postgres_fdw: Fix join push down with extensions

Objects in an extension are shippable to a foreign server if the
extension is part of the foreign server definition's shippable
extensions list.  But this was not properly considered in some cases
when checking whether a join condition can be pushed to a foreign server
and the join condition uses an object from a shippable extension.  So
the join would never be pushed down in those cases.

So, the list of extensions needs to be made available in fpinfo of the
relation being considered to be pushed down before any expressions are
assessed for being shippable.  Fix foreign_join_ok() to do that for a
join relation.

David Rowley and Ashutosh Bapat, per report from David Rowley

reduced version of patch 332bec1e60 for
backpatch to 9.6, first release with join push down

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -1501,6 +1501,35 @@ SELECT t1.c1, t2.c1 FROM ft4 t1 FULL JOIN ft5 t2 ON (t1.c1 = t2.c1) WHERE (t1.c1
     | 21
 (10 rows)
 
+-- full outer join + WHERE clause with shippable extensions set
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c2, t1.c3 FROM ft1 t1 FULL JOIN ft2 t2 ON (t1.c1 = t2.c1) WHERE postgres_fdw_abs(t1.c1) > 0 OFFSET 10 LIMIT 10;
+                                                                                  QUERY PLAN                                                                                   
+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Limit
+   Output: t1.c1, t2.c2, t1.c3
+   ->  Foreign Scan
+         Output: t1.c1, t2.c2, t1.c3
+         Relations: (public.ft1 t1) FULL JOIN (public.ft2 t2)
+         Remote SQL: SELECT r1."C 1", r1.c3, r2.c2 FROM ("S 1"."T 1" r1 FULL JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1")))) WHERE ((public.postgres_fdw_abs(r1."C 1") > 0))
+(6 rows)
+
+ALTER SERVER loopback OPTIONS (DROP extensions);
+-- full outer join + WHERE clause with shippable extensions not set
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c2, t1.c3 FROM ft1 t1 FULL JOIN ft2 t2 ON (t1.c1 = t2.c1) WHERE postgres_fdw_abs(t1.c1) > 0 OFFSET 10 LIMIT 10;
+                                                          QUERY PLAN                                                           
+-------------------------------------------------------------------------------------------------------------------------------
+ Limit
+   Output: t1.c1, t2.c2, t1.c3
+   ->  Foreign Scan
+         Output: t1.c1, t2.c2, t1.c3
+         Filter: (postgres_fdw_abs(t1.c1) > 0)
+         Relations: (public.ft1 t1) FULL JOIN (public.ft2 t2)
+         Remote SQL: SELECT r1."C 1", r1.c3, r2.c2 FROM ("S 1"."T 1" r1 FULL JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1"))))
+(7 rows)
+
+ALTER SERVER loopback OPTIONS (ADD extensions 'postgres_fdw');
 -- join two tables with FOR UPDATE clause
 -- tests whole-row reference for row marks
 EXPLAIN (VERBOSE, COSTS OFF)

contrib/postgres_fdw/postgres_fdw.c

@@ -3990,6 +3990,15 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
 		joinclauses = NIL;
 	}
 
+	/* Get foreign server */
+	fpinfo->server = fpinfo_o->server;
+
+	/*
+	 * Copy shippable_extensions before checking whether the foreign join is
+	 * OK, so that we know which quals can be evaluated on the foreign server.
+	 */
+	fpinfo->shippable_extensions = fpinfo_o->shippable_extensions;
+
 	/* Join quals must be safe to push down. */
 	foreach(lc, joinclauses)
 	{
@@ -4133,9 +4142,6 @@ foreign_join_ok(PlannerInfo *root, RelOptInfo *joinrel, JoinType jointype,
 	else
 		fpinfo->user = NULL;
 
-	/* Get foreign server */
-	fpinfo->server = fpinfo_o->server;
-
 	/*
 	 * Since both the joining relations come from the same server, the server
 	 * level options should have same value for both the relations. Pick from

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -422,6 +422,14 @@ SELECT t1.c1, t2.c2, t3.c3 FROM ft2 t1 LEFT JOIN ft2 t2 ON (t1.c1 = t2.c1) RIGHT
 EXPLAIN (VERBOSE, COSTS OFF)
 SELECT t1.c1, t2.c1 FROM ft4 t1 FULL JOIN ft5 t2 ON (t1.c1 = t2.c1) WHERE (t1.c1 = t2.c1 OR t1.c1 IS NULL) ORDER BY t1.c1, t2.c1 OFFSET 10 LIMIT 10;
 SELECT t1.c1, t2.c1 FROM ft4 t1 FULL JOIN ft5 t2 ON (t1.c1 = t2.c1) WHERE (t1.c1 = t2.c1 OR t1.c1 IS NULL) ORDER BY t1.c1, t2.c1 OFFSET 10 LIMIT 10;
+-- full outer join + WHERE clause with shippable extensions set
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c2, t1.c3 FROM ft1 t1 FULL JOIN ft2 t2 ON (t1.c1 = t2.c1) WHERE postgres_fdw_abs(t1.c1) > 0 OFFSET 10 LIMIT 10;
+ALTER SERVER loopback OPTIONS (DROP extensions);
+-- full outer join + WHERE clause with shippable extensions not set
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT t1.c1, t2.c2, t1.c3 FROM ft1 t1 FULL JOIN ft2 t2 ON (t1.c1 = t2.c1) WHERE postgres_fdw_abs(t1.c1) > 0 OFFSET 10 LIMIT 10;
+ALTER SERVER loopback OPTIONS (ADD extensions 'postgres_fdw');
 -- join two tables with FOR UPDATE clause
 -- tests whole-row reference for row marks
 EXPLAIN (VERBOSE, COSTS OFF)