Ensure that foreign scans with lateral refs are planned correctly.

As reported in bug #15613 from Srinivasan S A, file_fdw and postgres_fdw
neglected to mark plain baserel foreign paths as parameterized when the
relation has lateral_relids.  Other FDWs have surely copied this mistake,
so rather than just patching those two modules, install a band-aid fix
in create_foreignscan_path to rectify the mistake centrally.

Although the band-aid is enough to fix the visible symptom, correct
the calls in file_fdw and postgres_fdw anyway, so that they are valid
examples for external FDWs.

Also, since the band-aid isn't enough to make this work for parameterized
foreign joins, throw an elog(ERROR) if such a case is passed to
create_foreignscan_path.  This shouldn't pose much of a problem for
existing external FDWs, since it's likely they aren't trying to make such
paths anyway (though some of them may need a defense against joins with
lateral_relids, similar to the one this patch installs into postgres_fdw).

Add some assertions in relnode.c to catch future occurrences of the same
error --- in particular, as backstop against core-code mistakes like the
one fixed by commit bdd9a99aa.

Discussion: https://postgr.es/m/15613-092be1be9576c728@postgresql.org

contrib/file_fdw/file_fdw.c

@@ -556,6 +556,10 @@ fileGetForeignPaths(PlannerInfo *root,
 	 * Create a ForeignPath node and add it as only possible path.  We use the
 	 * fdw_private list of the path to carry the convert_selectively option;
 	 * it will be propagated into the fdw_private list of the Plan node.
+	 *
+	 * We don't support pushing join clauses into the quals of this path, but
+	 * it could still have required parameterization due to LATERAL refs in
+	 * its tlist.
 	 */
 	add_path(baserel, (Path *)
 			 create_foreignscan_path(root, baserel,
@@ -564,7 +568,7 @@ fileGetForeignPaths(PlannerInfo *root,
 									 startup_cost,
 									 total_cost,
 									 NIL,	/* no pathkeys */
-									 NULL,	/* no outer rel either */
+									 baserel->lateral_relids,
 									 NULL,	/* no extra plan */
 									 coptions));
 

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -3435,6 +3435,62 @@ select c2, sum from "S 1"."T 1" t1, lateral (select sum(t2.c1 + t1."C 1") sum fr
 (2 rows)
 
 reset enable_hashagg;
+-- bug #15613: bad plan for foreign table scan with lateral reference
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT ref_0.c2, subq_1.*
+FROM
+    "S 1"."T 1" AS ref_0,
+    LATERAL (
+        SELECT ref_0."C 1" c1, subq_0.*
+        FROM (SELECT ref_0.c2, ref_1.c3
+              FROM ft1 AS ref_1) AS subq_0
+             RIGHT JOIN ft2 AS ref_3 ON (subq_0.c3 = ref_3.c3)
+    ) AS subq_1
+WHERE ref_0."C 1" < 10 AND subq_1.c3 = '00001'
+ORDER BY ref_0."C 1";
+                                               QUERY PLAN                                                
+---------------------------------------------------------------------------------------------------------
+ Nested Loop
+   Output: ref_0.c2, ref_0."C 1", (ref_0.c2), ref_1.c3, ref_0."C 1"
+   ->  Nested Loop
+         Output: ref_0.c2, ref_0."C 1", ref_1.c3, (ref_0.c2)
+         ->  Index Scan using t1_pkey on "S 1"."T 1" ref_0
+               Output: ref_0."C 1", ref_0.c2, ref_0.c3, ref_0.c4, ref_0.c5, ref_0.c6, ref_0.c7, ref_0.c8
+               Index Cond: (ref_0."C 1" < 10)
+         ->  Foreign Scan on public.ft1 ref_1
+               Output: ref_1.c3, ref_0.c2
+               Remote SQL: SELECT c3 FROM "S 1"."T 1" WHERE ((c3 = '00001'::text))
+   ->  Materialize
+         Output: ref_3.c3
+         ->  Foreign Scan on public.ft2 ref_3
+               Output: ref_3.c3
+               Remote SQL: SELECT c3 FROM "S 1"."T 1" WHERE ((c3 = '00001'::text))
+(15 rows)
+
+SELECT ref_0.c2, subq_1.*
+FROM
+    "S 1"."T 1" AS ref_0,
+    LATERAL (
+        SELECT ref_0."C 1" c1, subq_0.*
+        FROM (SELECT ref_0.c2, ref_1.c3
+              FROM ft1 AS ref_1) AS subq_0
+             RIGHT JOIN ft2 AS ref_3 ON (subq_0.c3 = ref_3.c3)
+    ) AS subq_1
+WHERE ref_0."C 1" < 10 AND subq_1.c3 = '00001'
+ORDER BY ref_0."C 1";
+ c2 | c1 | c2 |  c3   
+----+----+----+-------
+  1 |  1 |  1 | 00001
+  2 |  2 |  2 | 00001
+  3 |  3 |  3 | 00001
+  4 |  4 |  4 | 00001
+  5 |  5 |  5 | 00001
+  6 |  6 |  6 | 00001
+  7 |  7 |  7 | 00001
+  8 |  8 |  8 | 00001
+  9 |  9 |  9 | 00001
+(9 rows)
+
 -- Check with placeHolderVars
 explain (verbose, costs off)
 select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.c1) from ft1 right join ft2 on (ft1.c1 = ft2.c1)) q(a, b, c) on (ft4.c1 <= q.b);

