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

9 files changed, 287 insertions, 15 deletions

diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index 11e9b4e8ccb..30e95f585fa 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -10222,6 +10222,31 @@ SELECT * FROM result_tbl ORDER BY a;
 (2 rows)
 
 DELETE FROM result_tbl;
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO result_tbl SELECT a, b, 'AAA' || c FROM async_pt WHERE b === 505;
+                                   QUERY PLAN                                    
+---------------------------------------------------------------------------------
+ Insert on public.result_tbl
+   ->  Append
+         ->  Async Foreign Scan on public.async_p1 async_pt_1
+               Output: async_pt_1.a, async_pt_1.b, ('AAA'::text || async_pt_1.c)
+               Filter: (async_pt_1.b === 505)
+               Remote SQL: SELECT a, b, c FROM public.base_tbl1
+         ->  Async Foreign Scan on public.async_p2 async_pt_2
+               Output: async_pt_2.a, async_pt_2.b, ('AAA'::text || async_pt_2.c)
+               Filter: (async_pt_2.b === 505)
+               Remote SQL: SELECT a, b, c FROM public.base_tbl2
+(10 rows)
+
+INSERT INTO result_tbl SELECT a, b, 'AAA' || c FROM async_pt WHERE b === 505;
+SELECT * FROM result_tbl ORDER BY a;
+  a   |  b  |    c    
+------+-----+---------
+ 1505 | 505 | AAA0505
+ 2505 | 505 | AAA0505
+(2 rows)
+
+DELETE FROM result_tbl;
 -- Check case where multiple partitions use the same connection
 CREATE TABLE base_tbl3 (a int, b int, c text);
 CREATE FOREIGN TABLE async_p3 PARTITION OF async_pt FOR VALUES FROM (3000) TO (4000)
@@ -10359,6 +10384,69 @@ SELECT * FROM join_tbl ORDER BY a1;
 (30 rows)
 
