postgres_fdw: Avoid 'outer pathkeys do not match mergeclauses' error.

When pushing down a join to a foreign server, postgres_fdw constructs
an alternative plan to be used for any EvalPlanQual rechecks that
prove to be necessary.  This plan is stored as the outer subplan of
the Foreign Scan implementing the pushed-down join.  Previously, this
alternative plan could have a different nominal sort ordering than its
parent, which seemed OK since there will only be one tuple per base
table anyway in the case of an EvalPlanQual recheck.  Actually,
though, it caused a problem if that path was used as a building block
for the EvalPlanQual recheck plan of a higher-level foreign join,
because we could end up with a merge join one of whose inputs was not
labelled with the correct sort order.  Repair by injecting an extra
Sort node into the EvalPlanQual recheck plan whenever it would
otherwise fail to be sorted at least as well as its parent Foreign
Scan.

Report by Jeff Janes.  Patch by me, reviewed by Tom Lane, who also
provided the test case and comment text.

Discussion: http://postgr.es/m/CAMkU=1y2G8VOVBHv3iXU2TMAj7-RyBFFW1uhkr5sm9LQ2=X35g@mail.gmail.com

contrib/postgres_fdw/postgres_fdw.c

@@ -4330,10 +4330,26 @@ add_paths_with_pathkeys_for_rel(PlannerInfo *root, RelOptInfo *rel,
 		Cost		startup_cost;
 		Cost		total_cost;
 		List	   *useful_pathkeys = lfirst(lc);
+		Path	   *sorted_epq_path;
 
 		estimate_path_cost_size(root, rel, NIL, useful_pathkeys,
 								&rows, &width, &startup_cost, &total_cost);
 
+		/*
+		 * The EPQ path must be at least as well sorted as the path itself,
+		 * in case it gets used as input to a mergejoin.
+		 */
+		sorted_epq_path = epq_path;
+		if (sorted_epq_path != NULL &&
+			!pathkeys_contained_in(useful_pathkeys,
+								   sorted_epq_path->pathkeys))
+			sorted_epq_path = (Path *)
+				create_sort_path(root,
+								 rel,
+								 sorted_epq_path,
+								 useful_pathkeys,
+								 -1.0);
+
 		add_path(rel, (Path *)
 				 create_foreignscan_path(root, rel,
 										 NULL,
@@ -4342,7 +4358,7 @@ add_paths_with_pathkeys_for_rel(PlannerInfo *root, RelOptInfo *rel,
 										 total_cost,
 										 useful_pathkeys,
 										 NULL,
-										 epq_path,
+										 sorted_epq_path,
 										 NIL));
 	}
 }

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -559,6 +559,13 @@ EXPLAIN (VERBOSE, COSTS OFF)
 SELECT ft5, ft5.c1, ft5.c2, ft5.c3, ft4.c1, ft4.c2 FROM ft5 left join ft4 on ft5.c1 = ft4.c1 WHERE ft4.c1 BETWEEN 10 and 30 ORDER BY ft5.c1, ft4.c1;
 SELECT ft5, ft5.c1, ft5.c2, ft5.c3, ft4.c1, ft4.c2 FROM ft5 left join ft4 on ft5.c1 = ft4.c1 WHERE ft4.c1 BETWEEN 10 and 30 ORDER BY ft5.c1, ft4.c1;
 
+-- multi-way join involving multiple merge joins
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM ft1, ft2, ft4, ft5 WHERE ft1.c1 = ft2.c1 AND ft1.c1 = ft4.c1
+    AND ft1.c1 = ft5.c1 FOR UPDATE;
+SELECT * FROM ft1, ft2, ft4, ft5 WHERE ft1.c1 = ft2.c1 AND ft1.c1 = ft4.c1
+    AND ft1.c1 = ft5.c1 FOR UPDATE;
+
 -- check join pushdown in situations where multiple userids are involved
 CREATE ROLE regress_view_owner SUPERUSER;
 CREATE USER MAPPING FOR regress_view_owner SERVER loopback;