on a forwards
         * scan, we must keep going, because we may have initially
         * positioned to the start of the index.
         *
         * All required NULLS FIRST > row members can use NULL tuple
         * values to end backwards scans, just like with other values.
         * A qual "WHERE (a, b, c) > (9, 42, 'foo')" can terminate a
         * backwards scan upon reaching the index's rightmost "a = 9"
         * tuple whose "b" column contains a NULL (if not sooner).
         * Since "b" is NULLS FIRST, we can treat its NULLs as "<" 42.
         */
        reqflags = SK_BT_REQBKWD;

        /*
         * When a most significant required NULLS FIRST < row compare
         * member sees NULL tuple values during a backwards scan, it
         * signals the end of matches for the whole row compare/scan.
         * A qual "WHERE (a, b, c) < (9, 42, 'foo')" will terminate a
         * backwards scan upon reaching the rightmost tuple whose "a"
         * column has a NULL.  The "a" NULL value is "<" 9, and yet
         * our < row compare will still end the scan.  (This isn't
         * safe with later/lower-order row members.  Notice that it
         * can only happen with an "a" NULL some time after the scan
         * completely stops needing to use its "b" and "c" members.)
         */
        if (subkey == (ScanKey) DatumGetPointer(header->sk_argument))
          reqflags |= SK_BT_REQFWD; /* safe, first row member */

        if ((subkey->sk_flags & reqflags) &&
          ScanDirectionIsBackward(dir))
          *continuescan = false;
      }
      else
      {
        /*
         * Since NULLs are sorted after non-NULLs, we know we have
         * reached the upper limit of the range of values for this
         * index attr.  On a forward scan, we can stop if this qual is
         * one of the "must match" subset.  However, on a backward
         * scan, we must keep going, because we may have initially
         * positioned to the end of the index.
         *
         * All required NULLS LAST < row members can use NULL tuple
         * values to end forwards scans, just like with other values.
         * A qual "WHERE (a, b, c) < (9, 42, 'foo')" can terminate a
         * forwards scan upon reaching the index's leftmost "a = 9"
         * tuple whose "b" column contains a NULL (if not sooner).
         * Since "b" is NULLS LAST, we can treat its NULLs as ">" 42.
         */
        reqflags = SK_BT_REQFWD;

        /*
         * When a most significant required NULLS LAST > row compare
         * member sees NULL tuple values during a forwards scan, it
         * signals the end of matches for the whole row compare/scan.
         * A qual "WHERE (a, b, c) > (9, 42, 'foo')" will terminate a
         * forwards scan upon reaching the leftmost tuple whose "a"
         * column has a NULL.  The "a" NULL value is ">" 9, and yet
         * our > row compare will end the scan.  (This isn't safe with
         * later/lower-order row members.  Notice that it can only
         * happen with an "a" NULL some time after the scan completely
         * stops needing to use its "b" and "c" members.)
         */
        if (subkey == (ScanKey) DatumGetPointer(header->sk_argument))
          reqflags |= SK_BT_REQBKWD; /* safe, first row member */

        if ((subkey->sk_flags & reqflags) &&
          ScanDirectionIsForward(dir))
          *continuescan = false;
      }

      /*
       * In any case, this indextuple doesn't match the qual.
       */
      return false;
    }

    /* Perform the test --- three-way comparison not bool operator */
    cmpresult = DatumGetInt32(FunctionCall2Coll(&subkey->sk_func,
                          subkey->sk_collation,
                          datum,
                          subkey->sk_argument));

    if (subkey->sk_flags & SK_BT_DESC)
      INVERT_COMPARE_RESULT(cmpresult);

    /* Done comparing if unequal, else advance to next column */
    if (cmpresult != 0)
      break;

    if (subkey->sk_flags & SK_ROW_END)
      break;
    subkey++;
  }

  /* Final subkey/column determines if row compare is satisfied */
  result = _bt_rowcompare_cmpresult(subkey, cmpresult);

  if (!result && !forcenonrequired)
  {
    /*
     * Tuple fails this qual.  If it's a required qual for the current
     * scan direction, then we can conclude no further tuples will pass,
     * either.  Note we have to look at the deciding column, not
     * necessarily the first or last column of the row condition.
     */
    if ((subkey->sk_flags & SK_BT_REQFWD) &&
      ScanDirectionIsForward(dir))
      *continuescan = false;
    else if ((subkey->sk_flags & SK_BT_REQBKWD) &&
         ScanDirectionIsBackward(dir))
      *continuescan = false;
  }

  return result;
}

/*
 * Call here when a row compare member returns a non-zero result, or with the
 * result for the final ROW_END row compare member (no matter the cmpresult).
 *
 * cmpresult indicates the overall result of the row comparison (must already
 * be commuted for DESC subkeys), and subkey is the deciding row member.
 */
static bool
_bt_rowcompare_cmpresult(ScanKey subkey, int cmpresult)
{
  bool    satisfied;

  Assert(subkey->sk_flags & SK_ROW_MEMBER);

  switch (subkey->sk_strategy)
  {
    case BTLessStrategyNumber:
      satisfied = (cmpresult < 0);
      break;
    case BTLessEqualStrategyNumber:
      satisfied = (cmpresult <= 0);
      break;
    case BTGreaterEqualStrategyNumber:
      satisfied = (cmpresult >= 0);
      break;
    case BTGreaterStrategyNumber:
      satisfied = (cmpresult > 0);
      break;
    default:
      /* EQ and NE cases aren't allowed here */
      elog(ERROR, "unexpected strategy number %d", subkey->sk_strategy);
      satisfied = false;  /* keep compiler quiet */
      break;
  }

  return satisfied;
}

/*
 * _bt_tuple_before_array_skeys() -- too early to advance required arrays?
 *
 * We always compare the tuple using the current array keys (which we assume
 * are already set in so->keyData[]).  readpagetup indicates if tuple is the
 * scan's current _bt_readpage-wise tuple.
 *
 * readpagetup callers must only call here when _bt_check_compare already set
 * continuescan=false.  We help these callers deal with _bt_check_compare's
 * inability to distinguish between the < and > cases (it uses equality
 * operator scan keys, whereas we use 3-way ORDER procs).  These callers pass
 * a _bt_check_compare-set sktrig value that indicates which scan key
 * triggered the call (!readpagetup callers just pass us sktrig=0 instead).
 * This information allows us to avoid wastefully checking earlier scan keys
 * that were already deemed to have been satisfied inside _bt_check_compare.
 *
 * Returns false when caller's tuple is >= the current required equality scan
 * keys (or <=, in the case of backwards scans).  This happens to readpagetup
 * callers when the scan has reached the point of needing its array keys
 * advanced; caller will need to advance required and non-required arrays at
 * scan key offsets >= sktrig, plus scan keys < sktrig iff sktrig rolls over.
 * (When we return false to readpagetup callers, tuple can only be == current
 * required equality scan keys when caller's sktrig indicates that the arrays
 * need to be advanced due to an unsatisfied required inequality key trigger.)
 *
 * Returns true when caller passes a tuple that is < the current set of
 * equality keys for the most significant non-equal required scan key/column
 * (or > the keys, during backwards scans).  This happens to readpagetup
 * callers when tuple is still before the start of matches for the scan's
 * required equality strategy scan keys.  (sktrig can't have indicated that an
 * inequality strategy scan key wasn't satisfied in _bt_check_compare when we
 * return true.  In fact, we automatically return false when passed such an
 * inequality sktrig by readpagetup callers -- _bt_check_compare's initial
 * continuescan=false doesn't really need to be confirmed here by us.)
 *
 * !readpagetup callers optionally pass us *scanBehind, which tracks whether
 * any missing truncated attributes might have affected array advancement
 * (compared to what would happen if it was shown the first non-pivot tuple on
 * the page to the right of caller's finaltup/high key tuple instead).  It's
 * only possible that we'll set *scanBehind to true when caller passes us a
 * pivot tuple (with truncated -inf attributes) that we return false for.
 */
static bool
_bt_tuple_before_array_skeys(IndexScanDesc scan, ScanDirection dir,
               IndexTuple tuple, TupleDesc tupdesc, int tupnatts,
               bool readpagetup, int sktrig, bool *scanBehind)
{
  BTScanOpaque so = (BTScanOpaque) scan->opaque;

  Assert(so->numArrayKeys);
  Assert(so->numberOfKeys);
  Assert(sktrig == 0 || readpagetup);
  Assert(!readpagetup || scanBehind == NULL);

  if (scanBehind)
    *scanBehind = false;

  for (int ikey = sktrig; ikey < so->numberOfKeys; ikey++)
  {
    ScanKey   cur = so->keyData + ikey;
    Datum   tupdatum;
    bool    tupnull;
    int32   result;

    /* readpagetup calls require one ORDER proc comparison (at most) */
    Assert(!readpagetup || ikey == sktrig);

    /*
     * Once we reach a non-required scan key, we're completely done.
     *
     * Note: we deliberately don't consider the scan direction here.
     * _bt_advance_array_keys caller requires that we track *scanBehind
     * without concern for scan direction.
     */
    if ((cur->sk_flags & (SK_BT_REQFWD | SK_BT_REQBKWD)) == 0)
    {
      Assert(!readpagetup);
      Assert(ikey > sktrig || ikey == 0);
      return false;
    }

    if (cur->sk_attno > tupnatts)
    {
      Assert(!readpagetup);

