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/expected/postgres_fdw.out

@@ -1582,22 +1582,25 @@ SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t
                Output: t1.c1, t2.c1, t1.c3, t1.*, t2.*
                Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2)
                Remote SQL: SELECT r1."C 1", r1.c3, CASE WHEN (r1.*)::text IS NOT NULL THEN ROW(r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8) END, r2."C 1", CASE WHEN (r2.*)::text IS NOT NULL THEN ROW(r2."C 1", r2.c2, r2.c3, r2.c4, r2.c5, r2.c6, r2.c7, r2.c8) END FROM ("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1")))) ORDER BY r1.c3 ASC NULLS LAST, r1."C 1" ASC NULLS LAST FOR UPDATE OF r1
-               ->  Merge Join
+               ->  Sort
                      Output: t1.c1, t1.c3, t1.*, t2.c1, t2.*
-                     Merge Cond: (t1.c1 = t2.c1)
-                     ->  Sort
-                           Output: t1.c1, t1.c3, t1.*
-                           Sort Key: t1.c1
-                           ->  Foreign Scan on public.ft1 t1
+                     Sort Key: t1.c3, t1.c1
+                     ->  Merge Join
+                           Output: t1.c1, t1.c3, t1.*, t2.c1, t2.*
+                           Merge Cond: (t1.c1 = t2.c1)
+                           ->  Sort
                                  Output: t1.c1, t1.c3, t1.*
-                                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR UPDATE
-                     ->  Sort
-                           Output: t2.c1, t2.*
-                           Sort Key: t2.c1
-                           ->  Foreign Scan on public.ft2 t2
+                                 Sort Key: t1.c1
+                                 ->  Foreign Scan on public.ft1 t1
+                                       Output: t1.c1, t1.c3, t1.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR UPDATE
+                           ->  Sort
                                  Output: t2.c1, t2.*
-                                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
-(23 rows)
+                                 Sort Key: t2.c1
+                                 ->  Foreign Scan on public.ft2 t2
+                                       Output: t2.c1, t2.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
+(26 rows)
 
 SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10 FOR UPDATE OF t1;
  c1  | c1  
@@ -1626,22 +1629,25 @@ SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t
                Output: t1.c1, t2.c1, t1.c3, t1.*, t2.*
                Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2)
                Remote SQL: SELECT r1."C 1", r1.c3, CASE WHEN (r1.*)::text IS NOT NULL THEN ROW(r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8) END, r2."C 1", CASE WHEN (r2.*)::text IS NOT NULL THEN ROW(r2."C 1", r2.c2, r2.c3, r2.c4, r2.c5, r2.c6, r2.c7, r2.c8) END FROM ("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1")))) ORDER BY r1.c3 ASC NULLS LAST, r1."C 1" ASC NULLS LAST FOR UPDATE OF r1 FOR UPDATE OF r2
-               ->  Merge Join
+               ->  Sort
                      Output: t1.c1, t1.c3, t1.*, t2.c1, t2.*
-                     Merge Cond: (t1.c1 = t2.c1)
-                     ->  Sort
-                           Output: t1.c1, t1.c3, t1.*
-                           Sort Key: t1.c1
-                           ->  Foreign Scan on public.ft1 t1
+                     Sort Key: t1.c3, t1.c1
+                     ->  Merge Join
+                           Output: t1.c1, t1.c3, t1.*, t2.c1, t2.*
+                           Merge Cond: (t1.c1 = t2.c1)
+                           ->  Sort
                                  Output: t1.c1, t1.c3, t1.*
-                                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR UPDATE
-                     ->  Sort
-                           Output: t2.c1, t2.*
-                           Sort Key: t2.c1
-                           ->  Foreign Scan on public.ft2 t2
+                                 Sort Key: t1.c1
+                                 ->  Foreign Scan on public.ft1 t1
+                                       Output: t1.c1, t1.c3, t1.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR UPDATE
+                           ->  Sort
                                  Output: t2.c1, t2.*
-                                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR UPDATE
-(23 rows)
+                                 Sort Key: t2.c1
+                                 ->  Foreign Scan on public.ft2 t2
+                                       Output: t2.c1, t2.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR UPDATE
+(26 rows)
 
 SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10 FOR UPDATE;
  c1  | c1  
@@ -1671,22 +1677,25 @@ SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t
                Output: t1.c1, t2.c1, t1.c3, t1.*, t2.*
                Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2)
                Remote SQL: SELECT r1."C 1", r1.c3, CASE WHEN (r1.*)::text IS NOT NULL THEN ROW(r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8) END, r2."C 1", CASE WHEN (r2.*)::text IS NOT NULL THEN ROW(r2."C 1", r2.c2, r2.c3, r2.c4, r2.c5, r2.c6, r2.c7, r2.c8) END FROM ("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1")))) ORDER BY r1.c3 ASC NULLS LAST, r1."C 1" ASC NULLS LAST FOR SHARE OF r1
-               ->  Merge Join
+               ->  Sort
                      Output: t1.c1, t1.c3, t1.*, t2.c1, t2.*
-                     Merge Cond: (t1.c1 = t2.c1)
-                     ->  Sort
-                           Output: t1.c1, t1.c3, t1.*
-                           Sort Key: t1.c1
-                           ->  Foreign Scan on public.ft1 t1
+                     Sort Key: t1.c3, t1.c1
+                     ->  Merge Join
+                           Output: t1.c1, t1.c3, t1.*, t2.c1, t2.*
+                           Merge Cond: (t1.c1 = t2.c1)
+                           ->  Sort
                                  Output: t1.c1, t1.c3, t1.*
-                                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR SHARE
-                     ->  Sort
-                           Output: t2.c1, t2.*
-                           Sort Key: t2.c1
-                           ->  Foreign Scan on public.ft2 t2
+                                 Sort Key: t1.c1
+                                 ->  Foreign Scan on public.ft1 t1
+                                       Output: t1.c1, t1.c3, t1.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR SHARE
+                           ->  Sort
                                  Output: t2.c1, t2.*
-                                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
-(23 rows)
+                                 Sort Key: t2.c1
+                                 ->  Foreign Scan on public.ft2 t2
+                                       Output: t2.c1, t2.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
+(26 rows)
 
 SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10 FOR SHARE OF t1;
  c1  | c1  
@@ -1715,22 +1724,25 @@ SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t
                Output: t1.c1, t2.c1, t1.c3, t1.*, t2.*
                Relations: (public.ft1 t1) INNER JOIN (public.ft2 t2)
                Remote SQL: SELECT r1."C 1", r1.c3, CASE WHEN (r1.*)::text IS NOT NULL THEN ROW(r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8) END, r2."C 1", CASE WHEN (r2.*)::text IS NOT NULL THEN ROW(r2."C 1", r2.c2, r2.c3, r2.c4, r2.c5, r2.c6, r2.c7, r2.c8) END FROM ("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1")))) ORDER BY r1.c3 ASC NULLS LAST, r1."C 1" ASC NULLS LAST FOR SHARE OF r1 FOR SHARE OF r2
-               ->  Merge Join
+               ->  Sort
                      Output: t1.c1, t1.c3, t1.*, t2.c1, t2.*
-                     Merge Cond: (t1.c1 = t2.c1)
-                     ->  Sort
-                           Output: t1.c1, t1.c3, t1.*
-                           Sort Key: t1.c1
-                           ->  Foreign Scan on public.ft1 t1
+                     Sort Key: t1.c3, t1.c1
+                     ->  Merge Join
+                           Output: t1.c1, t1.c3, t1.*, t2.c1, t2.*
+                           Merge Cond: (t1.c1 = t2.c1)
+                           ->  Sort
                                  Output: t1.c1, t1.c3, t1.*
-                                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR SHARE
-                     ->  Sort
-                           Output: t2.c1, t2.*
-                           Sort Key: t2.c1
-                           ->  Foreign Scan on public.ft2 t2
+                                 Sort Key: t1.c1
+                                 ->  Foreign Scan on public.ft1 t1
+                                       Output: t1.c1, t1.c3, t1.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR SHARE
+                           ->  Sort
                                  Output: t2.c1, t2.*
-                                 Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR SHARE
-(23 rows)
+                                 Sort Key: t2.c1
+                                 ->  Foreign Scan on public.ft2 t2
+                                       Output: t2.c1, t2.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR SHARE
+(26 rows)
 
 SELECT t1.c1, t2.c1 FROM ft1 t1 JOIN ft2 t2 ON (t1.c1 = t2.c1) ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10 FOR SHARE;
  c1  | c1  
@@ -2199,6 +2211,75 @@ SELECT ft5, ft5.c1, ft5.c2, ft5.c3, ft4.c1, ft4.c2 FROM ft5 left join ft4 on ft5
  (30,31,AAA030) | 30 | 31 | AAA030 | 30 | 31
 (4 rows)
 
