postgres_fdw: Fix interaction of PHVs with child joins.

Commit f49842d1 introduced the
concept of a child join, but did not update this code accordingly.

Ashutosh Bapat, with cosmetic changes by me

Discussion: http://postgr.es/m/CAFjFpRf=J_KPOtw+bhZeURYkbizr8ufSaXg6gPEF6DKpgH-t6g@mail.gmail.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -7800,4 +7800,44 @@ SELECT t1.a,t1.b FROM fprt1 t1, LATERAL (SELECT t2.a, t2.b FROM fprt2 t2 WHERE t
  400 | 400
 (4 rows)
 
+-- with PHVs, partition-wise join selected but no join pushdown
+EXPLAIN (COSTS OFF)
+SELECT t1.a, t1.phv, t2.b, t2.phv FROM (SELECT 't1_phv' phv, * FROM fprt1 WHERE a % 25 = 0) t1 FULL JOIN (SELECT 't2_phv' phv, * FROM fprt2 WHERE b % 25 = 0) t2 ON (t1.a = t2.b) ORDER BY t1.a, t2.b;
+                         QUERY PLAN                         
+------------------------------------------------------------
+ Sort
+   Sort Key: ftprt1_p1.a, ftprt2_p1.b
+   ->  Result
+         ->  Append
+               ->  Hash Full Join
+                     Hash Cond: (ftprt1_p1.a = ftprt2_p1.b)
+                     ->  Foreign Scan on ftprt1_p1
+                     ->  Hash
+                           ->  Foreign Scan on ftprt2_p1
+               ->  Hash Full Join
+                     Hash Cond: (ftprt1_p2.a = ftprt2_p2.b)
+                     ->  Foreign Scan on ftprt1_p2
+                     ->  Hash
+                           ->  Foreign Scan on ftprt2_p2
+(14 rows)
+
+SELECT t1.a, t1.phv, t2.b, t2.phv FROM (SELECT 't1_phv' phv, * FROM fprt1 WHERE a % 25 = 0) t1 FULL JOIN (SELECT 't2_phv' phv, * FROM fprt2 WHERE b % 25 = 0) t2 ON (t1.a = t2.b) ORDER BY t1.a, t2.b;
+  a  |  phv   |  b  |  phv   
+-----+--------+-----+--------
+   0 | t1_phv |   0 | t2_phv
+  50 | t1_phv |     | 
+ 100 | t1_phv |     | 
+ 150 | t1_phv | 150 | t2_phv
+ 200 | t1_phv |     | 
+ 250 | t1_phv | 250 | t2_phv
+ 300 | t1_phv |     | 
+ 350 | t1_phv |     | 
+ 400 | t1_phv | 400 | t2_phv
+ 450 | t1_phv |     | 
+     |        |  75 | t2_phv
+     |        | 225 | t2_phv
+     |        | 325 | t2_phv
+     |        | 475 | t2_phv
+(14 rows)
+
 RESET enable_partitionwise_join;

contrib/postgres_fdw/postgres_fdw.c

@@ -4565,7 +4565,11 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
 	foreach(lc, root->placeholder_list)
 	{
 		PlaceHolderInfo *phinfo = lfirst(lc);
-		Relids		relids = joinrel->relids;
+		Relids		relids;
+
+		/* PlaceHolderInfo refers to parent relids, not child relids. */
+		relids = IS_OTHER_REL(joinrel) ?
+			joinrel->top_parent_relids : joinrel->relids;
 
 		if (bms_is_subset(phinfo->ph_eval_at, relids) &&
 			bms_nonempty_difference(relids, phinfo->ph_eval_at))

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1913,4 +1913,9 @@ EXPLAIN (COSTS OFF)
 SELECT t1.a,t1.b FROM fprt1 t1, LATERAL (SELECT t2.a, t2.b FROM fprt2 t2 WHERE t1.a = t2.b AND t1.b = t2.a) q WHERE t1.a%25 = 0 ORDER BY 1,2;
 SELECT t1.a,t1.b FROM fprt1 t1, LATERAL (SELECT t2.a, t2.b FROM fprt2 t2 WHERE t1.a = t2.b AND t1.b = t2.a) q WHERE t1.a%25 = 0 ORDER BY 1,2;
 
+-- with PHVs, partition-wise join selected but no join pushdown
+EXPLAIN (COSTS OFF)
+SELECT t1.a, t1.phv, t2.b, t2.phv FROM (SELECT 't1_phv' phv, * FROM fprt1 WHERE a % 25 = 0) t1 FULL JOIN (SELECT 't2_phv' phv, * FROM fprt2 WHERE b % 25 = 0) t2 ON (t1.a = t2.b) ORDER BY t1.a, t2.b;
+SELECT t1.a, t1.phv, t2.b, t2.phv FROM (SELECT 't1_phv' phv, * FROM fprt1 WHERE a % 25 = 0) t1 FULL JOIN (SELECT 't2_phv' phv, * FROM fprt2 WHERE b % 25 = 0) t2 ON (t1.a = t2.b) ORDER BY t1.a, t2.b;
+
 RESET enable_partitionwise_join;