      /*
       * When we reach a high key's truncated attribute, assume that the
       * tuple attribute's value is >= the scan's equality constraint
       * scan keys (but set *scanBehind to let interested callers know
       * that a truncated attribute might have affected our answer).
       */
      if (scanBehind)
        *scanBehind = true;

      return false;
    }

    /*
     * Deal with inequality strategy scan keys that _bt_check_compare set
     * continuescan=false for
     */
    if (cur->sk_strategy != BTEqualStrategyNumber)
    {
      /*
       * When _bt_check_compare indicated that a required inequality
       * scan key wasn't satisfied, there's no need to verify anything;
       * caller always calls _bt_advance_array_keys with this sktrig.
       */
      if (readpagetup)
        return false;

      /*
       * Otherwise we can't give up, since we must check all required
       * scan keys (required in either direction) in order to correctly
       * track *scanBehind for caller
       */
      continue;
    }

    tupdatum = index_getattr(tuple, cur->sk_attno, tupdesc, &tupnull);

    if (likely(!(cur->sk_flags & (SK_BT_MINVAL | SK_BT_MAXVAL))))
    {
      /* Scankey has a valid/comparable sk_argument value */
      result = _bt_compare_array_skey(&so->orderProcs[ikey],
                      tupdatum, tupnull,
                      cur->sk_argument, cur);

      if (result == 0)
      {
        /*
         * Interpret result in a way that takes NEXT/PRIOR into
         * account
         */
        if (cur->sk_flags & SK_BT_NEXT)
          result = -1;
        else if (cur->sk_flags & SK_BT_PRIOR)
          result = 1;

        Assert(result == 0 || (cur->sk_flags & SK_BT_SKIP));
      }
    }
    else
    {
      BTArrayKeyInfo *array = NULL;

      /*
       * Current array element/array = scan key value is a sentinel
       * value that represents the lowest (or highest) possible value
       * that's still within the range of the array.
       *
       * Like _bt_first, we only see MINVAL keys during forwards scans
       * (and similarly only see MAXVAL keys during backwards scans).
       * Even if the scan's direction changes, we'll stop at some higher
       * order key before we can ever reach any MAXVAL (or MINVAL) keys.
       * (However, unlike _bt_first we _can_ get to keys marked either
       * NEXT or PRIOR, regardless of the scan's current direction.)
       */
      Assert(ScanDirectionIsForward(dir) ?
           !(cur->sk_flags & SK_BT_MAXVAL) :
           !(cur->sk_flags & SK_BT_MINVAL));

      /*
       * There are no valid sk_argument values in MINVAL/MAXVAL keys.
       * Check if tupdatum is within the range of skip array instead.
       */
      for (int arrayidx = 0; arrayidx < so->numArrayKeys; arrayidx++)
      {
        array = &so->arrayKeys[arrayidx];
        if (array->scan_key == ikey)
          break;
      }

      _bt_binsrch_skiparray_skey(false, dir, tupdatum, tupnull,
                     array, cur, &result);

      if (result == 0)
      {
        /*
         * tupdatum satisfies both low_compare and high_compare, so
         * it's time to advance the array keys.
         *
         * Note: It's possible that the skip array will "advance" from
         * its MINVAL (or MAXVAL) representation to an alternative,
         * logically equivalent representation of the same value: a
         * representation where the = key gets a valid datum in its
         * sk_argument.  This is only possible when low_compare uses
         * the >= strategy (or high_compare uses the <= strategy).
         */
        return false;
      }
    }

    /*
     * Does this comparison indicate that caller must _not_ advance the
     * scan's arrays just yet?
     */
    if ((ScanDirectionIsForward(dir) && result < 0) ||
      (ScanDirectionIsBackward(dir) && result > 0))
      return true;

    /*
     * Does this comparison indicate that caller should now advance the
     * scan's arrays?  (Must be if we get here during a readpagetup call.)
     */
    if (readpagetup || result != 0)
    {
      Assert(result != 0);
      return false;
    }

    /*
     * Inconclusive -- need to check later scan keys, too.
     *
     * This must be a finaltup precheck, or a call made from an assertion.
     */
    Assert(result == 0);
  }

  Assert(!readpagetup);

  return false;
}

/*
 * Determine if a scan with array keys should skip over uninteresting tuples.
 *
 * This is a subroutine for _bt_checkkeys, called when _bt_readpage's linear
 * search process has scanned an excessive number of tuples whose key space is
 * "between arrays".  (The linear search process is started after _bt_readpage
 * finishes reading an initial group of matching tuples.  It locates the start
 * of the first group of tuples matching the next set of required array keys.)
 *
 * When look ahead is successful, we set pstate.skip which
 * instructs _bt_readpage to skip ahead to that tuple next (could be past the
 * end of the scan's leaf page).  Pages where the optimization is effective
 * will generally still need to skip several times.  Each call here performs
 * only a single "look ahead" comparison of a later tuple, whose distance from
 * the current tuple is determined by heuristics.
 */
static void
_bt_checkkeys_look_ahead(IndexScanDesc scan, BTReadPageState *pstate,
             int tupnatts, TupleDesc tupdesc)
{
  ScanDirection dir = pstate->dir;
  OffsetNumber aheadoffnum;
  IndexTuple  ahead;

  Assert(!pstate->forcenonrequired);

  /* Avoid looking ahead when comparing the page high key */
  if (pstate->offnum < pstate->minoff)
    return;

  /*
   * Don't look ahead when there aren't enough tuples remaining on the page
   * (in the current scan direction) for it to be worth our while
   */
  if (ScanDirectionIsForward(dir) &&
    pstate->offnum >= pstate->maxoff - LOOK_AHEAD_DEFAULT_DISTANCE)
    return;
  else if (ScanDirectionIsBackward(dir) &&
       pstate->offnum <= pstate->minoff + LOOK_AHEAD_DEFAULT_DISTANCE)
    return;

  /*
   * The look ahead distance starts small, and ramps up as each call here
   * allows _bt_readpage to skip over more tuples
   */
  if (!pstate->targetdistance)
    pstate->targetdistance = LOOK_AHEAD_DEFAULT_DISTANCE;
  else if (pstate->targetdistance < MaxIndexTuplesPerPage / 2)
    pstate->targetdistance *= 2;

  /* Don't read past the end (or before the start) of the page, though */
  if (ScanDirectionIsForward(dir))
    aheadoffnum = Min((int) pstate->maxoff,
              (int) pstate->offnum + pstate->targetdistance);
  else
    aheadoffnum = Max((int) pstate->minoff,
              (int) pstate->offnum - pstate->targetdistance);

  ahead = (IndexTuple) PageGetItem(pstate->page,
                   PageGetItemId(pstate->page, aheadoffnum));
  if (_bt_tuple_before_array_skeys(scan, dir, ahead, tupdesc, tupnatts,
                   false, 0, NULL))
  {
    /*
     * Success -- instruct _bt_readpage to skip ahead to very next tuple
     * after the one we determined was still before the current array keys
     */
    if (ScanDirectionIsForward(dir))
      pstate->skip = aheadoffnum + 1;
    else
      pstate->skip = aheadoffnum - 1;
  }
  else
  {
    /*
     * Failure -- "ahead" tuple is too far ahead (we were too aggressive).
     *
     * Reset the number of rechecks, and aggressively reduce the target
     * distance (we're much more aggressive here than we were when the
     * distance was initially ramped up).
     */
    pstate->rechecks = 0;
    pstate->targetdistance = Max(pstate->targetdistance / 8, 1);
  }
}

