Always commute strategy when preprocessing DESC keys.

petergeoghegan · petergeoghegan · commit 796962922e69 · 2025-09-12T13:23:00.000-04:00
A recently added nbtree preprocessing step failed to account for the fact that DESC columns already had their B-Tree strategy number commuted at this point in preprocessing. As a result, preprocessing could output a set of scan keys where one or more keys had the correct strategy number, but used the wrong comparison routine. To fix, make the faulty code path that looks up a more restrictive replacement operator/comparison routine commute its requested inequality strategy (while outputting the transformed strategy number as before). This makes the final transformed scan key comport with the approach preprocessing has always used to deal with DESC columns (which is described by comments above _bt_fix_scankey_strategy). Oversight in commit commit b3f1a13, which made nbtree preprocessing perform transformations on skip array inequalities that can reduce the total number of index searches. Author: Peter Geoghegan <pg@bowt.ie> Reported-By: Natalya Aksman <natalya@timescale.com> Discussion: https://postgr.es/m/19049-b7df801e71de41b2@postgresql.org Backpatch-through: 18
diff --git a/src/backend/access/nbtree/nbtpreprocesskeys.c b/src/backend/access/nbtree/nbtpreprocesskeys.c
@@ -1412,6 +1412,7 @@ _bt_skiparray_strat_decrement(IndexScanDesc scan, ScanKey arraysk,
 	Datum		orig_sk_argument = high_compare->sk_argument,
 				new_sk_argument;
 	bool		uflow;
+	int16		lookupstrat;
 
 	Assert(high_compare->sk_strategy == BTLessStrategyNumber);
 
@@ -1433,9 +1434,14 @@ _bt_skiparray_strat_decrement(IndexScanDesc scan, ScanKey arraysk,
 		return;
 	}
 
-	/* Look up <= operator (might fail) */
-	leop = get_opfamily_member(opfamily, opcintype, opcintype,
-							   BTLessEqualStrategyNumber);
+	/*
+	 * Look up <= operator (might fail), accounting for the fact that a
+	 * high_compare on a DESC column already had its strategy commuted
+	 */
+	lookupstrat = BTLessEqualStrategyNumber;
+	if (high_compare->sk_flags & SK_BT_DESC)
+		lookupstrat = BTGreaterEqualStrategyNumber; /* commute this too */
+	leop = get_opfamily_member(opfamily, opcintype, opcintype, lookupstrat);
 	if (!OidIsValid(leop))
 		return;
 	cmp_proc = get_opcode(leop);
@@ -1464,6 +1470,7 @@ _bt_skiparray_strat_increment(IndexScanDesc scan, ScanKey arraysk,
 	Datum		orig_sk_argument = low_compare->sk_argument,
 				new_sk_argument;
 	bool		oflow;
+	int16		lookupstrat;
 
 	Assert(low_compare->sk_strategy == BTGreaterStrategyNumber);
 
@@ -1485,9 +1492,14 @@ _bt_skiparray_strat_increment(IndexScanDesc scan, ScanKey arraysk,
 		return;
 	}
 
-	/* Look up >= operator (might fail) */
-	geop = get_opfamily_member(opfamily, opcintype, opcintype,
-							   BTGreaterEqualStrategyNumber);
+	/*
+	 * Look up >= operator (might fail), accounting for the fact that a
+	 * low_compare on a DESC column already had its strategy commuted
+	 */
+	lookupstrat = BTGreaterEqualStrategyNumber;
+	if (low_compare->sk_flags & SK_BT_DESC)
+		lookupstrat = BTLessEqualStrategyNumber; /* commute this too */
+	geop = get_opfamily_member(opfamily, opcintype, opcintype, lookupstrat);
 	if (!OidIsValid(geop))
 		return;
 	cmp_proc = get_opcode(geop);
