postgres_fdw: Avoid pulling up restrict infos from subqueries

Semi-join joins below left/right join are deparsed as
subqueries.  Thus, we can't refer to subqueries vars from upper relations.
This commit avoids pulling conditions from them.

Reported-by: Robins Tharakan <tharakan@gmail.com>
Bug: #18852
Discussion: https://postgr.es/m/CAEP4nAzryLd3gwcUpFBAG9MWyDfMRX8ZjuyY2XXjyC_C6k%2B_Zw%40mail.gmail.com
Author: Alexander Pyhalov <a.pyhalov@postgrespro.ru>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Backpatch-through: 17

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -4936,6 +4936,44 @@ SELECT ft2.*, ft4.* FROM ft2 INNER JOIN
 ----+----+----+----+----+----+----+----+----+----+----
 (0 rows)
 
+-- Semi-join conditions shouldn't pop up as left/right join clauses.
+SET enable_material TO off;
+EXPLAIN (verbose, costs off)
+SELECT x1.c1 FROM
+		(SELECT * FROM ft2 WHERE EXISTS (SELECT 1 FROM ft4 WHERE ft4.c1 = ft2.c1 AND ft2.c2 < 10)) x1
+	RIGHT JOIN
+		(SELECT * FROM ft2 WHERE EXISTS (SELECT 1 FROM ft4 WHERE ft4.c1 = ft2.c1 AND ft2.c2 < 10)) x2
+	ON (x1.c1 = x2.c1)
+ORDER BY x1.c1 LIMIT 10;
+                                                                                                                                                                                 QUERY PLAN                                                                                                                                                                                 
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan
+   Output: ft2.c1
+   Relations: ((public.ft2 ft2_1) LEFT JOIN ((public.ft2) SEMI JOIN (public.ft4))) SEMI JOIN (public.ft4 ft4_1)
+   Remote SQL: SELECT s9.c1 FROM ("S 1"."T 1" r6 LEFT JOIN (SELECT r4."C 1" FROM "S 1"."T 1" r4 WHERE ((r4.c2 < 10)) AND EXISTS (SELECT NULL FROM "S 1"."T 3" r5 WHERE ((r4."C 1" = r5.c1)))) s9(c1) ON (((s9.c1 = r6."C 1")))) WHERE ((r6.c2 < 10)) AND EXISTS (SELECT NULL FROM "S 1"."T 3" r7 WHERE ((r6."C 1" = r7.c1))) ORDER BY s9.c1 ASC NULLS LAST LIMIT 10::bigint
+(4 rows)
+
+SELECT x1.c1 FROM
+		(SELECT * FROM ft2 WHERE EXISTS (SELECT 1 FROM ft4 WHERE ft4.c1 = ft2.c1 AND ft2.c2 < 10)) x1
+	RIGHT JOIN
+		(SELECT * FROM ft2 WHERE EXISTS (SELECT 1 FROM ft4 WHERE ft4.c1 = ft2.c1 AND ft2.c2 < 10)) x2
+	ON (x1.c1 = x2.c1)
+ORDER BY x1.c1 LIMIT 10;
+ c1 
+----
+  2
+  4
+  6
+  8
+ 10
+ 12
+ 14
+ 16
+ 18
+ 20
+(10 rows)
+
+RESET enable_material;
 -- Can't push down semi-join with inner rel vars in targetlist
 EXPLAIN (verbose, costs off)
 SELECT ft1.c1 FROM ft1 JOIN ft2 on ft1.c1 = ft2.c1 WHERE

contrib/postgres_fdw/postgres_fdw.c

@@ -5963,17 +5963,33 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
 			break;
 
 		case JOIN_LEFT:
-			fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
+
-											  fpinfo_i->remote_conds);
+			/*
-			fpinfo->remote_conds = list_concat(fpinfo->remote_conds,
+			 * When semi-join is involved in the inner or outer part of the
-											   fpinfo_o->remote_conds);
+			 * left join, it's deparsed as a subquery, and we can't refer to
+			 * its vars on the upper level.
+			 */
+			if (bms_is_empty(fpinfo_i->hidden_subquery_rels))
+				fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
+												  fpinfo_i->remote_conds);
+			if (bms_is_empty(fpinfo_o->hidden_subquery_rels))
+				fpinfo->remote_conds = list_concat(fpinfo->remote_conds,
+												   fpinfo_o->remote_conds);
 			break;
 
 		case JOIN_RIGHT:
-			fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
+
-											  fpinfo_o->remote_conds);
+			/*
-			fpinfo->remote_conds = list_concat(fpinfo->remote_conds,
+			 * When semi-join is involved in the inner or outer part of the
-											   fpinfo_i->remote_conds);
+			 * right join, it's deparsed as a subquery, and we can't refer to
+			 * its vars on the upper level.
+			 */
+			if (bms_is_empty(fpinfo_o->hidden_subquery_rels))
+				fpinfo->joinclauses = list_concat(fpinfo->joinclauses,
+												  fpinfo_o->remote_conds);
+			if (bms_is_empty(fpinfo_i->hidden_subquery_rels))
+				fpinfo->remote_conds = list_concat(fpinfo->remote_conds,
+												   fpinfo_i->remote_conds);
 			break;
 
 		case JOIN_SEMI:

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1455,6 +1455,23 @@ SELECT ft2.*, ft4.* FROM ft2 INNER JOIN
 	WHERE ft2.c1 > 900
 	ORDER BY ft2.c1 LIMIT 10;
 
+-- Semi-join conditions shouldn't pop up as left/right join clauses.
+SET enable_material TO off;
+EXPLAIN (verbose, costs off)
+SELECT x1.c1 FROM
+		(SELECT * FROM ft2 WHERE EXISTS (SELECT 1 FROM ft4 WHERE ft4.c1 = ft2.c1 AND ft2.c2 < 10)) x1
+	RIGHT JOIN
+		(SELECT * FROM ft2 WHERE EXISTS (SELECT 1 FROM ft4 WHERE ft4.c1 = ft2.c1 AND ft2.c2 < 10)) x2
+	ON (x1.c1 = x2.c1)
+ORDER BY x1.c1 LIMIT 10;
+SELECT x1.c1 FROM
+		(SELECT * FROM ft2 WHERE EXISTS (SELECT 1 FROM ft4 WHERE ft4.c1 = ft2.c1 AND ft2.c2 < 10)) x1
+	RIGHT JOIN
+		(SELECT * FROM ft2 WHERE EXISTS (SELECT 1 FROM ft4 WHERE ft4.c1 = ft2.c1 AND ft2.c2 < 10)) x2
+	ON (x1.c1 = x2.c1)
+ORDER BY x1.c1 LIMIT 10;
+RESET enable_material;
+
 -- Can't push down semi-join with inner rel vars in targetlist
 EXPLAIN (verbose, costs off)
 SELECT ft1.c1 FROM ft1 JOIN ft2 on ft1.c1 = ft2.c1 WHERE