 DELETE FROM join_tbl;
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO join_tbl SELECT t1.a, t1.b, 'AAA' || t1.c, t2.a, t2.b, 'AAA' || t2.c FROM async_pt t1, async_pt t2 WHERE t1.a = t2.a AND t1.b = t2.b AND t1.b % 100 = 0;
+                                                                                           QUERY PLAN                                                                                            
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Insert on public.join_tbl
+   ->  Append
+         ->  Async Foreign Scan
+               Output: t1_1.a, t1_1.b, ('AAA'::text || t1_1.c), t2_1.a, t2_1.b, ('AAA'::text || t2_1.c)
+               Relations: (public.async_p1 t1_1) INNER JOIN (public.async_p1 t2_1)
+               Remote SQL: SELECT r5.a, r5.b, r5.c, r8.a, r8.b, r8.c FROM (public.base_tbl1 r5 INNER JOIN public.base_tbl1 r8 ON (((r5.a = r8.a)) AND ((r5.b = r8.b)) AND (((r5.b % 100) = 0))))
+         ->  Async Foreign Scan
+               Output: t1_2.a, t1_2.b, ('AAA'::text || t1_2.c), t2_2.a, t2_2.b, ('AAA'::text || t2_2.c)
+               Relations: (public.async_p2 t1_2) INNER JOIN (public.async_p2 t2_2)
+               Remote SQL: SELECT r6.a, r6.b, r6.c, r9.a, r9.b, r9.c FROM (public.base_tbl2 r6 INNER JOIN public.base_tbl2 r9 ON (((r6.a = r9.a)) AND ((r6.b = r9.b)) AND (((r6.b % 100) = 0))))
+         ->  Hash Join
+               Output: t1_3.a, t1_3.b, ('AAA'::text || t1_3.c), t2_3.a, t2_3.b, ('AAA'::text || t2_3.c)
+               Hash Cond: ((t2_3.a = t1_3.a) AND (t2_3.b = t1_3.b))
+               ->  Seq Scan on public.async_p3 t2_3
+                     Output: t2_3.a, t2_3.b, t2_3.c
+               ->  Hash
+                     Output: t1_3.a, t1_3.b, t1_3.c
+                     ->  Seq Scan on public.async_p3 t1_3
+                           Output: t1_3.a, t1_3.b, t1_3.c
+                           Filter: ((t1_3.b % 100) = 0)
+(20 rows)
+
+INSERT INTO join_tbl SELECT t1.a, t1.b, 'AAA' || t1.c, t2.a, t2.b, 'AAA' || t2.c FROM async_pt t1, async_pt t2 WHERE t1.a = t2.a AND t1.b = t2.b AND t1.b % 100 = 0;
+SELECT * FROM join_tbl ORDER BY a1;
+  a1  | b1  |   c1    |  a2  | b2  |   c2    
+------+-----+---------+------+-----+---------
+ 1000 |   0 | AAA0000 | 1000 |   0 | AAA0000
+ 1100 | 100 | AAA0100 | 1100 | 100 | AAA0100
+ 1200 | 200 | AAA0200 | 1200 | 200 | AAA0200
+ 1300 | 300 | AAA0300 | 1300 | 300 | AAA0300
+ 1400 | 400 | AAA0400 | 1400 | 400 | AAA0400
+ 1500 | 500 | AAA0500 | 1500 | 500 | AAA0500
+ 1600 | 600 | AAA0600 | 1600 | 600 | AAA0600
+ 1700 | 700 | AAA0700 | 1700 | 700 | AAA0700
+ 1800 | 800 | AAA0800 | 1800 | 800 | AAA0800
+ 1900 | 900 | AAA0900 | 1900 | 900 | AAA0900
+ 2000 |   0 | AAA0000 | 2000 |   0 | AAA0000
+ 2100 | 100 | AAA0100 | 2100 | 100 | AAA0100
+ 2200 | 200 | AAA0200 | 2200 | 200 | AAA0200
+ 2300 | 300 | AAA0300 | 2300 | 300 | AAA0300
+ 2400 | 400 | AAA0400 | 2400 | 400 | AAA0400
+ 2500 | 500 | AAA0500 | 2500 | 500 | AAA0500
+ 2600 | 600 | AAA0600 | 2600 | 600 | AAA0600
+ 2700 | 700 | AAA0700 | 2700 | 700 | AAA0700
+ 2800 | 800 | AAA0800 | 2800 | 800 | AAA0800
+ 2900 | 900 | AAA0900 | 2900 | 900 | AAA0900
+ 3000 |   0 | AAA0000 | 3000 |   0 | AAA0000
+ 3100 | 100 | AAA0100 | 3100 | 100 | AAA0100
+ 3200 | 200 | AAA0200 | 3200 | 200 | AAA0200
+ 3300 | 300 | AAA0300 | 3300 | 300 | AAA0300
+ 3400 | 400 | AAA0400 | 3400 | 400 | AAA0400
+ 3500 | 500 | AAA0500 | 3500 | 500 | AAA0500
+ 3600 | 600 | AAA0600 | 3600 | 600 | AAA0600
+ 3700 | 700 | AAA0700 | 3700 | 700 | AAA0700
+ 3800 | 800 | AAA0800 | 3800 | 800 | AAA0800
+ 3900 | 900 | AAA0900 | 3900 | 900 | AAA0900
+(30 rows)
+
+DELETE FROM join_tbl;
 RESET enable_partitionwise_join;
 -- Test rescan of an async Append node with do_exec_prune=false
 SET enable_hashjoin TO false;
@@ -10536,6 +10624,88 @@ DROP TABLE local_tbl;
 DROP INDEX base_tbl1_idx;
 DROP INDEX base_tbl2_idx;
 DROP INDEX async_p3_idx;
+-- UNION queries
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO result_tbl
+(SELECT a, b, 'AAA' || c FROM async_p1 ORDER BY a LIMIT 10)
+UNION
+(SELECT a, b, 'AAA' || c FROM async_p2 WHERE b < 10);
+                                                   QUERY PLAN                                                    
+-----------------------------------------------------------------------------------------------------------------
+ Insert on public.result_tbl
+   ->  HashAggregate
+         Output: async_p1.a, async_p1.b, (('AAA'::text || async_p1.c))
+         Group Key: async_p1.a, async_p1.b, (('AAA'::text || async_p1.c))
+         ->  Append
+               ->  Async Foreign Scan on public.async_p1
+                     Output: async_p1.a, async_p1.b, ('AAA'::text || async_p1.c)
+                     Remote SQL: SELECT a, b, c FROM public.base_tbl1 ORDER BY a ASC NULLS LAST LIMIT 10::bigint
+               ->  Async Foreign Scan on public.async_p2
+                     Output: async_p2.a, async_p2.b, ('AAA'::text || async_p2.c)
+                     Remote SQL: SELECT a, b, c FROM public.base_tbl2 WHERE ((b < 10))
+(11 rows)
+
+INSERT INTO result_tbl
+(SELECT a, b, 'AAA' || c FROM async_p1 ORDER BY a LIMIT 10)
+UNION
+(SELECT a, b, 'AAA' || c FROM async_p2 WHERE b < 10);
+SELECT * FROM result_tbl ORDER BY a;
+  a   | b  |    c    
+------+----+---------
+ 1000 |  0 | AAA0000
+ 1005 |  5 | AAA0005
+ 1010 | 10 | AAA0010
+ 1015 | 15 | AAA0015
+ 1020 | 20 | AAA0020
+ 1025 | 25 | AAA0025
+ 1030 | 30 | AAA0030
+ 1035 | 35 | AAA0035
+ 1040 | 40 | AAA0040
+ 1045 | 45 | AAA0045
+ 2000 |  0 | AAA0000
+ 2005 |  5 | AAA0005
+(12 rows)
+
+DELETE FROM result_tbl;
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO result_tbl
+(SELECT a, b, 'AAA' || c FROM async_p1 ORDER BY a LIMIT 10)
+UNION ALL
+(SELECT a, b, 'AAA' || c FROM async_p2 WHERE b < 10);
+                                                QUERY PLAN                                                 
+-----------------------------------------------------------------------------------------------------------
+ Insert on public.result_tbl
+   ->  Append
+         ->  Async Foreign Scan on public.async_p1
+               Output: async_p1.a, async_p1.b, ('AAA'::text || async_p1.c)
+               Remote SQL: SELECT a, b, c FROM public.base_tbl1 ORDER BY a ASC NULLS LAST LIMIT 10::bigint
+         ->  Async Foreign Scan on public.async_p2
+               Output: async_p2.a, async_p2.b, ('AAA'::text || async_p2.c)
+               Remote SQL: SELECT a, b, c FROM public.base_tbl2 WHERE ((b < 10))
+(8 rows)
+
+INSERT INTO result_tbl
+(SELECT a, b, 'AAA' || c FROM async_p1 ORDER BY a LIMIT 10)
+UNION ALL
+(SELECT a, b, 'AAA' || c FROM async_p2 WHERE b < 10);
+SELECT * FROM result_tbl ORDER BY a;
+  a   | b  |    c    
+------+----+---------
+ 1000 |  0 | AAA0000
+ 1005 |  5 | AAA0005
+ 1010 | 10 | AAA0010
+ 1015 | 15 | AAA0015
+ 1020 | 20 | AAA0020
+ 1025 | 25 | AAA0025
+ 1030 | 30 | AAA0030
+ 1035 | 35 | AAA0035
+ 1040 | 40 | AAA0040
+ 1045 | 45 | AAA0045
+ 2000 |  0 | AAA0000
+ 2005 |  5 | AAA0005
+(12 rows)
+
+DELETE FROM result_tbl;
 -- Test that pending requests are processed properly
 SET enable_mergejoin TO false;
 SET enable_hashjoin TO false;
diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql
index 6b5de89e144..ea35e61eb8a 100644
--- a/contrib/postgres_fdw/sql/postgres_fdw.sql
+++ b/contrib/postgres_fdw/sql/postgres_fdw.sql
@@ -3245,6 +3245,13 @@ INSERT INTO result_tbl SELECT * FROM async_pt WHERE b === 505;
 SELECT * FROM result_tbl ORDER BY a;
 DELETE FROM result_tbl;
 