contrib/postgres_fdw/postgres_fdw.c

@@ -930,6 +930,9 @@ postgresGetForeignPaths(PlannerInfo *root,
 	 * baserestrict conditions we were able to send to remote, there might
 	 * actually be an indexscan happening there).  We already did all the work
 	 * to estimate cost and size of this path.
+	 *
+	 * Although this path uses no join clauses, it could still have required
+	 * parameterization due to LATERAL refs in its tlist.
 	 */
 	path = create_foreignscan_path(root, baserel,
 								   NULL,	/* default pathtarget */
@@ -937,7 +940,7 @@ postgresGetForeignPaths(PlannerInfo *root,
 								   fpinfo->startup_cost,
 								   fpinfo->total_cost,
 								   NIL, /* no pathkeys */
-								   NULL,	/* no outer rel either */
+								   baserel->lateral_relids,
 								   NULL,	/* no extra plan */
 								   NIL);	/* no fdw_private list */
 	add_path(baserel, (Path *) path);
@@ -4978,7 +4981,7 @@ add_paths_with_pathkeys_for_rel(PlannerInfo *root, RelOptInfo *rel,
 										 startup_cost,
 										 total_cost,
 										 useful_pathkeys,
-										 NULL,
+										 rel->lateral_relids,
 										 sorted_epq_path,
 										 NIL));
 	}
@@ -5115,6 +5118,13 @@ postgresGetForeignJoinPaths(PlannerInfo *root,
 	if (joinrel->fdw_private)
 		return;
 
+	/*
+	 * This code does not work for joins with lateral references, since those
+	 * must have parameterized paths, which we don't generate yet.
+	 */
+	if (!bms_is_empty(joinrel->lateral_relids))
+		return;
+
 	/*
 	 * Create unfinished PgFdwRelationInfo entry which is used to indicate
 	 * that the join relation is already considered, so that we won't waste
@@ -5206,7 +5216,7 @@ postgresGetForeignJoinPaths(PlannerInfo *root,
 									   startup_cost,
 									   total_cost,
 									   NIL, /* no pathkeys */
-									   NULL,	/* no required_outer */
+									   joinrel->lateral_relids,
 									   epq_path,
 									   NIL);	/* no fdw_private */
 
@@ -5534,7 +5544,7 @@ add_foreign_grouping_paths(PlannerInfo *root, RelOptInfo *input_rel,
 										startup_cost,
 										total_cost,
 										NIL,	/* no pathkeys */
-										NULL,	/* no required_outer */
+										grouped_rel->lateral_relids,
 										NULL,
 										NIL);	/* no fdw_private */
 

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -884,6 +884,32 @@ select c2, sum from "S 1"."T 1" t1, lateral (select sum(t2.c1 + t1."C 1") sum fr
 select c2, sum from "S 1"."T 1" t1, lateral (select sum(t2.c1 + t1."C 1") sum from ft2 t2 group by t2.c1) qry where t1.c2 * 2 = qry.sum and t1.c2 < 3 and t1."C 1" < 100 order by 1;
 reset enable_hashagg;
 
