diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index b787c6f81a..777077c125 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -1427,6 +1427,28 @@ cost_subqueryscan(SubqueryScanPath *path, PlannerInfo *root,
 	startup_cost += path->path.pathtarget->cost.startup;
 	run_cost += path->path.pathtarget->cost.per_tuple * path->path.rows;
 
+	/* Adjust costing for parallelism, if used. */
+	if (path->path.parallel_workers > 0)
+	{
+		double		parallel_divisor = get_parallel_divisor(&path->path);
+
+		/* The CPU cost is divided among all the workers. */
+		run_cost /= parallel_divisor;
+
+		/*
+		 * It may be possible to amortize some of the I/O cost, but probably
+		 * not very much, because most operating systems already do aggressive
+		 * prefetching.  For now, we assume that the disk run cost can't be
+		 * amortized at all.
+		 */
+
+		/*
+		 * In the case of a parallel plan, the row count needs to represent
+		 * the number of tuples processed per worker.
+		 */
+		path->path.rows = clamp_row_est(path->path.rows / parallel_divisor);
+	}
+
 	path->path.startup_cost += startup_cost;
 	path->path.total_cost += startup_cost + run_cost;
 }
diff --git a/src/test/regress/expected/incremental_sort.out b/src/test/regress/expected/incremental_sort.out
index 21c429226f..0d8d77140a 100644
--- a/src/test/regress/expected/incremental_sort.out
+++ b/src/test/regress/expected/incremental_sort.out
@@ -1487,14 +1487,12 @@ explain (costs off) select * from t union select * from t order by 1,3;
    ->  Unique
          ->  Sort
                Sort Key: t.a, t.b, t.c
-               ->  Append
-                     ->  Gather
-                           Workers Planned: 2
+               ->  Gather
+                     Workers Planned: 2
+                     ->  Parallel Append
                            ->  Parallel Seq Scan on t
-                     ->  Gather
-                           Workers Planned: 2
                            ->  Parallel Seq Scan on t t_1
-(13 rows)
+(11 rows)
 
 -- Full sort, not just incremental sort can be pushed below a gather merge path
 -- by generate_useful_gather_paths.