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO result_tbl SELECT a, b, 'AAA' || c FROM async_pt WHERE b === 505;
+INSERT INTO result_tbl SELECT a, b, 'AAA' || c FROM async_pt WHERE b === 505;
+
+SELECT * FROM result_tbl ORDER BY a;
+DELETE FROM result_tbl;
+
 -- Check case where multiple partitions use the same connection
 CREATE TABLE base_tbl3 (a int, b int, c text);
 CREATE FOREIGN TABLE async_p3 PARTITION OF async_pt FOR VALUES FROM (3000) TO (4000)
@@ -3286,6 +3293,13 @@ INSERT INTO join_tbl SELECT * FROM async_pt t1, async_pt t2 WHERE t1.a = t2.a AN
 SELECT * FROM join_tbl ORDER BY a1;
 DELETE FROM join_tbl;
 
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO join_tbl SELECT t1.a, t1.b, 'AAA' || t1.c, t2.a, t2.b, 'AAA' || t2.c FROM async_pt t1, async_pt t2 WHERE t1.a = t2.a AND t1.b = t2.b AND t1.b % 100 = 0;
+INSERT INTO join_tbl SELECT t1.a, t1.b, 'AAA' || t1.c, t2.a, t2.b, 'AAA' || t2.c FROM async_pt t1, async_pt t2 WHERE t1.a = t2.a AND t1.b = t2.b AND t1.b % 100 = 0;
+
+SELECT * FROM join_tbl ORDER BY a1;
+DELETE FROM join_tbl;
+
 RESET enable_partitionwise_join;
 
 -- Test rescan of an async Append node with do_exec_prune=false
@@ -3357,6 +3371,33 @@ DROP INDEX base_tbl1_idx;
 DROP INDEX base_tbl2_idx;
 DROP INDEX async_p3_idx;
 