+-- 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;
+                                                                                                                                                                                                                                                                                                                                                                                                         QUERY PLAN                                                                                                                                                                                                                                                                                                                                                                                                         
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ LockRows
+   Output: ft1.c1, ft1.c2, ft1.c3, ft1.c4, ft1.c5, ft1.c6, ft1.c7, ft1.c8, ft2.c1, ft2.c2, ft2.c3, ft2.c4, ft2.c5, ft2.c6, ft2.c7, ft2.c8, ft4.c1, ft4.c2, ft4.c3, ft5.c1, ft5.c2, ft5.c3, ft1.*, ft2.*, ft4.*, ft5.*
+   ->  Foreign Scan
+         Output: ft1.c1, ft1.c2, ft1.c3, ft1.c4, ft1.c5, ft1.c6, ft1.c7, ft1.c8, ft2.c1, ft2.c2, ft2.c3, ft2.c4, ft2.c5, ft2.c6, ft2.c7, ft2.c8, ft4.c1, ft4.c2, ft4.c3, ft5.c1, ft5.c2, ft5.c3, ft1.*, ft2.*, ft4.*, ft5.*
+         Relations: (((public.ft1) INNER JOIN (public.ft2)) INNER JOIN (public.ft4)) INNER JOIN (public.ft5)
+         Remote SQL: SELECT r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8, CASE WHEN (r1.*)::text IS NOT NULL THEN ROW(r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8) END, r2."C 1", r2.c2, r2.c3, r2.c4, r2.c5, r2.c6, r2.c7, r2.c8, CASE WHEN (r2.*)::text IS NOT NULL THEN ROW(r2."C 1", r2.c2, r2.c3, r2.c4, r2.c5, r2.c6, r2.c7, r2.c8) END, r3.c1, r3.c2, r3.c3, CASE WHEN (r3.*)::text IS NOT NULL THEN ROW(r3.c1, r3.c2, r3.c3) END, r4.c1, r4.c2, r4.c3, CASE WHEN (r4.*)::text IS NOT NULL THEN ROW(r4.c1, r4.c2, r4.c3) END FROM ((("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1")))) INNER JOIN "S 1"."T 3" r3 ON (((r1."C 1" = r3.c1)))) INNER JOIN "S 1"."T 4" r4 ON (((r1."C 1" = r4.c1)))) FOR UPDATE OF r1 FOR UPDATE OF r2 FOR UPDATE OF r3 FOR UPDATE OF r4
+         ->  Merge Join
+               Output: ft1.c1, ft1.c2, ft1.c3, ft1.c4, ft1.c5, ft1.c6, ft1.c7, ft1.c8, ft1.*, ft2.c1, ft2.c2, ft2.c3, ft2.c4, ft2.c5, ft2.c6, ft2.c7, ft2.c8, ft2.*, ft4.c1, ft4.c2, ft4.c3, ft4.*, ft5.c1, ft5.c2, ft5.c3, ft5.*
+               Merge Cond: (ft1.c1 = ft5.c1)
+               ->  Merge Join
+                     Output: ft1.c1, ft1.c2, ft1.c3, ft1.c4, ft1.c5, ft1.c6, ft1.c7, ft1.c8, ft1.*, ft2.c1, ft2.c2, ft2.c3, ft2.c4, ft2.c5, ft2.c6, ft2.c7, ft2.c8, ft2.*, ft4.c1, ft4.c2, ft4.c3, ft4.*
+                     Merge Cond: (ft1.c1 = ft4.c1)
+                     ->  Merge Join
+                           Output: ft1.c1, ft1.c2, ft1.c3, ft1.c4, ft1.c5, ft1.c6, ft1.c7, ft1.c8, ft1.*, ft2.c1, ft2.c2, ft2.c3, ft2.c4, ft2.c5, ft2.c6, ft2.c7, ft2.c8, ft2.*
+                           Merge Cond: (ft1.c1 = ft2.c1)
+                           ->  Sort
