Fix deparsing of Consts in postgres_fdw ORDER BY

For UNION ALL queries where a union child query contained a foreign
table, if the targetlist of that query contained a constant, and the
top-level query performed an ORDER BY which contained the column for the
constant value, then postgres_fdw would find the EquivalenceMember with
the Const and then try to produce an ORDER BY containing that Const.

This caused problems with INT typed Consts as these could appear to be
requests to order by an ordinal column position rather than the constant
value.  This could lead to either an error such as:

ERROR:  ORDER BY position <int const> is not in select list

or worse, if the constant value is a valid column, then we could just
sort by the wrong column altogether.

Here we fix this issue by just not including these Consts in the ORDER
BY clause.

In passing, add a new section for testing ORDER BY in the postgres_fdw
tests and move two existing tests which were misplaced in the WHERE
clause testing section into it.

Reported-by: Michał Kłeczek
Reviewed-by: Ashutosh Bapat, Richard Guo
Bug: #18381
Discussion: https://postgr.es/m/0714C8B8-8D82-4ABB-9F8D-A0C3657E7B6E%40kleczek.org
Discussion: https://postgr.es/m/18381-137456acd168bf93%40postgresql.org
Backpatch-through: 12, oldest supported version

1 files changed, 67 insertions, 26 deletions

diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out
index c355e8f3f7d..58a603ac56f 100644
--- a/contrib/postgres_fdw/expected/postgres_fdw.out
+++ b/contrib/postgres_fdw/expected/postgres_fdw.out
@@ -893,32 +893,6 @@ SELECT * FROM ft2 WHERE c1 = ANY (ARRAY(SELECT c1 FROM ft1 WHERE c1 < 5));
   4 |  4 | 00004 | Mon Jan 05 00:00:00 1970 PST | Mon Jan 05 00:00:00 1970 | 4  | 4          | foo
 (4 rows)
 
--- we should not push order by clause with volatile expressions or unsafe
--- collations
-EXPLAIN (VERBOSE, COSTS OFF)
-	SELECT * FROM ft2 ORDER BY ft2.c1, random();
-                                  QUERY PLAN                                   
--------------------------------------------------------------------------------
- Sort
-   Output: c1, c2, c3, c4, c5, c6, c7, c8, (random())
-   Sort Key: ft2.c1, (random())
-   ->  Foreign Scan on public.ft2
-         Output: c1, c2, c3, c4, c5, c6, c7, c8, random()
-         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
-(6 rows)
-
-EXPLAIN (VERBOSE, COSTS OFF)
-	SELECT * FROM ft2 ORDER BY ft2.c1, ft2.c3 collate "C";
-                                  QUERY PLAN                                   
--------------------------------------------------------------------------------
- Sort
-   Output: c1, c2, c3, c4, c5, c6, c7, c8, ((c3)::text)
-   Sort Key: ft2.c1, ft2.c3 COLLATE "C"
-   ->  Foreign Scan on public.ft2
-         Output: c1, c2, c3, c4, c5, c6, c7, c8, c3
-         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
-(6 rows)
-
 -- user-defined operator/function
 CREATE FUNCTION postgres_fdw_abs(int) RETURNS int AS $$
 BEGIN
@@ -1207,6 +1181,73 @@ WHERE c1 = 642 AND length(to_tsvector('custom_search'::regconfig, c3)) > 0;
 (1 row)
 
 -- ===================================================================
+-- ORDER BY queries
+-- ===================================================================
+-- we should not push order by clause with volatile expressions or unsafe
+-- collations
+EXPLAIN (VERBOSE, COSTS OFF)
+	SELECT * FROM ft2 ORDER BY ft2.c1, random();
+                                  QUERY PLAN                                   
+-------------------------------------------------------------------------------
+ Sort
+   Output: c1, c2, c3, c4, c5, c6, c7, c8, (random())
+   Sort Key: ft2.c1, (random())
+   ->  Foreign Scan on public.ft2
+         Output: c1, c2, c3, c4, c5, c6, c7, c8, random()
+         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
+(6 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF)
+	SELECT * FROM ft2 ORDER BY ft2.c1, ft2.c3 collate "C";
+                                  QUERY PLAN                                   
+-------------------------------------------------------------------------------
+ Sort
+   Output: c1, c2, c3, c4, c5, c6, c7, c8, ((c3)::text)
+   Sort Key: ft2.c1, ft2.c3 COLLATE "C"
+   ->  Foreign Scan on public.ft2
+         Output: c1, c2, c3, c4, c5, c6, c7, c8, c3
+         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
+(6 rows)
+
+-- Ensure we don't push ORDER BY expressions which are Consts at the UNION
+-- child level to the foreign server.
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM (
+    SELECT 1 AS type,c1 FROM ft1
+    UNION ALL
+    SELECT 2 AS type,c1 FROM ft2
+) a ORDER BY type,c1;
+                                   QUERY PLAN                                    
+---------------------------------------------------------------------------------
+ Merge Append
+   Sort Key: (1), ft1.c1
+   ->  Foreign Scan on public.ft1
+         Output: 1, ft1.c1
+         Remote SQL: SELECT "C 1" FROM "S 1"."T 1" ORDER BY "C 1" ASC NULLS LAST
+   ->  Foreign Scan on public.ft2
+         Output: 2, ft2.c1
+         Remote SQL: SELECT "C 1" FROM "S 1"."T 1" ORDER BY "C 1" ASC NULLS LAST
+(8 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM (
+    SELECT 1 AS type,c1 FROM ft1
+    UNION ALL
+    SELECT 2 AS type,c1 FROM ft2
+) a ORDER BY type;
+                    QUERY PLAN                     
+---------------------------------------------------
+ Merge Append
+   Sort Key: (1)
+   ->  Foreign Scan on public.ft1
+         Output: 1, ft1.c1
+         Remote SQL: SELECT "C 1" FROM "S 1"."T 1"
+   ->  Foreign Scan on public.ft2
+         Output: 2, ft2.c1
+         Remote SQL: SELECT "C 1" FROM "S 1"."T 1"
+(8 rows)
+
+-- ===================================================================
 -- JOIN queries
 -- ===================================================================
 -- Analyze ft4 and ft5 so that we have better statistics. These tables do not