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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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26
src/backend/access/nbtree/nbtsearch.c
View File

@@ -1162,23 +1162,23 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
if (cur->sk_flags & SK_ROW_HEADER)
{
/*
* Row comparison header: look to the first row member instead.
*
* The member scankeys are already in insertion format (ie, they
* have sk_func = 3-way-comparison function), but we have to watch
* out for nulls, which _bt_preprocess_keys didn't check. A null
* in the first row member makes the condition unmatchable, just
* like qual_ok = false.
* Row comparison header: look to the first row member instead
*/
ScanKey subkey = (ScanKey) DatumGetPointer(cur->sk_argument);
/*
* Cannot be a NULL in the first row member: _bt_preprocess_keys
* would've marked the qual as unsatisfiable, preventing us from
* ever getting this far
*/
Assert(subkey->sk_flags & SK_ROW_MEMBER);
if (subkey->sk_flags & SK_ISNULL)
{
Assert(!so->needPrimScan);
_bt_parallel_done(scan);
return false;
}
Assert(subkey->sk_attno == cur->sk_attno);
Assert(!(subkey->sk_flags & SK_ISNULL));
/*
* The member scankeys are already in insertion format (ie, they
* have sk_func = 3-way-comparison function)
*/
memcpy(inskey.scankeys + i, subkey, sizeof(ScanKeyData));
/*

14
src/backend/access/nbtree/nbtutils.c
View File

@@ -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;