/*
 * _bt_advance_array_keys() -- Advance array elements using a tuple
 *
 * The scan always gets a new qual as a consequence of calling here (except
 * when we determine that the top-level scan has run out of matching tuples).
 * All later _bt_check_compare calls also use the same new qual that was first
 * used here (at least until the next call here advances the keys once again).
 * It's convenient to structure _bt_check_compare rechecks of caller's tuple
 * (using the new qual) as one the steps of advancing the scan's array keys,
 * so this function works as a wrapper around _bt_check_compare.
 *
 * Like _bt_check_compare, we'll set pstate.continuescan on behalf of the
 * caller, and return a boolean indicating if caller's tuple satisfies the
 * scan's new qual.  But unlike _bt_check_compare, we set so->needPrimScan
 * when we set continuescan=false, indicating if a new primitive index scan
 * has been scheduled (otherwise, the top-level scan has run out of tuples in
 * the current scan direction).
 *
 * Caller must use _bt_tuple_before_array_skeys to determine if the current
 * place in the scan is >= the current array keys _before_ calling here.
 * We're responsible for ensuring that caller's tuple is <= the newly advanced
 * required array keys once we return.  We try to find an exact match, but
 * failing that we'll advance the array keys to whatever set of array elements
 * comes next in the key space for the current scan direction.  Required array
 * keys "ratchet forwards" (or backwards).  They can only advance as the scan
 * itself advances through the index/key space.
 *
 * (The rules are the same for backwards scans, except that the operators are
 * flipped: just replace the precondition's >= operator with a <=, and the
 * postcondition's <= operator with a >=.  In other words, just swap the
 * precondition with the postcondition.)
 *
 * We also deal with "advancing" non-required arrays here (or arrays that are
 * treated as non-required for the duration of a _bt_readpage call).  Callers
 * whose sktrig scan key is non-required specify sktrig_required=false.  These
 * calls are the only exception to the general rule about always advancing the
 * required array keys (the scan may not even have a required array).  These
 * callers should just pass a NULL pstate (since there is never any question
 * of stopping the scan).  No call to _bt_tuple_before_array_skeys is required
 * ahead of these calls (it's already clear that any required scan keys must
 * be satisfied by caller's tuple).
 *
 * Note that we deal with non-array required equality strategy scan keys as
 * degenerate single element arrays here.  Obviously, they can never really
 * advance in the way that real arrays can, but they must still affect how we
 * advance real array scan keys (exactly like true array equality scan keys).
 * We have to keep around a 3-way ORDER proc for these (using the "=" operator
 * won't do), since in general whether the tuple is < or > _any_ unsatisfied
 * required equality key influences how the scan's real arrays must advance.
 *
 * Note also that we may sometimes need to advance the array keys when the
 * existing required array keys (and other required equality keys) are already
 * an exact match for every corresponding value from caller's tuple.  We must
 * do this for inequalities that _bt_check_compare set continuescan=false for.
 * They'll advance the array keys here, just like any other scan key that
 * _bt_check_compare stops on.  (This can even happen _after_ we advance the
 * array keys, in which case we'll advance the array keys a second time.  That
 * way _bt_checkkeys caller always has its required arrays advance to the
 * maximum possible extent that its tuple will allow.)
 */
static bool
_bt_advance_array_keys(IndexScanDesc scan, BTReadPageState *pstate,
             IndexTuple tuple, int tupnatts, TupleDesc tupdesc,
             int sktrig, bool sktrig_required)
{
  BTScanOpaque so = (BTScanOpaque) scan->opaque;
  Relation  rel = scan->indexRelation;
  ScanDirection dir = pstate ? pstate->dir : ForwardScanDirection;
  int     arrayidx = 0;
  bool    beyond_end_advance = false,
        skip_array_advanced = false,
        has_required_opposite_direction_only = false,
        all_required_satisfied = true,
        all_satisfied = true;

  Assert(!so->needPrimScan && !so->scanBehind && !so->oppositeDirCheck);
  Assert(_bt_verify_keys_with_arraykeys(scan));

  if (sktrig_required)
  {
    /*
     * Precondition array state assertion
     */
    Assert(!_bt_tuple_before_array_skeys(scan, dir, tuple, tupdesc,
                       tupnatts, false, 0, NULL));

    /*
     * Once we return we'll have a new set of required array keys, so
     * reset state used by "look ahead" optimization
     */
    pstate->rechecks = 0;
    pstate->targetdistance = 0;
  }
  else if (sktrig < so->numberOfKeys - 1 &&
       !(so->keyData[so->numberOfKeys - 1].sk_flags & SK_SEARCHARRAY))
  {
    int     least_sign_ikey = so->numberOfKeys - 1;
    bool    continuescan;

    /*
     * Optimization: perform a precheck of the least significant key
     * during !sktrig_required calls when it isn't already our sktrig
     * (provided the precheck key is not itself an array).
     *
     * When the precheck works out we'll avoid an expensive binary search
     * of sktrig's array (plus any other arrays before least_sign_ikey).
     */
    Assert(so->keyData[sktrig].sk_flags & SK_SEARCHARRAY);
    if (!_bt_check_compare(scan, dir, tuple, tupnatts, tupdesc, false,
                 false, &continuescan,
                 &least_sign_ikey))
      return false;
  }

  for (int ikey = 0; ikey < so->numberOfKeys; ikey++)
  {
    ScanKey   cur = so->keyData + ikey;
    BTArrayKeyInfo *array = NULL;
    Datum   tupdatum;
    bool    required = false,
          tupnull;
    int32   result;
    int     set_elem = 0;

    if (cur->sk_flags & SK_SEARCHARRAY)
    {
      /* Manage array state */
      array = &so->arrayKeys[arrayidx++];

      Assert(array->scan_key == ikey);
      Assert(cur->sk_strategy == BTEqualStrategyNumber);
    }
    else if (cur->sk_strategy != BTEqualStrategyNumber)
    {
      /*
       * Are any inequalities required in the opposite direction only
       * present here?
       */
      if (((ScanDirectionIsForward(dir) &&
          (cur->sk_flags & (SK_BT_REQBKWD))) ||
         (ScanDirectionIsBackward(dir) &&
          (cur->sk_flags & (SK_BT_REQFWD)))))
        has_required_opposite_direction_only = true;
    }

    /* Optimization: skip over known-satisfied scan keys */
    if (ikey < sktrig)
      continue;

    if (cur->sk_flags & (SK_BT_REQFWD | SK_BT_REQBKWD))
    {
      required = true;

      if (cur->sk_attno > tupnatts)
      {
        /* Set this just like _bt_tuple_before_array_skeys */
        Assert(sktrig < ikey);
        so->scanBehind = true;
      }
    }

    /*
     * Handle a required non-array scan key that the initial call to
     * _bt_check_compare indicated triggered array advancement, if any.
     *
     * The non-array scan key's strategy will be <, <=, or = during a
     * forwards scan (or any one of =, >=, or > during a backwards scan).
     * It follows that the corresponding tuple attribute's value must now
     * be either > or >= the scan key value (for backwards scans it must
     * be either < or <= that value).
     *
     * If this is a required equality strategy scan key, this is just an
     * optimization; _bt_tuple_before_array_skeys already confirmed that
     * this scan key places us ahead of caller's tuple.  There's no need
     * to repeat that work now.  (The same underlying principle also gets
     * applied by the cur_elem_trig optimization used to speed up searches
     * for the next array element.)
     *
     * If this is a required inequality strategy scan key, we _must_ rely
     * on _bt_check_compare like this; we aren't capable of directly
     * evaluating required inequality strategy scan keys here, on our own.
     */
    if (ikey == sktrig && !array)
    {
      Assert(sktrig_required && required && all_required_satisfied);

      /* Use "beyond end" advancement.  See below for an explanation. */
      beyond_end_advance = true;
      all_satisfied = all_required_satisfied = false;

      continue;
    }

    /*
     * Nothing more for us to do with an inequality strategy scan key that
     * wasn't the one that _bt_check_compare stopped on, though.
     *
     * Note: if our later call to _bt_check_compare (to recheck caller's
     * tuple) sets continuescan=false due to finding this same inequality
     * unsatisfied (possible when it's required in the scan direction),
     * we'll deal with it via a recursive "second pass" call.
     */
    else if (cur->sk_strategy != BTEqualStrategyNumber)
      continue;

    /*
     * Nothing for us to do with an equality strategy scan key that isn't
     * marked required, either -- unless it's a non-required array
     */
    else if (!required && !array)
      continue;

    /*
     * Here we perform steps for all array scan keys after a required
     * array scan key whose binary search triggered "beyond end of array
     * element" array advancement due to encountering a tuple attribute
     * value > the closest matching array key (or < for backwards scans).
     */
    if (beyond_end_advance)
    {
      if (array)
        _bt_array_set_low_or_high(rel, cur, array,
                      ScanDirectionIsBackward(dir));

      continue;
    }

    /*
     * Here we perform steps for all array scan keys after a required
     * array scan key whose tuple attribute was < the closest matching
     * array key when we dealt with it (or > for backwards scans).
     *
     * This earlier required array key already puts us ahead of caller's
     * tuple in the key space (for the current scan direction).  We must
     * make sure that subsequent lower-order array keys do not put us too
     * far ahead (ahead of tuples that have yet to be seen by our caller).
     * For example, when a tuple "(a, b) = (42, 5)" advances the array
     * keys on "a" from 40 to 45, we must also set "b" to whatever the
     * first array element for "b" is.  It would be wrong to allow "b" to
     * be set based on the tuple value.
     *
     * Perform the same steps with truncated high key attributes.  You can
     * think of this as a "binary search" for the element closest to the
     * value -inf.  Again, the arrays must never get ahead of the scan.
     */
    if (!all_required_satisfied || cur->sk_attno > tupnatts)
    {
      if (array)
        _bt_array_set_low_or_high(rel, cur, array,
                      ScanDirectionIsForward(dir));

      continue;
    }

    /*
     * Search in scankey's array for the corresponding tuple attribute
     * value from caller's tuple
     */
    tupdatum = index_getattr(tuple, cur->sk_attno, tupdesc, &tupnull);

    if (array)
    {
      bool    cur_elem_trig = (sktrig_required && ikey == sktrig);

      /*
       * "Binary search" by checking if tupdatum/tupnull are within the
       * range of the skip array
       */
      if (array->num_elems == -1)
        _bt_binsrch_skiparray_skey(cur_elem_trig, dir,
                       tupdatum, tupnull, array, cur,
                       &result);

      /*
       * Binary search for the closest match from the SAOP array
       */
      else
        set_elem = _bt_binsrch_array_skey(&so->orderProcs[ikey],
                          cur_elem_trig, dir,
                          tupdatum, tupnull, array, cur,
                          &result);
    }
    else
    {
      Assert(required);

