Simplify and generalize PrepareSortSupportFromIndexRel()

PrepareSortSupportFromIndexRel() was accepting btree strategy numbers
purely for the purpose of comparing it later against btree strategies
to determine if the sort direction was forward or reverse.  Change
that.  Instead, pass a bool directly, to indicate the same without an
unfortunate assumption that a strategy number refers specifically to a
btree strategy.  (This is similar in spirit to commits 0d2aa4d and
c594f1a.)

(This could arguably be simplfied further by having the callers fill
in ssup_reverse directly.  But this way, it preserves consistency by
having all PrepareSortSupport*() variants be responsible for filling
in ssup_reverse.)

Moreover, remove the hardcoded check against BTREE_AM_OID, and check
against amcanorder instead, which is the actual requirement.

Co-authored-by: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com

Author: petere

src/backend/access/nbtree/nbtsort.c

@@ -1171,7 +1171,7 @@ _bt_load(BTWriteState *wstate, BTSpool *btspool, BTSpool *btspool2)
 		{
 			SortSupport sortKey = sortKeys + i;
 			ScanKey		scanKey = wstate->inskey->scankeys + i;
-			int16		strategy;
+			bool		reverse;
 
 			sortKey->ssup_cxt = CurrentMemoryContext;
 			sortKey->ssup_collation = scanKey->sk_collation;
@@ -1183,10 +1183,9 @@ _bt_load(BTWriteState *wstate, BTSpool *btspool, BTSpool *btspool2)
 
 			Assert(sortKey->ssup_attno != 0);
 
-			strategy = (scanKey->sk_flags & SK_BT_DESC) != 0 ?
-				BTGreaterStrategyNumber : BTLessStrategyNumber;
+			reverse = (scanKey->sk_flags & SK_BT_DESC) != 0;
 
-			PrepareSortSupportFromIndexRel(wstate->index, strategy, sortKey);
+			PrepareSortSupportFromIndexRel(wstate->index, reverse, sortKey);
 		}
 
 		for (;;)

src/backend/utils/sort/sortsupport.c

@@ -150,28 +150,25 @@ PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup)
 }
 
 /*
- * Fill in SortSupport given an index relation, attribute, and strategy.
+ * Fill in SortSupport given an index relation and attribute.
  *
  * Caller must previously have zeroed the SortSupportData structure and then
  * filled in ssup_cxt, ssup_attno, ssup_collation, and ssup_nulls_first.  This
- * will fill in ssup_reverse (based on the supplied strategy), as well as the
+ * will fill in ssup_reverse (based on the supplied argument), as well as the
  * comparator function pointer.
  */
 void
-PrepareSortSupportFromIndexRel(Relation indexRel, int16 strategy,
+PrepareSortSupportFromIndexRel(Relation indexRel, bool reverse,
 							   SortSupport ssup)
 {
 	Oid			opfamily = indexRel->rd_opfamily[ssup->ssup_attno - 1];
 	Oid			opcintype = indexRel->rd_opcintype[ssup->ssup_attno - 1];
 
 	Assert(ssup->comparator == NULL);
 
-	if (indexRel->rd_rel->relam != BTREE_AM_OID)
-		elog(ERROR, "unexpected non-btree AM: %u", indexRel->rd_rel->relam);
-	if (strategy != BTGreaterStrategyNumber &&
-		strategy != BTLessStrategyNumber)
-		elog(ERROR, "unexpected sort support strategy: %d", strategy);
-	ssup->ssup_reverse = (strategy == BTGreaterStrategyNumber);
+	if (!indexRel->rd_indam->amcanorder)
+		elog(ERROR, "unexpected non-amcanorder AM: %u", indexRel->rd_rel->relam);
+	ssup->ssup_reverse = reverse;
 
 	FinishSortSupportFunction(opfamily, opcintype, ssup);
 }

src/backend/utils/sort/tuplesortvariants.c

@@ -329,7 +329,7 @@ tuplesort_begin_cluster(TupleDesc tupDesc,
 	{
 		SortSupport sortKey = base->sortKeys + i;
 		ScanKey		scanKey = indexScanKey->scankeys + i;
-		int16		strategy;
+		bool		reverse;
 
 		sortKey->ssup_cxt = CurrentMemoryContext;
 		sortKey->ssup_collation = scanKey->sk_collation;
@@ -341,10 +341,9 @@ tuplesort_begin_cluster(TupleDesc tupDesc,
 
 		Assert(sortKey->ssup_attno != 0);
 
-		strategy = (scanKey->sk_flags & SK_BT_DESC) != 0 ?
-			BTGreaterStrategyNumber : BTLessStrategyNumber;
+		reverse = (scanKey->sk_flags & SK_BT_DESC) != 0;
 
-		PrepareSortSupportFromIndexRel(indexRel, strategy, sortKey);
+		PrepareSortSupportFromIndexRel(indexRel, reverse, sortKey);
 	}
 
 	pfree(indexScanKey);
@@ -412,7 +411,7 @@ tuplesort_begin_index_btree(Relation heapRel,
 	{
 		SortSupport sortKey = base->sortKeys + i;
 		ScanKey		scanKey = indexScanKey->scankeys + i;
-		int16		strategy;
+		bool		reverse;
 
 		sortKey->ssup_cxt = CurrentMemoryContext;
 		sortKey->ssup_collation = scanKey->sk_collation;
@@ -424,10 +423,9 @@ tuplesort_begin_index_btree(Relation heapRel,
 
 		Assert(sortKey->ssup_attno != 0);
 
-		strategy = (scanKey->sk_flags & SK_BT_DESC) != 0 ?
-			BTGreaterStrategyNumber : BTLessStrategyNumber;
+		reverse = (scanKey->sk_flags & SK_BT_DESC) != 0;
 
-		PrepareSortSupportFromIndexRel(indexRel, strategy, sortKey);
+		PrepareSortSupportFromIndexRel(indexRel, reverse, sortKey);
 	}
 
 	pfree(indexScanKey);

src/include/utils/sortsupport.h

@@ -384,7 +384,7 @@ extern int	ssup_datum_int32_cmp(Datum x, Datum y, SortSupport ssup);
 /* Other functions in utils/sort/sortsupport.c */
 extern void PrepareSortSupportComparisonShim(Oid cmpFunc, SortSupport ssup);
 extern void PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup);
-extern void PrepareSortSupportFromIndexRel(Relation indexRel, int16 strategy,
+extern void PrepareSortSupportFromIndexRel(Relation indexRel, bool reverse,
 										   SortSupport ssup);
 extern void PrepareSortSupportFromGistIndexRel(Relation indexRel, SortSupport ssup);