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Improve nbtree unsatisfiable RowCompare detection.

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Move nbtree's detection of RowCompare quals that are unsatisfiable due
to having a NULL in their first row element: rather than detecting these
cases at the point where _bt_first builds its insertion scan key, do so
earlier, during preprocessing proper.  This brings the RowCompare case
in line every other case involving an unsatisfiable-due-to-NULL qual.

nbtree now consistently detects such unsatisfiable quals -- even when
they happen to involve a key that isn't examined by _bt_first at all.
Affected cases thereby avoid useless full index scans that cannot
possibly return any matching rows.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/CAH2-WzmySVXst2hFrOATC-zw1Byg1XC-jYUS314=mzuqsNwk+Q@mail.gmail.com
This commit is contained in:
Peter Geoghegan
2025-01-07 10:38:30 -05:00
parent 428a99b589
commit ec986020de
6 changed files with 248 additions and 16 deletions
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14
src/backend/access/nbtree/nbtutils.c
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@@ -3371,6 +3371,13 @@ _bt_fix_scankey_strategy(ScanKey skey, int16 *indoption)
{
ScanKey subkey = (ScanKey) DatumGetPointer(skey->sk_argument);
if (subkey->sk_flags & SK_ISNULL)
{
/* First row member is NULL, so RowCompare is unsatisfiable */
Assert(subkey->sk_flags & SK_ROW_MEMBER);
return false;
}
for (;;)
{
Assert(subkey->sk_flags & SK_ROW_MEMBER);
@@ -3982,13 +3989,14 @@ _bt_check_rowcompare(ScanKey skey, IndexTuple tuple, int tupnatts,
if (subkey->sk_flags & SK_ISNULL)
{
/*
* Unlike the simple-scankey case, this isn't a disallowed case.
* 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.
*/
if (subkey != (ScanKey) DatumGetPointer(skey->sk_argument))
subkey--;
Assert(subkey != (ScanKey) DatumGetPointer(skey->sk_argument));
subkey--;
if ((subkey->sk_flags & SK_BT_REQFWD) &&
ScanDirectionIsForward(dir))
*continuescan = false;