+                                 Output: ft1.c1, ft1.c2, ft1.c3, ft1.c4, ft1.c5, ft1.c6, ft1.c7, ft1.c8, ft1.*
+                                 Sort Key: ft1.c1
+                                 ->  Foreign Scan on public.ft1
+                                       Output: ft1.c1, ft1.c2, ft1.c3, ft1.c4, ft1.c5, ft1.c6, ft1.c7, ft1.c8, ft1.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR UPDATE
+                           ->  Sort
+                                 Output: ft2.c1, ft2.c2, ft2.c3, ft2.c4, ft2.c5, ft2.c6, ft2.c7, ft2.c8, ft2.*
+                                 Sort Key: ft2.c1
+                                 ->  Foreign Scan on public.ft2
+                                       Output: ft2.c1, ft2.c2, ft2.c3, ft2.c4, ft2.c5, ft2.c6, ft2.c7, ft2.c8, ft2.*
+                                       Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" FOR UPDATE
+                     ->  Sort
+                           Output: ft4.c1, ft4.c2, ft4.c3, ft4.*
+                           Sort Key: ft4.c1
+                           ->  Foreign Scan on public.ft4
+                                 Output: ft4.c1, ft4.c2, ft4.c3, ft4.*
+                                 Remote SQL: SELECT c1, c2, c3 FROM "S 1"."T 3" FOR UPDATE
+               ->  Sort
+                     Output: ft5.c1, ft5.c2, ft5.c3, ft5.*
+                     Sort Key: ft5.c1
+                     ->  Foreign Scan on public.ft5
+                           Output: ft5.c1, ft5.c2, ft5.c3, ft5.*
+                           Remote SQL: SELECT c1, c2, c3 FROM "S 1"."T 4" FOR UPDATE
+(39 rows)
+
+SELECT * FROM ft1, ft2, ft4, ft5 WHERE ft1.c1 = ft2.c1 AND ft1.c1 = ft4.c1
+    AND ft1.c1 = ft5.c1 FOR UPDATE;
+ c1 | c2 |  c3   |              c4              |            c5            | c6 |     c7     | c8  | c1 | c2 |  c3   |              c4              |            c5            | c6 |     c7     | c8  | c1 | c2 |   c3   | c1 | c2 |   c3   
+----+----+-------+------------------------------+--------------------------+----+------------+-----+----+----+-------+------------------------------+--------------------------+----+------------+-----+----+----+--------+----+----+--------
+  6 |  6 | 00006 | Wed Jan 07 00:00:00 1970 PST | Wed Jan 07 00:00:00 1970 | 6  | 6          | foo |  6 |  6 | 00006 | Wed Jan 07 00:00:00 1970 PST | Wed Jan 07 00:00:00 1970 | 6  | 6          | foo |  6 |  7 | AAA006 |  6 |  7 | AAA006
+ 12 |  2 | 00012 | Tue Jan 13 00:00:00 1970 PST | Tue Jan 13 00:00:00 1970 | 2  | 2          | foo | 12 |  2 | 00012 | Tue Jan 13 00:00:00 1970 PST | Tue Jan 13 00:00:00 1970 | 2  | 2          | foo | 12 | 13 | AAA012 | 12 | 13 | AAA012
+ 18 |  8 | 00018 | Mon Jan 19 00:00:00 1970 PST | Mon Jan 19 00:00:00 1970 | 8  | 8          | foo | 18 |  8 | 00018 | Mon Jan 19 00:00:00 1970 PST | Mon Jan 19 00:00:00 1970 | 8  | 8          | foo | 18 | 19 | AAA018 | 18 | 19 | 
+ 24 |  4 | 00024 | Sun Jan 25 00:00:00 1970 PST | Sun Jan 25 00:00:00 1970 | 4  | 4          | foo | 24 |  4 | 00024 | Sun Jan 25 00:00:00 1970 PST | Sun Jan 25 00:00:00 1970 | 4  | 4          | foo | 24 | 25 | AAA024 | 24 | 25 | AAA024
+ 30 |  0 | 00030 | Sat Jan 31 00:00:00 1970 PST | Sat Jan 31 00:00:00 1970 | 0  | 0          | foo | 30 |  0 | 00030 | Sat Jan 31 00:00:00 1970 PST | Sat Jan 31 00:00:00 1970 | 0  | 0          | foo | 30 | 31 | AAA030 | 30 | 31 | AAA030
