Don't require sort support functions to provide a comparator.

This could be useful for datatypes like text, where we might want
to optimize for some collations but not others.  However, this patch
doesn't introduce any new sortsupport functions that work this way;
it merely revises the code so that future patches may do so.

Patch by me.  Review by Peter Geoghegan.

src/backend/executor/nodeMergejoin.c

@@ -230,19 +230,19 @@ MJExamineQuals(List *mergeclauses,
 				 qual->opno);
 
 		/* And get the matching support or comparison function */
+		Assert(clause->ssup.comparator == NULL);
 		sortfunc = get_opfamily_proc(opfamily,
 									 op_lefttype,
 									 op_righttype,
 									 BTSORTSUPPORT_PROC);
 		if (OidIsValid(sortfunc))
 		{
-			/* The sort support function should provide a comparator */
+			/* The sort support function can provide a comparator */
 			OidFunctionCall1(sortfunc, PointerGetDatum(&clause->ssup));
-			Assert(clause->ssup.comparator != NULL);
 		}
-		else
+		if (clause->ssup.comparator == NULL)
 		{
-			/* opfamily doesn't provide sort support, get comparison func */
+			/* support not available, get comparison func */
 			sortfunc = get_opfamily_proc(opfamily,
 										 op_lefttype,
 										 op_righttype,

src/backend/utils/cache/lsyscache.c

@@ -245,62 +245,6 @@ get_ordering_op_properties(Oid opno,
 	return result;
 }
 
-/*
- * get_sort_function_for_ordering_op
- *		Get the OID of the datatype-specific btree sort support function,
- *		or if there is none, the btree comparison function,
- *		associated with an ordering operator (a "<" or ">" operator).
- *
- * *sortfunc receives the support or comparison function OID.
- * *issupport is set TRUE if it's a support func, FALSE if a comparison func.
- * *reverse is set FALSE if the operator is "<", TRUE if it's ">"
- * (indicating that comparison results must be negated before use).
- *
- * Returns TRUE if successful, FALSE if no btree function can be found.
- * (This indicates that the operator is not a valid ordering operator.)
- */
-bool
-get_sort_function_for_ordering_op(Oid opno, Oid *sortfunc,
-								  bool *issupport, bool *reverse)
-{
-	Oid			opfamily;
-	Oid			opcintype;
-	int16		strategy;
-
-	/* Find the operator in pg_amop */
-	if (get_ordering_op_properties(opno,
-								   &opfamily, &opcintype, &strategy))
-	{
-		/* Found a suitable opfamily, get matching support function */
-		*sortfunc = get_opfamily_proc(opfamily,
-									  opcintype,
-									  opcintype,
-									  BTSORTSUPPORT_PROC);
-		if (OidIsValid(*sortfunc))
-			*issupport = true;
-		else
-		{
-			/* opfamily doesn't provide sort support, get comparison func */
-			*sortfunc = get_opfamily_proc(opfamily,
-										  opcintype,
-										  opcintype,
-										  BTORDER_PROC);
-			if (!OidIsValid(*sortfunc)) /* should not happen */
-				elog(ERROR, "missing support function %d(%u,%u) in opfamily %u",
-					 BTORDER_PROC, opcintype, opcintype, opfamily);
-			*issupport = false;
-		}
-		*reverse = (strategy == BTGreaterStrategyNumber);
-		return true;
-	}
-
-	/* ensure outputs are set on failure */
-	*sortfunc = InvalidOid;
-	*issupport = false;
-	*reverse = false;
-	return false;
-}
-
 /*
  * get_equality_op_for_ordering_op
  *		Get the OID of the datatype-specific btree equality operator

src/backend/utils/sort/sortsupport.c

@@ -18,6 +18,7 @@
 /* See sortsupport.h */
 #define SORTSUPPORT_INCLUDE_DEFINITIONS
 
+#include "access/nbtree.h"
 #include "fmgr.h"
 #include "utils/lsyscache.h"
 #include "utils/sortsupport.h"
@@ -94,24 +95,43 @@ PrepareSortSupportComparisonShim(Oid cmpFunc, SortSupport ssup)
 void
 PrepareSortSupportFromOrderingOp(Oid orderingOp, SortSupport ssup)
 {
-	Oid			sortFunction;
-	bool		issupport;
+	Oid			sortSupportFunction;
+	Oid			opfamily;
+	Oid			opcintype;
+	int16		strategy;
+
+	Assert(ssup->comparator == NULL);
 
-	if (!get_sort_function_for_ordering_op(orderingOp,
-										   &sortFunction,
-										   &issupport,
-										   &ssup->ssup_reverse))
+	/* Find the operator in pg_amop */
+	if (!get_ordering_op_properties(orderingOp, &opfamily, &opcintype,
+									&strategy))
 		elog(ERROR, "operator %u is not a valid ordering operator",
 			 orderingOp);
+	ssup->ssup_reverse = (strategy == BTGreaterStrategyNumber);
 
-	if (issupport)
+	/* Look for a sort support function */
+	sortSupportFunction = get_opfamily_proc(opfamily, opcintype, opcintype,
+											BTSORTSUPPORT_PROC);
+	if (OidIsValid(sortSupportFunction))
 	{
-		/* The sort support function should provide a comparator */
-		OidFunctionCall1(sortFunction, PointerGetDatum(ssup));
-		Assert(ssup->comparator != NULL);
+		/*
+		 * The sort support function can provide a comparator, but it can
+		 * also choose not to so (e.g. based on the selected collation).
+		 */
+		OidFunctionCall1(sortSupportFunction, PointerGetDatum(ssup));
 	}
-	else
+
+	if (ssup->comparator == NULL)
 	{
+		Oid			sortFunction;
+
+		sortFunction = get_opfamily_proc(opfamily, opcintype, opcintype,
+										 BTORDER_PROC);
+
+		if (!OidIsValid(sortFunction))
+			elog(ERROR, "missing support function %d(%u,%u) in opfamily %u",
+				 BTORDER_PROC, opcintype, opcintype, opfamily);
+
 		/* We'll use a shim to call the old-style btree comparator */
 		PrepareSortSupportComparisonShim(sortFunction, ssup);
 	}

src/include/utils/lsyscache.h

@@ -50,8 +50,6 @@ extern Oid get_opfamily_member(Oid opfamily, Oid lefttype, Oid righttype,
 					int16 strategy);
 extern bool get_ordering_op_properties(Oid opno,
 						   Oid *opfamily, Oid *opcintype, int16 *strategy);
-extern bool get_sort_function_for_ordering_op(Oid opno, Oid *sortfunc,
-								  bool *issupport, bool *reverse);
 extern Oid	get_equality_op_for_ordering_op(Oid opno, bool *reverse);
 extern Oid	get_ordering_op_for_equality_op(Oid opno, bool use_lhs_type);
 extern List *get_mergejoin_opfamilies(Oid opno);