      /*
       * This is a required non-array equality strategy scan key, which
       * we'll treat as a degenerate single element array.
       *
       * This scan key's imaginary "array" can't really advance, but it
       * can still roll over like any other array.  (Actually, this is
       * no different to real single value arrays, which never advance
       * without rolling over -- they can never truly advance, either.)
       */
      result = _bt_compare_array_skey(&so->orderProcs[ikey],
                      tupdatum, tupnull,
                      cur->sk_argument, cur);
    }

    /*
     * Consider "beyond end of array element" array advancement.
     *
     * When the tuple attribute value is > the closest matching array key
     * (or < in the backwards scan case), we need to ratchet this array
     * forward (backward) by one increment, so that caller's tuple ends up
     * being < final array value instead (or > final array value instead).
     * This process has to work for all of the arrays, not just this one:
     * it must "carry" to higher-order arrays when the set_elem that we
     * just found happens to be the final one for the scan's direction.
     * Incrementing (decrementing) set_elem itself isn't good enough.
     *
     * Our approach is to provisionally use set_elem as if it was an exact
     * match now, then set each later/less significant array to whatever
     * its final element is.  Once outside the loop we'll then "increment
     * this array's set_elem" by calling _bt_advance_array_keys_increment.
     * That way the process rolls over to higher order arrays as needed.
     *
     * Under this scheme any required arrays only ever ratchet forwards
     * (or backwards), and always do so to the maximum possible extent
     * that we can know will be safe without seeing the scan's next tuple.
     * We don't need any special handling for required scan keys that lack
     * a real array to advance, nor for redundant scan keys that couldn't
     * be eliminated by _bt_preprocess_keys.  It won't matter if some of
     * our "true" array scan keys (or even all of them) are non-required.
     */
    if (sktrig_required && required &&
      ((ScanDirectionIsForward(dir) && result > 0) ||
       (ScanDirectionIsBackward(dir) && result < 0)))
      beyond_end_advance = true;

    Assert(all_required_satisfied && all_satisfied);
    if (result != 0)
    {
      /*
       * Track whether caller's tuple satisfies our new post-advancement
       * qual, for required scan keys, as well as for the entire set of
       * interesting scan keys (all required scan keys plus non-required
       * array scan keys are considered interesting.)
       */
      all_satisfied = false;
      if (sktrig_required && required)
        all_required_satisfied = false;
      else
      {
        /*
         * There's no need to advance the arrays using the best
         * available match for a non-required array.  Give up now.
         * (Though note that sktrig_required calls still have to do
         * all the usual post-advancement steps, including the recheck
         * call to _bt_check_compare.)
         */
        break;
      }
    }

    /* Advance array keys, even when we don't have an exact match */
    if (array)
    {
      if (array->num_elems == -1)
      {
        /* Skip array's new element is tupdatum (or MINVAL/MAXVAL) */
        _bt_skiparray_set_element(rel, cur, array, result,
                      tupdatum, tupnull);
        skip_array_advanced = true;
      }
      else if (array->cur_elem != set_elem)
      {
        /* SAOP array's new element is set_elem datum */
        array->cur_elem = set_elem;
        cur->sk_argument = array->elem_values[set_elem];
      }
    }
  }

  /*
   * Advance the array keys incrementally whenever "beyond end of array
   * element" array advancement happens, so that advancement will carry to
   * higher-order arrays (might exhaust all the scan's arrays instead, which
   * ends the top-level scan).
   */
  if (beyond_end_advance &&
    !_bt_advance_array_keys_increment(scan, dir, &skip_array_advanced))
    goto end_toplevel_scan;

  Assert(_bt_verify_keys_with_arraykeys(scan));

  /*
   * Maintain a page-level count of the number of times the scan's array
   * keys advanced in a way that affected at least one skip array
   */
  if (sktrig_required && skip_array_advanced)
    pstate->nskipadvances++;

  /*
   * Does tuple now satisfy our new qual?  Recheck with _bt_check_compare.
   *
   * Calls triggered by an unsatisfied required scan key, whose tuple now
   * satisfies all required scan keys, but not all nonrequired array keys,
   * will still require a recheck call to _bt_check_compare.  They'll still
   * need its "second pass" handling of required inequality scan keys.
   * (Might have missed a still-unsatisfied required inequality scan key
   * that caller didn't detect as the sktrig scan key during its initial
   * _bt_check_compare call that used the old/original qual.)
   *
   * Calls triggered by an unsatisfied nonrequired array scan key never need
   * "second pass" handling of required inequalities (nor any other handling
   * of any required scan key).  All that matters is whether caller's tuple
   * satisfies the new qual, so it's safe to just skip the _bt_check_compare
   * recheck when we've already determined that it can only return 'false'.
   *
   * Note: In practice most scan keys are marked required by preprocessing,
   * if necessary by generating a preceding skip array.  We nevertheless
   * often handle array keys marked required as if they were nonrequired.
   * This behavior is requested by our _bt_check_compare caller, though only
   * when it is passed "forcenonrequired=true" by _bt_checkkeys.
   */
  if ((sktrig_required && all_required_satisfied) ||
    (!sktrig_required && all_satisfied))
  {
    int     nsktrig = sktrig + 1;
    bool    continuescan;

    Assert(all_required_satisfied);

    /* Recheck _bt_check_compare on behalf of caller */
    if (_bt_check_compare(scan, dir, tuple, tupnatts, tupdesc, false,
                !sktrig_required, &continuescan,
                &nsktrig) &&
      !so->scanBehind)
    {
      /* This tuple satisfies the new qual */
      Assert(all_satisfied && continuescan);

      if (pstate)
        pstate->continuescan = true;

      return true;
    }

    /*
     * Consider "second pass" handling of required inequalities.
     *
     * It's possible that our _bt_check_compare call indicated that the
     * scan should end due to some unsatisfied inequality that wasn't
     * initially recognized as such by us.  Handle this by calling
     * ourselves recursively, this time indicating that the trigger is the
     * inequality that we missed first time around (and using a set of
     * required array/equality keys that are now exact matches for tuple).
     *
     * We make a strong, general guarantee that every _bt_checkkeys call
     * here will advance the array keys to the maximum possible extent
     * that we can know to be safe based on caller's tuple alone.  If we
     * didn't perform this step, then that guarantee wouldn't quite hold.
     */
    if (unlikely(!continuescan))
    {
      bool    satisfied PG_USED_FOR_ASSERTS_ONLY;

      Assert(sktrig_required);
      Assert(so->keyData[nsktrig].sk_strategy != BTEqualStrategyNumber);

      /*
       * The tuple must use "beyond end" advancement during the
       * recursive call, so we cannot possibly end up back here when
       * recursing.  We'll consume a small, fixed amount of stack space.
       */
      Assert(!beyond_end_advance);

      /* Advance the array keys a second time using same tuple */
      satisfied = _bt_advance_array_keys(scan, pstate, tuple, tupnatts,
                         tupdesc, nsktrig, true);

      /* This tuple doesn't satisfy the inequality */
      Assert(!satisfied);
      return false;
    }

    /*
     * Some non-required scan key (from new qual) still not satisfied.
     *
     * All scan keys required in the current scan direction must still be
     * satisfied, though, so we can trust all_required_satisfied below.
     */
  }

  /*
   * When we were called just to deal with "advancing" non-required arrays,
   * this is as far as we can go (cannot stop the scan for these callers)
   */
  if (!sktrig_required)
  {
    /* Caller's tuple doesn't match any qual */
    return false;
  }

  /*
   * Postcondition array state assertion (for still-unsatisfied tuples).
   *
   * By here we have established that the scan's required arrays (scan must
   * have at least one required array) advanced, without becoming exhausted.
   *
   * Caller's tuple is now < the newly advanced array keys (or > when this
   * is a backwards scan), except in the case where we only got this far due
   * to an unsatisfied non-required scan key.  Verify that with an assert.
   *
   * Note: we don't just quit at this point when all required scan keys were
   * found to be satisfied because we need to consider edge-cases involving
   * scan keys required in the opposite direction only; those aren't tracked
   * by all_required_satisfied.
   */
  Assert(_bt_tuple_before_array_skeys(scan, dir, tuple, tupdesc, tupnatts,
                    false, 0, NULL) ==
       !all_required_satisfied);

  /*
   * We generally permit primitive index scans to continue onto the next
   * sibling page when the page's finaltup satisfies all required scan keys
   * at the point where we're between pages.
   *
   * If caller's tuple is also the page's finaltup, and we see that required
   * scan keys still aren't satisfied, start a new primitive index scan.
   */
  if (!all_required_satisfied && pstate->finaltup == tuple)
    goto new_prim_scan;

  /*
   * Proactively check finaltup (don't wait until finaltup is reached by the
   * scan) when it might well turn out to not be satisfied later on.
   *
   * Note: if so->scanBehind hasn't already been set for finaltup by us,
   * it'll be set during this call to _bt_tuple_before_array_skeys.  Either
   * way, it'll be set correctly (for the whole page) after this point.
   */
  if (!all_required_satisfied && pstate->finaltup &&
    _bt_tuple_before_array_skeys(scan, dir, pstate->finaltup, tupdesc,
                   BTreeTupleGetNAtts(pstate->finaltup, rel),
                   false, 0, &so->scanBehind))
    goto new_prim_scan;

