Support non-btree indexes in get_actual_variable_range()

This was previously not supported because the btree strategy numbers
were hardcoded.  Now we can support this for any index that has the
required strategy mapping support and the required operators.

If an index scan used for get_actual_variable_range() requires
recheck, we now just ignore it instead of erroring out.  With btree we
knew this couldn't happen, but now it might.

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

src/backend/utils/adt/selfuncs.c

@@ -6327,10 +6327,7 @@ get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata,
 	{
 		IndexOptInfo *index = (IndexOptInfo *) lfirst(lc);
 		ScanDirection indexscandir;
-
-		/* Ignore non-btree indexes */
-		if (index->relam != BTREE_AM_OID)
-			continue;
+		StrategyNumber strategy;
 
 		/*
 		 * Ignore partial indexes --- we only want stats that cover the entire
@@ -6354,15 +6351,16 @@ get_actual_variable_range(PlannerInfo *root, VariableStatData *vardata,
 			continue;			/* test first 'cause it's cheapest */
 		if (!match_index_to_operand(vardata->var, 0, index))
 			continue;
-		switch (get_op_opfamily_strategy(sortop, index->sortopfamily[0]))
+		strategy = get_op_opfamily_strategy(sortop, index->sortopfamily[0]);
+		switch (IndexAmTranslateStrategy(strategy, index->relam, index->sortopfamily[0], true))
 		{
-			case BTLessStrategyNumber:
+			case COMPARE_LT:
 				if (index->reverse_sort[0])
 					indexscandir = BackwardScanDirection;
 				else
 					indexscandir = ForwardScanDirection;
 				break;
-			case BTGreaterStrategyNumber:
+			case COMPARE_GT:
 				if (index->reverse_sort[0])
 					indexscandir = ForwardScanDirection;
 				else
@@ -6602,13 +6600,17 @@ get_actual_variable_endpoint(Relation heapRel,
 		}
 
 		/*
-		 * We expect that btree will return data in IndexTuple not HeapTuple
-		 * format.  It's not lossy either.
+		 * We expect that the index will return data in IndexTuple not
+		 * HeapTuple format.
 		 */
 		if (!index_scan->xs_itup)
 			elog(ERROR, "no data returned for index-only scan");
+
+		/*
+		 * We do not yet support recheck here.
+		 */
 		if (index_scan->xs_recheck)
-			elog(ERROR, "unexpected recheck indication from btree");
+			break;
 
 		/* OK to deconstruct the index tuple */
 		index_deform_tuple(index_scan->xs_itup,