Re: fix cost subqueryscan wrong parallel cost

[Tom Lane]

I wrote:
> So perhaps we should do it more like the attached, which produces
> this plan for the UNION case:

sigh ... actually attached this time.

                        regards, tom lane


diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index b787c6f81a..18749e842d 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -1395,6 +1395,7 @@ cost_subqueryscan(SubqueryScanPath *path, PlannerInfo *root,
 {
 	Cost		startup_cost;
 	Cost		run_cost;
+	List	   *qpquals;
 	QualCost	qpqual_cost;
 	Cost		cpu_per_tuple;
 
@@ -1402,11 +1403,24 @@ cost_subqueryscan(SubqueryScanPath *path, PlannerInfo *root,
 	Assert(baserel->relid > 0);
 	Assert(baserel->rtekind == RTE_SUBQUERY);
 
-	/* Mark the path with the correct row estimate */
+	/*
+	 * We compute the rowcount estimate as the subplan's estimate times the
+	 * selectivity of relevant restriction clauses.  In simple cases this will
+	 * come out the same as baserel->rows; but when dealing with parallelized
+	 * paths we must do it like this to get the right answer.
+	 */
 	if (param_info)
-		path->path.rows = param_info->ppi_rows;
+		qpquals = list_concat_copy(param_info->ppi_clauses,
+								   baserel->baserestrictinfo);
 	else
-		path->path.rows = baserel->rows;
+		qpquals = baserel->baserestrictinfo;
+
+	path->path.rows = clamp_row_est(path->subpath->rows *
+									clauselist_selectivity(root,
+														   qpquals,
+														   0,
+														   JOIN_INNER,
+														   NULL));
 
 	/*
 	 * Cost of path is cost of evaluating the subplan, plus cost of evaluating
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.