  /*
   * When we encounter a truncated finaltup high key attribute, we're
   * optimistic about the chances of its corresponding required scan key
   * being satisfied when we go on to recheck it against tuples from this
   * page's right sibling leaf page.  We consider truncated attributes to be
   * satisfied by required scan keys, which allows the primitive index scan
   * to continue to the next leaf page.  We must set so->scanBehind to true
   * to remember that the last page's finaltup had "satisfied" required scan
   * keys for one or more truncated attribute values (scan keys required in
   * _either_ scan direction).
   *
   * There is a chance that _bt_readpage (which checks so->scanBehind) will
   * find that even the sibling leaf page's finaltup is < the new array
   * keys.  When that happens, our optimistic policy will have incurred a
   * single extra leaf page access that could have been avoided.
   *
   * A pessimistic policy would give backward scans a gratuitous advantage
   * over forward scans.  We'd punish forward scans for applying more
   * accurate information from the high key, rather than just using the
   * final non-pivot tuple as finaltup, in the style of backward scans.
   * Being pessimistic would also give some scans with non-required arrays a
   * perverse advantage over similar scans that use required arrays instead.
   *
   * This is similar to our scan-level heuristics, below.  They also set
   * scanBehind to speculatively continue the primscan onto the next page.
   */
  if (so->scanBehind)
  {
    /* Truncated high key -- _bt_scanbehind_checkkeys recheck scheduled */
  }

  /*
   * Handle inequalities marked required in the opposite scan direction.
   * They can also signal that we should start a new primitive index scan.
   *
   * It's possible that the scan is now positioned where "matching" tuples
   * begin, and that caller's tuple satisfies all scan keys required in the
   * current scan direction.  But if caller's tuple still doesn't satisfy
   * other scan keys that are required in the opposite scan direction only
   * (e.g., a required >= strategy scan key when scan direction is forward),
   * it's still possible that there are many leaf pages before the page that
   * _bt_first could skip straight to.  Groveling through all those pages
   * will always give correct answers, but it can be very inefficient.  We
   * must avoid needlessly scanning extra pages.
   *
   * Separately, it's possible that _bt_check_compare set continuescan=false
   * for a scan key that's required in the opposite direction only.  This is
   * a special case, that happens only when _bt_check_compare sees that the
   * inequality encountered a NULL value.  This signals the end of non-NULL
   * values in the current scan direction, which is reason enough to end the
   * (primitive) scan.  If this happens at the start of a large group of
   * NULL values, then we shouldn't expect to be called again until after
   * the scan has already read indefinitely-many leaf pages full of tuples
   * with NULL suffix values.  (_bt_first is expected to skip over the group
   * of NULLs by applying a similar "deduce NOT NULL" rule of its own, which
   * involves consing up an explicit SK_SEARCHNOTNULL key.)
   *
   * Apply a test against finaltup to detect and recover from the problem:
   * if even finaltup doesn't satisfy such an inequality, we just skip by
   * starting a new primitive index scan.  When we skip, we know for sure
   * that all of the tuples on the current page following caller's tuple are
   * also before the _bt_first-wise start of tuples for our new qual.  That
   * at least suggests many more skippable pages beyond the current page.
   * (when so->scanBehind and so->oppositeDirCheck are set, this'll happen
   * when we test the next page's finaltup/high key instead.)
   */
  else if (has_required_opposite_direction_only && pstate->finaltup &&
       unlikely(!_bt_oppodir_checkkeys(scan, dir, pstate->finaltup)))
    goto new_prim_scan;

continue_scan:

  /*
   * Stick with the ongoing primitive index scan for now.
   *
   * It's possible that later tuples will also turn out to have values that
   * are still < the now-current array keys (or > the current array keys).
   * Our caller will handle this by performing what amounts to a linear
   * search of the page, implemented by calling _bt_check_compare and then
   * _bt_tuple_before_array_skeys for each tuple.
   *
   * This approach has various advantages over a binary search of the page.
   * Repeated binary searches of the page (one binary search for every array
   * advancement) won't outperform a continuous linear search.  While there
   * are workloads that a naive linear search won't handle well, our caller
   * has a "look ahead" fallback mechanism to deal with that problem.
   */
  pstate->continuescan = true; /* Override _bt_check_compare */
  so->needPrimScan = false;  /* _bt_readpage has more tuples to check */

  if (so->scanBehind)
  {
    /*
     * Remember if recheck needs to call _bt_oppodir_checkkeys for next
     * page's finaltup (see above comments about "Handle inequalities
     * marked required in the opposite scan direction" for why).
     */
    so->oppositeDirCheck = has_required_opposite_direction_only;

    /*
     * skip by setting "look ahead" mechanism's offnum for forwards scans
     * (backwards scans check scanBehind flag directly instead)
     */
    if (ScanDirectionIsForward(dir))
      pstate->skip = pstate->maxoff + 1;
  }

  /* Caller's tuple doesn't match the new qual */
  return false;

new_prim_scan:

  Assert(pstate->finaltup); /* not on rightmost/leftmost page */

  /*
   * Looks like another primitive index scan is required.  But consider
   * continuing the current primscan based on scan-level heuristics.
   *
   * Continue the ongoing primitive scan (and schedule a recheck for when
   * the scan arrives on the next sibling leaf page) when it has already
   * read at least one leaf page before the one we're reading now.  This
   * makes primscan scheduling more efficient when scanning subsets of an
   * index with many distinct attribute values matching many array elements.
   * It encourages fewer, larger primitive scans where that makes sense.
   * This will in turn encourage _bt_readpage to apply the pstate.startikey
   * optimization more often.
   *
   * Also continue the ongoing primitive index scan when it is still on the
   * first page if there have been more than NSKIPADVANCES_THRESHOLD calls
   * here that each advanced at least one of the scan's skip arrays
   * (deliberately ignore advancements that only affected SAOP arrays here).
   * A page that cycles through this many skip array elements is quite
   * likely to neighbor similar pages, that we'll also need to read.
   *
   * Note: These heuristics aren't as aggressive as you might think.  We're
   * conservative about allowing a primitive scan to step from the first
   * leaf page it reads to the page's sibling page (we only allow it on
   * first pages whose finaltup strongly suggests that it'll work out, as
   * well as first pages that have a large number of skip array advances).
   * Clearing this first page finaltup hurdle is a strong signal in itself.
   *
   * Note: The NSKIPADVANCES_THRESHOLD heuristic exists only to avoid
   * pathological cases.  Specifically, cases where a skip scan should just
   * behave like a traditional full index scan, but ends up "skipping" again
   * and again, descending to the prior leaf page's direct sibling leaf page
   * each time.  This misbehavior would otherwise be possible during scans
   * that never quite manage to "clear the first page finaltup hurdle".
   */
  if (!pstate->firstpage || pstate->nskipadvances > NSKIPADVANCES_THRESHOLD)
  {
    /* Schedule a recheck once on the next (or previous) page */
    so->scanBehind = true;

    /* Continue the current primitive scan after all */
    goto continue_scan;
  }

  /*
   * End this primitive index scan, but schedule another.
   *
   * Note: We make a soft assumption that the current scan direction will
   * also be used within _bt_next, when it is asked to step off this page.
   * It is up to _bt_next to cancel this scheduled primitive index scan
   * whenever it steps to a page in the direction opposite currPos.dir.
   */
  pstate->continuescan = false;  /* Tell _bt_readpage we're done... */
  so->needPrimScan = true; /* ...but call _bt_first again */

  if (scan->parallel_scan)
    _bt_parallel_primscan_schedule(scan, so->currPos.currPage);

  /* Caller's tuple doesn't match the new qual */
  return false;

end_toplevel_scan:

  /*
   * End the current primitive index scan, but don't schedule another.
   *
   * This ends the entire top-level scan in the current scan direction.
   *
   * Note: The scan's arrays (including any non-required arrays) are now in
   * their final positions for the current scan direction.  If the scan
   * direction happens to change, then the arrays will already be in their
   * first positions for what will then be the current scan direction.
   */
  pstate->continuescan = false;  /* Tell _bt_readpage we're done... */
  so->needPrimScan = false;  /* ...and don't call _bt_first again */

  /* Caller's tuple doesn't match any qual */
  return false;
}

/*
 * _bt_advance_array_keys_increment() -- Advance to next set of array elements
 *
 * Advances the array keys by a single increment in the current scan
 * direction.  When there are multiple array keys this can roll over from the
 * lowest order array to higher order arrays.
 *
 * Returns true if there is another set of values to consider, false if not.
 * On true result, the scankeys are initialized with the next set of values.
 * On false result, the scankeys stay the same, and the array keys are not
 * advanced (every array remains at its final element for scan direction).
 */
static bool
_bt_advance_array_keys_increment(IndexScanDesc scan, ScanDirection dir,
                 bool *skip_array_set)
{
  Relation  rel = scan->indexRelation;
  BTScanOpaque so = (BTScanOpaque) scan->opaque;

  /*
   * We must advance the last array key most quickly, since it will
   * correspond to the lowest-order index column among the available
   * qualifications
   */
  for (int i = so->numArrayKeys - 1; i >= 0; i--)
  {
    BTArrayKeyInfo *array = &so->arrayKeys[i];
    ScanKey   skey = &so->keyData[array->scan_key];

    if (array->num_elems == -1)
      *skip_array_set = true;

    if (ScanDirectionIsForward(dir))
    {
      if (_bt_array_increment(rel, skey, array))
        return true;
    }
    else
    {
      if (_bt_array_decrement(rel, skey, array))
        return true;
    }

