diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c
index 919d54dd79..1095b73dac 100644
--- a/src/backend/optimizer/path/pathkeys.c
+++ b/src/backend/optimizer/path/pathkeys.c
@@ -489,8 +489,9 @@ get_useful_group_keys_orderings(PlannerInfo *root, Path *path)
 		n = group_keys_reorder_by_pathkeys(path->pathkeys, &pathkeys, &clauses,
 										   root->num_groupby_pathkeys);
 
-		if (n > 0 && compare_pathkeys(pathkeys, root->group_pathkeys) != PATHKEYS_EQUAL &&
-			(enable_incremental_sort || n == list_length(path->pathkeys)))
+		if (n > 0 &&
+			(enable_incremental_sort || n == root->num_groupby_pathkeys) &&
+			compare_pathkeys(pathkeys, root->group_pathkeys) != PATHKEYS_EQUAL)
 		{
 			info = makeNode(PathKeyInfo);
 			info->pathkeys = pathkeys;
diff --git a/src/test/regress/expected/aggregates.out b/src/test/regress/expected/aggregates.out
index ca38e78f21..67dd20f375 100644
--- a/src/test/regress/expected/aggregates.out
+++ b/src/test/regress/expected/aggregates.out
@@ -2856,6 +2856,23 @@ explain (COSTS OFF) SELECT x,w FROM btg GROUP BY w,x,y,z ORDER BY x*x,z;
                ->  Index Scan using abc on btg
 (8 rows)
 
+SET enable_incremental_sort = off;
+-- The case when the number of incoming subtree path keys is more than
+-- the number of grouping keys.
+CREATE INDEX idx_y_x_z ON btg(y,x,w);
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT y,x,array_agg(distinct w) FROM btg WHERE y < 0 GROUP BY x,y;
+                     QUERY PLAN                      
+-----------------------------------------------------
+ GroupAggregate
+   Output: y, x, array_agg(DISTINCT w)
+   Group Key: btg.y, btg.x
+   ->  Index Only Scan using idx_y_x_z on public.btg
+         Output: y, x, w
+         Index Cond: (btg.y < 0)
+(6 rows)
+
+RESET enable_incremental_sort;
 DROP TABLE btg;
 -- The case, when scanning sort order correspond to aggregate sort order but
 -- can not be found in the group-by list
diff --git a/src/test/regress/sql/aggregates.sql b/src/test/regress/sql/aggregates.sql
index 9880a188d3..524bdfa67d 100644
--- a/src/test/regress/sql/aggregates.sql
+++ b/src/test/regress/sql/aggregates.sql
@@ -1182,7 +1182,6 @@ SELECT balk(hundred) FROM tenk1;
 ROLLBACK;
 
 -- GROUP BY optimization by reorder columns
-
 CREATE TABLE btg AS SELECT
   i % 100 AS x,
   i % 100 AS y,
@@ -1223,8 +1222,11 @@ explain (COSTS OFF) SELECT x,y FROM btg GROUP BY w,x,z,y ORDER BY y,x,z,w;
 explain (COSTS OFF) SELECT x,w FROM btg GROUP BY w,x,y,z ORDER BY x*x,z;
 
 SET enable_incremental_sort = off;
-EXPLAIN (VERBOSE, COSTS ON)
-SELECT x,y, array_agg(z) FROM btg WHERE x < 2 GROUP BY y,x;
+-- The case when the number of incoming subtree path keys is more than
+-- the number of grouping keys.
+CREATE INDEX idx_y_x_z ON btg(y,x,w);
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT y,x,array_agg(distinct w) FROM btg WHERE y < 0 GROUP BY x,y;
 RESET enable_incremental_sort;
 
 DROP TABLE btg;