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

src/backend/optimizer/plan/createplan.c

@@ -82,7 +82,7 @@ static List *get_gating_quals(PlannerInfo *root, List *quals);
 static Plan *create_gating_plan(PlannerInfo *root, Path *path, Plan *plan,
 								List *gating_quals);
 static Plan *create_join_plan(PlannerInfo *root, JoinPath *best_path);
-static bool is_async_capable_path(Path *path);
+static bool mark_async_capable_plan(Plan *plan, Path *path);
 static Plan *create_append_plan(PlannerInfo *root, AppendPath *best_path,
 								int flags);
 static Plan *create_merge_append_plan(PlannerInfo *root, MergeAppendPath *best_path,
@@ -1110,14 +1110,30 @@ create_join_plan(PlannerInfo *root, JoinPath *best_path)
 }
 
 /*
- * is_async_capable_path
- *		Check whether a given Path node is async-capable.
+ * mark_async_capable_plan
+ *		Check whether a given Path node is async-capable, and if so, mark the
+ *		Plan node created from it as such and return true, otherwise return
+ *		false.
  */
 static bool
-is_async_capable_path(Path *path)
+mark_async_capable_plan(Plan *plan, Path *path)
 {
 	switch (nodeTag(path))
 	{
+		case T_SubqueryScanPath:
+			{
+				SubqueryScan *scan_plan = (SubqueryScan *) plan;
+
+				/*
+				 * If a SubqueryScan node atop of an async-capable plan node
+				 * is deletable, consider it as async-capable.
+				 */
+				if (trivial_subqueryscan(scan_plan) &&
+					mark_async_capable_plan(scan_plan->subplan,
+											((SubqueryScanPath *) path)->subpath))
+					break;
+				return false;
+			}
 		case T_ForeignPath:
 			{
 				FdwRoutine *fdwroutine = path->parent->fdwroutine;
@@ -1125,13 +1141,27 @@ is_async_capable_path(Path *path)
 				Assert(fdwroutine != NULL);
 				if (fdwroutine->IsForeignPathAsyncCapable != NULL &&
 					fdwroutine->IsForeignPathAsyncCapable((ForeignPath *) path))
-					return true;
+					break;
+				return false;
 			}
-			break;
+		case T_ProjectionPath:
+
+			/*
+			 * If the generated plan node doesn't include a Result node,
+			 * consider it as async-capable if the subpath is async-capable.
+			 */
+			if (!IsA(plan, Result) &&
+				mark_async_capable_plan(plan,
+										((ProjectionPath *) path)->subpath))
+				return true;
+			return false;
 		default:
-			break;
+			return false;
 	}
-	return false;
+
+	plan->async_capable = true;
+
+	return true;
 }
 
 /*
@@ -1294,14 +1324,14 @@ create_append_plan(PlannerInfo *root, AppendPath *best_path, int flags)
 			}
 		}
 
-		subplans = lappend(subplans, subplan);
-
-		/* Check to see if subplan can be executed asynchronously */
-		if (consider_async && is_async_capable_path(subpath))
+		/* If needed, check to see if subplan can be executed asynchronously */
+		if (consider_async && mark_async_capable_plan(subplan, subpath))
 		{
-			subplan->async_capable = true;
+			Assert(subplan->async_capable);
 			++nasyncplans;
 		}
+
+		subplans = lappend(subplans, subplan);
 	}
 
 	/*
@@ -5598,6 +5628,7 @@ make_subqueryscan(List *qptlist,
 	plan->righttree = NULL;
 	node->scan.scanrelid = scanrelid;
 	node->subplan = subplan;
+	node->scanstatus = SUBQUERY_SCAN_UNKNOWN;
 
 	return node;
 }

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;
 }