+-- bug #15613: bad plan for foreign table scan with lateral reference
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT ref_0.c2, subq_1.*
+FROM
+    "S 1"."T 1" AS ref_0,
+    LATERAL (
+        SELECT ref_0."C 1" c1, subq_0.*
+        FROM (SELECT ref_0.c2, ref_1.c3
+              FROM ft1 AS ref_1) AS subq_0
+             RIGHT JOIN ft2 AS ref_3 ON (subq_0.c3 = ref_3.c3)
+    ) AS subq_1
+WHERE ref_0."C 1" < 10 AND subq_1.c3 = '00001'
+ORDER BY ref_0."C 1";
+
+SELECT ref_0.c2, subq_1.*
+FROM
+    "S 1"."T 1" AS ref_0,
+    LATERAL (
+        SELECT ref_0."C 1" c1, subq_0.*
+        FROM (SELECT ref_0.c2, ref_1.c3
+              FROM ft1 AS ref_1) AS subq_0
+             RIGHT JOIN ft2 AS ref_3 ON (subq_0.c3 = ref_3.c3)
+    ) AS subq_1
+WHERE ref_0."C 1" < 10 AND subq_1.c3 = '00001'
+ORDER BY ref_0."C 1";
+
 -- Check with placeHolderVars
 explain (verbose, costs off)
 select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.c1) from ft1 right join ft2 on (ft1.c1 = ft2.c1)) q(a, b, c) on (ft4.c1 <= q.b);

src/backend/optimizer/util/pathnode.c

@@ -2069,6 +2069,27 @@ create_foreignscan_path(PlannerInfo *root, RelOptInfo *rel,
 {
 	ForeignPath *pathnode = makeNode(ForeignPath);
 
+	/*
+	 * Since the path's required_outer should always include all the rel's
+	 * lateral_relids, forcibly add those if necessary.  This is a bit of a
+	 * hack, but up till early 2019 the contrib FDWs failed to ensure that,
+	 * and it's likely that the same error has propagated into many external
+	 * FDWs.  Don't risk modifying the passed-in relid set here.
+	 */
+	if (rel->lateral_relids && !bms_is_subset(rel->lateral_relids,
+											  required_outer))
+		required_outer = bms_union(required_outer, rel->lateral_relids);
+
+	/*
+	 * Although this function is only designed to be used for scans of
+	 * baserels, before v12 postgres_fdw abused it to make paths for join and
+	 * upper rels.  It will work for such cases as long as required_outer is
+	 * empty (otherwise get_baserel_parampathinfo does the wrong thing), which
+	 * fortunately is the expected case for now.
+	 */
+	if (!bms_is_empty(required_outer) && !IS_SIMPLE_REL(rel))
+		elog(ERROR, "parameterized foreign joins are not supported yet");
+
 	pathnode->path.pathtype = T_ForeignScan;
 	pathnode->path.parent = rel;
 	pathnode->path.pathtarget = target ? target : rel->reltarget;

src/backend/optimizer/util/relnode.c

@@ -1247,6 +1247,9 @@ get_baserel_parampathinfo(PlannerInfo *root, RelOptInfo *baserel,
 	double		rows;
 	ListCell   *lc;
 
+	/* If rel has LATERAL refs, every path for it should account for them */
+	Assert(bms_is_subset(baserel->lateral_relids, required_outer));
+
 	/* Unparameterized paths have no ParamPathInfo */
 	if (bms_is_empty(required_outer))
 		return NULL;
@@ -1342,6 +1345,9 @@ get_joinrel_parampathinfo(PlannerInfo *root, RelOptInfo *joinrel,
 	double		rows;
 	ListCell   *lc;
 
+	/* If rel has LATERAL refs, every path for it should account for them */
+	Assert(bms_is_subset(joinrel->lateral_relids, required_outer));
+
 	/* Unparameterized paths have no ParamPathInfo or extra join clauses */
 	if (bms_is_empty(required_outer))
 		return NULL;
@@ -1533,6 +1539,9 @@ get_appendrel_parampathinfo(RelOptInfo *appendrel, Relids required_outer)
 {
 	ParamPathInfo *ppi;
 
+	/* If rel has LATERAL refs, every path for it should account for them */
+	Assert(bms_is_subset(appendrel->lateral_relids, required_outer));
+
 	/* Unparameterized paths have no ParamPathInfo */
 	if (bms_is_empty(required_outer))
 		return NULL;