+ 36 |  6 | 00036 | Fri Feb 06 00:00:00 1970 PST | Fri Feb 06 00:00:00 1970 | 6  | 6          | foo | 36 |  6 | 00036 | Fri Feb 06 00:00:00 1970 PST | Fri Feb 06 00:00:00 1970 | 6  | 6          | foo | 36 | 37 | AAA036 | 36 | 37 | 
+ 42 |  2 | 00042 | Thu Feb 12 00:00:00 1970 PST | Thu Feb 12 00:00:00 1970 | 2  | 2          | foo | 42 |  2 | 00042 | Thu Feb 12 00:00:00 1970 PST | Thu Feb 12 00:00:00 1970 | 2  | 2          | foo | 42 | 43 | AAA042 | 42 | 43 | AAA042
+ 48 |  8 | 00048 | Wed Feb 18 00:00:00 1970 PST | Wed Feb 18 00:00:00 1970 | 8  | 8          | foo | 48 |  8 | 00048 | Wed Feb 18 00:00:00 1970 PST | Wed Feb 18 00:00:00 1970 | 8  | 8          | foo | 48 | 49 | AAA048 | 48 | 49 | AAA048
+ 54 |  4 | 00054 | Tue Feb 24 00:00:00 1970 PST | Tue Feb 24 00:00:00 1970 | 4  | 4          | foo | 54 |  4 | 00054 | Tue Feb 24 00:00:00 1970 PST | Tue Feb 24 00:00:00 1970 | 4  | 4          | foo | 54 | 55 | AAA054 | 54 | 55 | 
+ 60 |  0 | 00060 | Mon Mar 02 00:00:00 1970 PST | Mon Mar 02 00:00:00 1970 | 0  | 0          | foo | 60 |  0 | 00060 | Mon Mar 02 00:00:00 1970 PST | Mon Mar 02 00:00:00 1970 | 0  | 0          | foo | 60 | 61 | AAA060 | 60 | 61 | AAA060
+ 66 |  6 | 00066 | Sun Mar 08 00:00:00 1970 PST | Sun Mar 08 00:00:00 1970 | 6  | 6          | foo | 66 |  6 | 00066 | Sun Mar 08 00:00:00 1970 PST | Sun Mar 08 00:00:00 1970 | 6  | 6          | foo | 66 | 67 | AAA066 | 66 | 67 | AAA066
+ 72 |  2 | 00072 | Sat Mar 14 00:00:00 1970 PST | Sat Mar 14 00:00:00 1970 | 2  | 2          | foo | 72 |  2 | 00072 | Sat Mar 14 00:00:00 1970 PST | Sat Mar 14 00:00:00 1970 | 2  | 2          | foo | 72 | 73 | AAA072 | 72 | 73 | 
+ 78 |  8 | 00078 | Fri Mar 20 00:00:00 1970 PST | Fri Mar 20 00:00:00 1970 | 8  | 8          | foo | 78 |  8 | 00078 | Fri Mar 20 00:00:00 1970 PST | Fri Mar 20 00:00:00 1970 | 8  | 8          | foo | 78 | 79 | AAA078 | 78 | 79 | AAA078
+ 84 |  4 | 00084 | Thu Mar 26 00:00:00 1970 PST | Thu Mar 26 00:00:00 1970 | 4  | 4          | foo | 84 |  4 | 00084 | Thu Mar 26 00:00:00 1970 PST | Thu Mar 26 00:00:00 1970 | 4  | 4          | foo | 84 | 85 | AAA084 | 84 | 85 | AAA084
+ 90 |  0 | 00090 | Wed Apr 01 00:00:00 1970 PST | Wed Apr 01 00:00:00 1970 | 0  | 0          | foo | 90 |  0 | 00090 | Wed Apr 01 00:00:00 1970 PST | Wed Apr 01 00:00:00 1970 | 0  | 0          | foo | 90 | 91 | AAA090 | 90 | 91 | 
+ 96 |  6 | 00096 | Tue Apr 07 00:00:00 1970 PST | Tue Apr 07 00:00:00 1970 | 6  | 6          | foo | 96 |  6 | 00096 | Tue Apr 07 00:00:00 1970 PST | Tue Apr 07 00:00:00 1970 | 6  | 6          | foo | 96 | 97 | AAA096 | 96 | 97 | AAA096
+(16 rows)
+
 -- check join pushdown in situations where multiple userids are involved
 CREATE ROLE regress_view_owner;
 CREATE USER MAPPING FOR regress_view_owner SERVER loopback;

contrib/postgres_fdw/postgres_fdw.c

@@ -4197,10 +4197,26 @@ 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,
@@ -4209,7 +4225,7 @@ 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

@@ -534,6 +534,13 @@ 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;
 CREATE USER MAPPING FOR regress_view_owner SERVER loopback;