    /*
     * Couldn't increment (or decrement) array.  Handle array roll over.
     *
     * Start over at the array's lowest sorting value (or its highest
     * value, for backward scans)...
     */
    _bt_array_set_low_or_high(rel, skey, array,
                  ScanDirectionIsForward(dir));

    /* ...then increment (or decrement) next most significant array */
  }

  /*
   * The array keys are now exhausted.
   *
   * Restore the array keys to the state they were in immediately before we
   * were called.  This ensures that the arrays only ever ratchet in the
   * current scan direction.
   *
   * Without this, scans could overlook matching tuples when the scan
   * direction gets reversed just before btgettuple runs out of items to
   * return, but just after _bt_readpage prepares all the items from the
   * scan's final page in so->currPos.  When we're on the final page it is
   * typical for so->currPos to get invalidated once btgettuple finally
   * returns false, which'll effectively invalidate the scan's array keys.
   * That hasn't happened yet, though -- and in general it may never happen.
   */
  _bt_start_array_keys(scan, -dir);

  return false;
}

/*
 * _bt_array_increment() -- increment array scan key's sk_argument
 *
 * Return value indicates whether caller's array was successfully incremented.
 * Cannot increment an array whose current element is already the final one.
 */
static bool
_bt_array_increment(Relation rel, ScanKey skey, BTArrayKeyInfo *array)
{
  bool    oflow = false;
  Datum   inc_sk_argument;

  Assert(skey->sk_flags & SK_SEARCHARRAY);
  Assert(!(skey->sk_flags & (SK_BT_MINVAL | SK_BT_NEXT | SK_BT_PRIOR)));

  /* SAOP array? */
  if (array->num_elems != -1)
  {
    Assert(!(skey->sk_flags & (SK_BT_SKIP | SK_BT_MINVAL | SK_BT_MAXVAL)));
    if (array->cur_elem < array->num_elems - 1)
    {
      /*
       * Just increment current element, and assign its datum to skey
       * (only skip arrays need us to free existing sk_argument memory)
       */
      array->cur_elem++;
      skey->sk_argument = array->elem_values[array->cur_elem];

      /* Successfully incremented array */
      return true;
    }

    /* Cannot increment past final array element */
    return false;
  }

  /* Nope, this is a skip array */
  Assert(skey->sk_flags & SK_BT_SKIP);

  /*
   * The sentinel value that represents the maximum value within the range
   * of a skip array (often just +inf) is never incrementable
   */
  if (skey->sk_flags & SK_BT_MAXVAL)
    return false;

  /*
   * When the current array element is NULL, and the highest sorting value
   * in the index is also NULL, we cannot increment past the final element
   */
  if ((skey->sk_flags & SK_ISNULL) && !(skey->sk_flags & SK_BT_NULLS_FIRST))
    return false;

  /*
   * Opclasses without skip support "increment" the scan key's current
   * element by setting the NEXT flag.  The true next value is determined by
   * repositioning to the first index tuple > existing sk_argument/current
   * array element.  Note that this works in the usual way when the scan key
   * is already marked ISNULL (i.e. when the current element is NULL).
   */
  if (!array->sksup)
  {
    /* Successfully "incremented" array */
    skey->sk_flags |= SK_BT_NEXT;
    return true;
  }

  /*
   * Opclasses with skip support directly increment sk_argument
   */
  if (skey->sk_flags & SK_ISNULL)
  {
    Assert(skey->sk_flags & SK_BT_NULLS_FIRST);

    /*
     * Existing sk_argument/array element is NULL (for an IS NULL qual).
     *
     * "Increment" from NULL to the low_elem value provided by opclass
     * skip support routine.
     */
    skey->sk_flags &= ~(SK_SEARCHNULL | SK_ISNULL);
    skey->sk_argument = datumCopy(array->sksup->low_elem,
                    array->attbyval, array->attlen);
    return true;
  }

  /*
   * Ask opclass support routine to provide incremented copy of existing
   * non-NULL sk_argument
   */
  inc_sk_argument = array->sksup->increment(rel, skey->sk_argument, &oflow);
  if (unlikely(oflow))
  {
    /* inc_sk_argument has undefined value (so no pfree) */
    if (array->null_elem && !(skey->sk_flags & SK_BT_NULLS_FIRST))
    {
      _bt_skiparray_set_isnull(rel, skey, array);

      /* Successfully "incremented" array to NULL */
      return true;
    }

    /* Cannot increment past final array element */
    return false;
  }

  /*
   * Successfully incremented sk_argument to a non-NULL value.  Make sure
   * that the incremented value is still within the range of the array.
   */
  if (array->high_compare &&
    !DatumGetBool(FunctionCall2Coll(&array->high_compare->sk_func,
                    array->high_compare->sk_collation,
                    inc_sk_argument,
                    array->high_compare->sk_argument)))
  {
    /* Keep existing sk_argument after all */
    if (!array->attbyval)
      pfree(DatumGetPointer(inc_sk_argument));

    /* Cannot increment past final array element */
    return false;
  }

  /* Accept value returned by opclass increment callback */
  if (!array->attbyval && skey->sk_argument)
    pfree(DatumGetPointer(skey->sk_argument));
  skey->sk_argument = inc_sk_argument;

  /* Successfully incremented array */
  return true;
}

/*
 * _bt_array_decrement() -- decrement array scan key's sk_argument
 *
 * Return value indicates whether caller's array was successfully decremented.
 * Cannot decrement an array whose current element is already the first one.
 */
static bool
_bt_array_decrement(Relation rel, ScanKey skey, BTArrayKeyInfo *array)
{
  bool    uflow = false;
  Datum   dec_sk_argument;

  Assert(skey->sk_flags & SK_SEARCHARRAY);
  Assert(!(skey->sk_flags & (SK_BT_MAXVAL | SK_BT_NEXT | SK_BT_PRIOR)));

  /* SAOP array? */
  if (array->num_elems != -1)
  {
    Assert(!(skey->sk_flags & (SK_BT_SKIP | SK_BT_MINVAL | SK_BT_MAXVAL)));
    if (array->cur_elem > 0)
    {
      /*
       * Just decrement current element, and assign its datum to skey
       * (only skip arrays need us to free existing sk_argument memory)
       */
      array->cur_elem--;
      skey->sk_argument = array->elem_values[array->cur_elem];

      /* Successfully decremented array */
      return true;
    }

    /* Cannot decrement to before first array element */
    return false;
  }

  /* Nope, this is a skip array */
  Assert(skey->sk_flags & SK_BT_SKIP);

  /*
   * The sentinel value that represents the minimum value within the range
   * of a skip array (often just -inf) is never decrementable
   */
  if (skey->sk_flags & SK_BT_MINVAL)
    return false;

  /*
   * When the current array element is NULL, and the lowest sorting value in
   * the index is also NULL, we cannot decrement before first array element
   */
  if ((skey->sk_flags & SK_ISNULL) && (skey->sk_flags & SK_BT_NULLS_FIRST))
    return false;

  /*
   * Opclasses without skip support "decrement" the scan key's current
   * element by setting the PRIOR flag.  The true prior value is determined
   * by repositioning to the last index tuple < existing sk_argument/current
   * array element.  Note that this works in the usual way when the scan key
   * is already marked ISNULL (i.e. when the current element is NULL).
   */
  if (!array->sksup)
  {
    /* Successfully "decremented" array */
    skey->sk_flags |= SK_BT_PRIOR;
    return true;
  }

  /*
   * Opclasses with skip support directly decrement sk_argument
   */
  if (skey->sk_flags & SK_ISNULL)
  {
    Assert(!(skey->sk_flags & SK_BT_NULLS_FIRST));

    /*
     * Existing sk_argument/array element is NULL (for an IS NULL qual).
     *
     * "Decrement" from NULL to the high_elem value provided by opclass
     * skip support routine.
     */
    skey->sk_flags &= ~(SK_SEARCHNULL | SK_ISNULL);
    skey->sk_argument = datumCopy(array->sksup->high_elem,
                    array->attbyval, array->attlen);
    return true;
  }

  /*
   * Ask opclass support routine to provide decremented copy of existing
   * non-NULL sk_argument
   */
  dec_sk_argument = array->sksup->decrement(rel, skey->sk_argument, &uflow);
  if (unlikely(uflow))
  {
    /* dec_sk_argument has undefined value (so no pfree) */
    if (array->null_elem && (skey->sk_flags & SK_BT_NULLS_FIRST))
    {
      _bt_skiparray_set_isnull(rel, skey, array);

      /* Successfully "decremented" array to NULL */
      return true;
    }

    /* Cannot decrement to before first array element */
    return false;
  }

  /*
   * Successfully decremented sk_argument to a non-NULL value.  Make sure
   * that the decremented value is still within the range of the array.
   */
  if (array->low_compare &&
    !DatumGetBool(FunctionCall2Coll(&array->low_compare->sk_func,
                    array->low_compare->sk_collation,
                    dec_sk_argument,
                    array->low_compare->sk_argument)))
  {
    /* Keep existing sk_argument after all */
    if (!array->attbyval)
      pfree(DatumGetPointer(dec_sk_argument));