+-- UNION queries
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO result_tbl
+(SELECT a, b, 'AAA' || c FROM async_p1 ORDER BY a LIMIT 10)
+UNION
+(SELECT a, b, 'AAA' || c FROM async_p2 WHERE b < 10);
+INSERT INTO result_tbl
+(SELECT a, b, 'AAA' || c FROM async_p1 ORDER BY a LIMIT 10)
+UNION
+(SELECT a, b, 'AAA' || c FROM async_p2 WHERE b < 10);
+
+SELECT * FROM result_tbl ORDER BY a;
+DELETE FROM result_tbl;
+
+EXPLAIN (VERBOSE, COSTS OFF)
+INSERT INTO result_tbl
+(SELECT a, b, 'AAA' || c FROM async_p1 ORDER BY a LIMIT 10)
+UNION ALL
+(SELECT a, b, 'AAA' || c FROM async_p2 WHERE b < 10);
+INSERT INTO result_tbl
+(SELECT a, b, 'AAA' || c FROM async_p1 ORDER BY a LIMIT 10)
+UNION ALL
+(SELECT a, b, 'AAA' || c FROM async_p2 WHERE b < 10);
+
+SELECT * FROM result_tbl ORDER BY a;
+DELETE FROM result_tbl;
+
 -- Test that pending requests are processed properly
 SET enable_mergejoin TO false;
 SET enable_hashjoin TO false;
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index d5760b10067..46a1943d97a 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -632,6 +632,7 @@ _copySubqueryScan(const SubqueryScan *from)
 	 * copy remainder of node
 	 */
 	COPY_NODE_FIELD(subplan);
+	COPY_SCALAR_FIELD(scanstatus);
 
 	return newnode;
 }
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index abb1f787eff..13e16435303 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -638,6 +638,7 @@ _outSubqueryScan(StringInfo str, const SubqueryScan *node)
 	_outScanInfo(str, (const Scan *) node);
 
 	WRITE_NODE_FIELD(subplan);
+	WRITE_ENUM_FIELD(scanstatus, SubqueryScanStatus);
 }
 
 static void
diff --git a/src/backend/nodes/readfuncs.c b/src/backend/nodes/readfuncs.c
index e7d008b2c51..48f7216c9ed 100644
--- a/src/backend/nodes/readfuncs.c
+++ b/src/backend/nodes/readfuncs.c
@@ -2194,6 +2194,7 @@ _readSubqueryScan(void)
 	ReadCommonScan(&local_node->scan);
 
 	READ_NODE_FIELD(subplan);
+	READ_ENUM_FIELD(scanstatus, SubqueryScanStatus);
 
 	READ_DONE();
 }
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 179c87c6714..51591bb8128 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/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;
 }
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;
 }
 
diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h
index 50ef3dda051..10dd35f0112 100644
--- a/src/include/nodes/plannodes.h
+++ b/src/include/nodes/plannodes.h
@@ -536,16 +536,28 @@ typedef struct TidRangeScan
  * relation, we make this a descendant of Scan anyway for code-sharing
  * purposes.
  *
+ * SubqueryScanStatus caches the trivial_subqueryscan property of the node.
+ * SUBQUERY_SCAN_UNKNOWN means not yet determined.  This is only used during
+ * planning.
+ *
  * Note: we store the sub-plan in the type-specific subplan field, not in
  * the generic lefttree field as you might expect.  This is because we do
  * not want plan-tree-traversal routines to recurse into the subplan without
  * knowing that they are changing Query contexts.
  * ----------------
  */
+typedef enum SubqueryScanStatus
+{
+	SUBQUERY_SCAN_UNKNOWN,
+	SUBQUERY_SCAN_TRIVIAL,
+	SUBQUERY_SCAN_NONTRIVIAL
+} SubqueryScanStatus;
+
 typedef struct SubqueryScan
 {
 	Scan		scan;
 	Plan	   *subplan;
+	SubqueryScanStatus scanstatus;
 } SubqueryScan;
 
 /* ----------------
diff --git a/src/include/optimizer/planmain.h b/src/include/optimizer/planmain.h
index 54a0d4c188d..6947bc65d1e 100644
--- a/src/include/optimizer/planmain.h
+++ b/src/include/optimizer/planmain.h
@@ -112,6 +112,7 @@ extern bool innerrel_is_unique(PlannerInfo *root,
  * prototypes for plan/setrefs.c
  */
 extern Plan *set_plan_references(PlannerInfo *root, Plan *plan);
+extern bool trivial_subqueryscan(SubqueryScan *plan);
 extern void record_plan_function_dependency(PlannerInfo *root, Oid funcid);
 extern void record_plan_type_dependency(PlannerInfo *root, Oid typid);
 extern bool extract_query_dependencies_walker(Node *node, PlannerInfo *root);