Allow asynchronous execution in more cases.

In commit 27e1f1456, create_append_plan() only allowed the subplan
created from a given subpath to be executed asynchronously when it was
an async-capable ForeignPath.  To extend coverage, this patch handles
cases when the given subpath includes some other Path types as well that
can be omitted in the plan processing, such as a ProjectionPath directly
atop an async-capable ForeignPath, allowing asynchronous execution in
partitioned-scan/partitioned-join queries with non-Var tlist expressions
and more UNION queries.

Andrey Lepikhov and Etsuro Fujita, reviewed by Alexander Pyhalov and
Zhihong Yu.

Discussion: https://postgr.es/m/659c37a8-3e71-0ff2-394c-f04428c76f08%40postgrespro.ru

1 files changed, 16 insertions, 2 deletions

diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c
index bf4c722c028..7519723081d 100644
--- a/src/backend/optimizer/plan/setrefs.c
+++ b/src/backend/optimizer/plan/setrefs.c
@@ -115,7 +115,6 @@ static Plan *set_indexonlyscan_references(PlannerInfo *root,
 static Plan *set_subqueryscan_references(PlannerInfo *root,
 										 SubqueryScan *plan,
 										 int rtoffset);
-static bool trivial_subqueryscan(SubqueryScan *plan);
 static Plan *clean_up_removed_plan_level(Plan *parent, Plan *child);
 static void set_foreignscan_references(PlannerInfo *root,
 									   ForeignScan *fscan,
@@ -1319,14 +1318,26 @@ set_subqueryscan_references(PlannerInfo *root,
  *
  * We can delete it if it has no qual to check and the targetlist just
  * regurgitates the output of the child plan.
+ *
+ * This might be called repeatedly on a SubqueryScan node, so we cache the
+ * result in the SubqueryScan node to avoid repeated computation.
  */
-static bool
+bool
 trivial_subqueryscan(SubqueryScan *plan)
 {
 	int			attrno;
 	ListCell   *lp,
 			   *lc;
 
+	/* We might have detected this already (see mark_async_capable_plan) */
+	if (plan->scanstatus == SUBQUERY_SCAN_TRIVIAL)
+		return true;
+	if (plan->scanstatus == SUBQUERY_SCAN_NONTRIVIAL)
+		return false;
+	Assert(plan->scanstatus == SUBQUERY_SCAN_UNKNOWN);
+	/* Initially, mark the SubqueryScan as non-deletable from the plan tree */
+	plan->scanstatus = SUBQUERY_SCAN_NONTRIVIAL;
+
 	if (plan->scan.plan.qual != NIL)
 		return false;
 
@@ -1368,6 +1379,9 @@ trivial_subqueryscan(SubqueryScan *plan)
 		attrno++;
 	}
 
+	/* Re-mark the SubqueryScan as deletable from the plan tree */
+	plan->scanstatus = SUBQUERY_SCAN_TRIVIAL;
+
 	return true;
 }