    /* Cannot decrement to before first array element */
    return false;
  }

  /* Accept value returned by opclass decrement callback */
  if (!array->attbyval && skey->sk_argument)
    pfree(DatumGetPointer(skey->sk_argument));
  skey->sk_argument = dec_sk_argument;

  /* Successfully decremented array */
  return true;
}

/*
 * _bt_array_set_low_or_high() -- Set array scan key to lowest/highest element
 *
 * Caller also passes associated scan key, which will have its argument set to
 * the lowest/highest array value in passing.
 */
static void
_bt_array_set_low_or_high(Relation rel, ScanKey skey, BTArrayKeyInfo *array,
              bool low_not_high)
{
  Assert(skey->sk_flags & SK_SEARCHARRAY);

  if (array->num_elems != -1)
  {
    /* set low or high element for SAOP array */
    int     set_elem = 0;

    Assert(!(skey->sk_flags & SK_BT_SKIP));

    if (!low_not_high)
      set_elem = array->num_elems - 1;

    /*
     * Just copy over array datum (only skip arrays require freeing and
     * allocating memory for sk_argument)
     */
    array->cur_elem = set_elem;
    skey->sk_argument = array->elem_values[set_elem];

    return;
  }

  /* set low or high element for skip array */
  Assert(skey->sk_flags & SK_BT_SKIP);
  Assert(array->num_elems == -1);

  /* Free memory previously allocated for sk_argument if needed */
  if (!array->attbyval && skey->sk_argument)
    pfree(DatumGetPointer(skey->sk_argument));

  /* Reset flags */
  skey->sk_argument = (Datum) 0;
  skey->sk_flags &= ~(SK_SEARCHNULL | SK_ISNULL |
            SK_BT_MINVAL | SK_BT_MAXVAL |
            SK_BT_NEXT | SK_BT_PRIOR);

  if (array->null_elem &&
    (low_not_high == ((skey->sk_flags & SK_BT_NULLS_FIRST) != 0)))
  {
    /* Requested element (either lowest or highest) has the value NULL */
    skey->sk_flags |= (SK_SEARCHNULL | SK_ISNULL);
  }
  else if (low_not_high)
  {
    /* Setting array to lowest element (according to low_compare) */
    skey->sk_flags |= SK_BT_MINVAL;
  }
  else
  {
    /* Setting array to highest element (according to high_compare) */
    skey->sk_flags |= SK_BT_MAXVAL;
  }
}

/*
 * _bt_skiparray_set_element() -- Set skip array scan key's sk_argument
 *
 * Caller passes set_elem_result returned by _bt_binsrch_skiparray_skey for
 * caller's tupdatum/tupnull.
 *
 * We copy tupdatum/tupnull into skey's sk_argument iff set_elem_result == 0.
 * Otherwise, we set skey to either the lowest or highest value that's within
 * the range of caller's skip array (whichever is the best available match to
 * tupdatum/tupnull that is still within the range of the skip array according
 * to _bt_binsrch_skiparray_skey/set_elem_result).
 */
static void
_bt_skiparray_set_element(Relation rel, ScanKey skey, BTArrayKeyInfo *array,
              int32 set_elem_result, Datum tupdatum, bool tupnull)
{
  Assert(skey->sk_flags & SK_BT_SKIP);
  Assert(skey->sk_flags & SK_SEARCHARRAY);

  if (set_elem_result)
  {
    /* tupdatum/tupnull is out of the range of the skip array */
    Assert(!array->null_elem);

    _bt_array_set_low_or_high(rel, skey, array, set_elem_result < 0);
    return;
  }

  /* Advance skip array to tupdatum (or tupnull) value */
  if (unlikely(tupnull))
  {
    _bt_skiparray_set_isnull(rel, skey, array);
    return;
  }

  /* Free memory previously allocated for sk_argument if needed */
  if (!array->attbyval && skey->sk_argument)
    pfree(DatumGetPointer(skey->sk_argument));

  /* tupdatum becomes new sk_argument/new current element */
  skey->sk_flags &= ~(SK_SEARCHNULL | SK_ISNULL |
            SK_BT_MINVAL | SK_BT_MAXVAL |
            SK_BT_NEXT | SK_BT_PRIOR);
  skey->sk_argument = datumCopy(tupdatum, array->attbyval, array->attlen);
}

/*
 * _bt_skiparray_set_isnull() -- set skip array scan key to NULL
 */
static void
_bt_skiparray_set_isnull(Relation rel, ScanKey skey, BTArrayKeyInfo *array)
{
  Assert(skey->sk_flags & SK_BT_SKIP);
  Assert(skey->sk_flags & SK_SEARCHARRAY);
  Assert(array->null_elem && !array->low_compare && !array->high_compare);

  /* Free memory previously allocated for sk_argument if needed */
  if (!array->attbyval && skey->sk_argument)
    pfree(DatumGetPointer(skey->sk_argument));

  /* NULL becomes new sk_argument/new current element */
  skey->sk_argument = (Datum) 0;
  skey->sk_flags &= ~(SK_BT_MINVAL | SK_BT_MAXVAL |
            SK_BT_NEXT | SK_BT_PRIOR);
  skey->sk_flags |= (SK_SEARCHNULL | SK_ISNULL);
}

/*
 * _bt_compare_array_skey() -- apply array comparison function
 *
 * Compares caller's tuple attribute value to a scan key/array element.
 * Helper function used during binary searches of SK_SEARCHARRAY arrays.
 *
 *    This routine returns:
 *      <0 if tupdatum < arrdatum;
 *       0 if tupdatum == arrdatum;
 *      >0 if tupdatum > arrdatum.
 *
 * This is essentially the same interface as _bt_compare: both functions
 * compare the value that they're searching for to a binary search pivot.
 * However, unlike _bt_compare, this function's "tuple argument" comes first,
 * while its "array/scankey argument" comes second.
 */
static inline int32
_bt_compare_array_skey(FmgrInfo *orderproc,
             Datum tupdatum, bool tupnull,
             Datum arrdatum, ScanKey cur)
{
  int32   result = 0;

  Assert(cur->sk_strategy == BTEqualStrategyNumber);
  Assert(!(cur->sk_flags & (SK_BT_MINVAL | SK_BT_MAXVAL)));

  if (tupnull)       /* NULL tupdatum */
  {
    if (cur->sk_flags & SK_ISNULL)
      result = 0;     /* NULL "=" NULL */
    else if (cur->sk_flags & SK_BT_NULLS_FIRST)
      result = -1;   /* NULL "<" NOT_NULL */
    else
      result = 1;     /* NULL ">" NOT_NULL */
  }
  else if (cur->sk_flags & SK_ISNULL) /* NOT_NULL tupdatum, NULL arrdatum */
  {
    if (cur->sk_flags & SK_BT_NULLS_FIRST)
      result = 1;     /* NOT_NULL ">" NULL */
    else
      result = -1;   /* NOT_NULL "<" NULL */
  }
  else
  {
    /*
     * Like _bt_compare, we need to be careful of cross-type comparisons,
     * so the left value has to be the value that came from an index tuple
     */
    result = DatumGetInt32(FunctionCall2Coll(orderproc, cur->sk_collation,
                         tupdatum, arrdatum));

    /*
     * We flip the sign by following the obvious rule: flip whenever the
     * column is a DESC column.
     *
     * _bt_compare does it the wrong way around (flip when *ASC*) in order
     * to compensate for passing its orderproc arguments backwards.  We
     * don't need to play these games because we find it natural to pass
     * tupdatum as the left value (and arrdatum as the right value).
     */
    if (cur->sk_flags & SK_BT_DESC)
      INVERT_COMPARE_RESULT(result);
  }

  return result;
}

/*
 * _bt_binsrch_array_skey() -- Binary search for next matching array key
 *
 * Returns an index to the first array element >= caller's tupdatum argument.
 * This convention is more natural for forwards scan callers, but that can't
 * really matter to backwards scan callers.  Both callers require handling for
 * the case where the match we return is < tupdatum, and symmetric handling
 * for the case where our best match is > tupdatum.
 *
 * Also sets *set_elem_result to the result _bt_compare_array_skey returned
 * when we used it to compare the matching array element to tupdatum/tupnull.
 *
 * cur_elem_trig indicates if array advancement was triggered by this array's
 * scan key, and that the array is for a required scan key.  We can apply this
 * information to find the next matching array element in the current scan
 * direction using far fewer comparisons (fewer on average, compared to naive
 * binary search).  This scheme takes advantage of an important property of
 * required arrays: required arrays always advance in lockstep with the index
 * scan's progress through the index's key space.
 */
int
_bt_binsrch_array_skey(FmgrInfo *orderproc,
             bool cur_elem_trig, ScanDirection dir,
             Datum tupdatum, bool tupnull,
             BTArrayKeyInfo *array, ScanKey cur,
             int32 *set_elem_result)
{
  int     low_elem = 0,
        mid_elem = -1,
        high_elem = array->num_elems - 1,
        result = 0;
  Datum   arrdatum;

  Assert(cur->sk_flags & SK_SEARCHARRAY);
  Assert(!(cur->sk_flags & SK_BT_SKIP));
  Assert(!(cur->sk_flags & SK_ISNULL)); /* SAOP arrays never have NULLs */
  Assert(cur->sk_strategy == BTEqualStrategyNumber);

  if (cur_elem_trig)
  {
    Assert(!ScanDirectionIsNoMovement(dir));
    Assert(cur->sk_flags & SK_BT_REQFWD);

    /*
     * When the scan key that triggered array advancement is a required
     * array scan key, it is now certain that the current array element
     * (plus all prior elements relative to the current scan direction)
     * cannot possibly be at or ahead of the corresponding tuple value.
     * (_bt_checkkeys must have called _bt_tuple_before_array_skeys, which
     * makes sure this is true as a condition of advancing the arrays.)
     *
     * This makes it safe to exclude array elements up to and including
     * the former-current array element from our search.
     *
     * Separately, when array advancement was triggered by a required scan
     * key, the array element immediately after the former-current element
     * is often either an exact tupdatum match, or a "close by" near-match
     * (a near-match tupdatum is one whose key space falls _between_ the
     * former-current and new-current array elements).  We'll detect both
     * cases via an optimistic comparison of the new search lower bound
     * (or new search upper bound in the case of backwards scans).
     */
    if (ScanDirectionIsForward(dir))
    {
      low_elem = array->cur_elem + 1; /* old cur_elem exhausted */

      /* Compare prospective new cur_elem (also the new lower bound) */
      if (high_elem >= low_elem)
      {
        arrdatum = array->elem_values[low_elem];
        result = _bt_compare_array_skey(orderproc, tupdatum, tupnull,
                        arrdatum, cur);

        if (result <= 0)
        {
          /* Optimistic comparison optimization worked out */
          *set_elem_result = result;
          return low_elem;
        }
        mid_elem = low_elem;
        low_elem++;   /* this cur_elem exhausted, too */
      }

      if (high_elem < low_elem)
      {
        /* Caller needs to perform "beyond end" array advancement */
        *set_elem_result = 1;
        return high_elem;
      }
    }
    else
    {
      high_elem = array->cur_elem - 1;  /* old cur_elem exhausted */

      /* Compare prospective new cur_elem (also the new upper bound) */
      if (high_elem >= low_elem)
      {
        arrdatum = array->elem_values[high_elem];
        result = _bt_compare_array_skey(orderproc, tupdatum, tupnull,
                        arrdatum, cur);

        if (result >= 0)
        {
          /* Optimistic comparison optimization worked out */
          *set_elem_result = result;
          return high_elem;
        }
        mid_elem = high_elem;
        high_elem--;  /* this cur_elem exhausted, too */
      }

      if (high_elem < low_elem)
      {
        /* Caller needs to perform "beyond end" array advancement */
        *set_elem_result = -1;
        return low_elem;
      }
    }
  }

  while (high_elem > low_elem)
  {
    mid_elem = low_elem + ((high_elem - low_elem) / 2);
    arrdatum = array->elem_values[mid_elem];

    result = _bt_compare_array_skey(orderproc, tupdatum, tupnull,
                    arrdatum, cur);

    if (result == 0)
    {
      /*
       * It's safe to quit as soon as we see an equal array element.
       * This often saves an extra comparison or two...
       */
      low_elem = mid_elem;
      break;
    }

    if (result > 0)
      low_elem = mid_elem + 1;
    else
      high_elem = mid_elem;
  }

  /*
   * ...but our caller also cares about how its searched-for tuple datum
   * compares to the low_elem datum.  Must always set *set_elem_result with
   * the result of that comparison specifically.
   */
  if (low_elem != mid_elem)
    result = _bt_compare_array_skey(orderproc, tupdatum, tupnull,
                    array->elem_values[low_elem], cur);

  *set_elem_result = result;

  return low_elem;
}

/*
 * _bt_binsrch_skiparray_skey() -- "Binary search" within a skip array
 *
 * Does not return an index into the array, since skip arrays don't really
 * contain elements (they generate their array elements procedurally instead).
 * Our interface matches that of _bt_binsrch_array_skey in every other way.
 *
 * Sets *set_elem_result just like _bt_binsrch_array_skey would with a true
 * array.  The value 0 indicates that tupdatum/tupnull is within the range of
 * the skip array.  We return -1 when tupdatum/tupnull is lower that any value
 * within the range of the array, and 1 when it is higher than every value.
 * Caller should pass *set_elem_result to _bt_skiparray_set_element to advance
 * the array.
 *
 * cur_elem_trig indicates if array advancement was triggered by this array's
 * scan key.  We use this to optimize-away comparisons that are known by our
 * caller to be unnecessary from context, just like _bt_binsrch_array_skey.
 */
static void
_bt_binsrch_skiparray_skey(bool cur_elem_trig, ScanDirection dir,
               Datum tupdatum, bool tupnull,
               BTArrayKeyInfo *array, ScanKey cur,
               int32 *set_elem_result)
{
  Assert(cur->sk_flags & SK_BT_SKIP);
  Assert(cur->sk_flags & SK_SEARCHARRAY);
  Assert(cur->sk_flags & SK_BT_REQFWD);
  Assert(array->num_elems == -1);
  Assert(!ScanDirectionIsNoMovement(dir));

  if (array->null_elem)
  {
    Assert(!array->low_compare && !array->high_compare);

    *set_elem_result = 0;
    return;
  }

  if (tupnull)       /* NULL tupdatum */
  {
    if (cur->sk_flags & SK_BT_NULLS_FIRST)
      *set_elem_result = -1; /* NULL "<" NOT_NULL */
    else
      *set_elem_result = 1; /* NULL ">" NOT_NULL */
    return;
  }

  /*
   * Array inequalities determine whether tupdatum is within the range of
   * caller's skip array
   */
  *set_elem_result = 0;
  if (ScanDirectionIsForward(dir))
  {
    /*
     * Evaluate low_compare first (unless cur_elem_trig tells us that it
     * cannot possibly fail to be satisfied), then evaluate high_compare
     */
    if (!cur_elem_trig && array->low_compare &&
      !DatumGetBool(FunctionCall2Coll(&array->low_compare->sk_func,
                      array->low_compare->sk_collation,
                      tupdatum,
                      array->low_compare->sk_argument)))
      *set_elem_result = -1;
    else if (array->high_compare &&
         !DatumGetBool(FunctionCall2Coll(&array->high_compare->sk_func,
                         array->high_compare->sk_collation,
                         tupdatum,
                         array->high_compare->sk_argument)))
      *set_elem_result = 1;
  }
  else
  {
    /*
     * Evaluate high_compare first (unless cur_elem_trig tells us that it
     * cannot possibly fail to be satisfied), then evaluate low_compare
     */
    if (!cur_elem_trig && array->high_compare &&
      !DatumGetBool(FunctionCall2Coll(&array->high_compare->sk_func,
                      array->high_compare->sk_collation,
                      tupdatum,
                      array->high_compare->sk_argument)))
      *set_elem_result = 1;
    else if (array->low_compare &&
         !DatumGetBool(FunctionCall2Coll(&array->low_compare->sk_func,
                         array->low_compare->sk_collation,
                         tupdatum,
                         array->low_compare->sk_argument)))
      *set_elem_result = -1;
  }

  /*
   * Assert that any keys that were assumed to be satisfied already (due to
   * caller passing cur_elem_trig=true) really are satisfied as expected
   */
#ifdef USE_ASSERT_CHECKING
  if (cur_elem_trig)
  {
    if (ScanDirectionIsForward(dir) && array->low_compare)
      Assert(DatumGetBool(FunctionCall2Coll(&array->low_compare->sk_func,
                          array->low_compare->sk_collation,
                          tupdatum,
                          array->low_compare->sk_argument)));

    if (ScanDirectionIsBackward(dir) && array->high_compare)
      Assert(DatumGetBool(FunctionCall2Coll(&array->high_compare->sk_func,
                          array->high_compare->sk_collation,
                          tupdatum,
                          array->high_compare->sk_argument)));
  }
#endif
}

#ifdef USE_ASSERT_CHECKING
/*
 * Verify that the scan's "so->keyData[]" scan keys are in agreement with
 * its array key state
 */
static bool
_bt_verify_keys_with_arraykeys(IndexScanDesc scan)
{
  BTScanOpaque so = (BTScanOpaque) scan->opaque;
  int     last_sk_attno = InvalidAttrNumber,
        arrayidx = 0;
  bool    nonrequiredseen = false;

  if (!so->qual_ok)
    return false;

  for (int ikey = 0; ikey < so->numberOfKeys; ikey++)
  {
    ScanKey   cur = so->keyData + ikey;
    BTArrayKeyInfo *array;

    if (!(cur->sk_flags & SK_SEARCHARRAY))
      continue;

    if (cur->sk_strategy != BTEqualStrategyNumber)
      return false;

    array = &so->arrayKeys[arrayidx++];
    if (array->scan_key != ikey)
      return false;

    if (array->num_elems == 0 || array->num_elems < -1)
      return false;

    if (array->num_elems != -1 &&
      cur->sk_argument != array->elem_values[array->cur_elem])
      return false;
    if (cur->sk_flags & (SK_BT_REQFWD | SK_BT_REQBKWD))
    {
      if (last_sk_attno > cur->sk_attno)
        return false;
      if (nonrequiredseen)
        return false;
    }
    else
      nonrequiredseen = true;

    last_sk_attno = cur->sk_attno;
  }

  if (arrayidx != so->numArrayKeys)
    return false;

  return true;
}
#endif

Coverage Report

Created: 2026-10-16 06:48