In postgres_fdw, allow CASE expressions to be pushed to the remote server.

This is simple enough except for the need to check whether CaseTestExpr
nodes have a collation that is not derived from a remote Var.  For that,
examine the CASE's "arg" expression and then pass that info down into the
recursive examination of the WHEN expressions.

Alexander Pyhalov, reviewed by Gilles Darold and myself

Discussion: https://postgr.es/m/fda09032e90d85d9b726a41e03f9097f@postgrespro.ru

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -1067,6 +1067,96 @@ SELECT * FROM ft1 t1 WHERE t1.c1 === t1.c2 order by t1.c2 limit 1;
   1 |  1 | 00001 | Fri Jan 02 00:00:00 1970 PST | Fri Jan 02 00:00:00 1970 | 1  | 1          | foo
 (1 row)
 
+-- Test CASE pushdown
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT c1,c2,c3 FROM ft2 WHERE CASE WHEN c1 > 990 THEN c1 END < 1000 ORDER BY c1;
+                                                                           QUERY PLAN                                                                           
+----------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan on public.ft2
+   Output: c1, c2, c3
+   Remote SQL: SELECT "C 1", c2, c3 FROM "S 1"."T 1" WHERE (((CASE WHEN ("C 1" > 990) THEN "C 1" ELSE NULL::integer END) < 1000)) ORDER BY "C 1" ASC NULLS LAST
+(3 rows)
+
+SELECT c1,c2,c3 FROM ft2 WHERE CASE WHEN c1 > 990 THEN c1 END < 1000 ORDER BY c1;
+ c1  | c2 |  c3   
+-----+----+-------
+ 991 |  1 | 00991
+ 992 |  2 | 00992
+ 993 |  3 | 00993
+ 994 |  4 | 00994
+ 995 |  5 | 00995
+ 996 |  6 | 00996
+ 997 |  7 | 00997
+ 998 |  8 | 00998
+ 999 |  9 | 00999
+(9 rows)
+
+-- Nested CASE
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT c1,c2,c3 FROM ft2 WHERE CASE CASE WHEN c2 > 0 THEN c2 END WHEN 100 THEN 601 WHEN c2 THEN c2 ELSE 0 END > 600 ORDER BY c1;
+                                                                                                QUERY PLAN                                                                                                 
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan on public.ft2
+   Output: c1, c2, c3
+   Remote SQL: SELECT "C 1", c2, c3 FROM "S 1"."T 1" WHERE (((CASE (CASE WHEN (c2 > 0) THEN c2 ELSE NULL::integer END) WHEN 100 THEN 601 WHEN c2 THEN c2 ELSE 0 END) > 600)) ORDER BY "C 1" ASC NULLS LAST
+(3 rows)
+
+SELECT c1,c2,c3 FROM ft2 WHERE CASE CASE WHEN c2 > 0 THEN c2 END WHEN 100 THEN 601 WHEN c2 THEN c2 ELSE 0 END > 600 ORDER BY c1;
+ c1 | c2 | c3 
+----+----+----
+(0 rows)
+
+-- CASE arg WHEN
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM ft1 WHERE c1 > (CASE mod(c1, 4) WHEN 0 THEN 1 WHEN 2 THEN 50 ELSE 100 END);
+                                                                        QUERY PLAN                                                                        
+----------------------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan on public.ft1
+   Output: c1, c2, c3, c4, c5, c6, c7, c8
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE (("C 1" > (CASE mod("C 1", 4) WHEN 0 THEN 1 WHEN 2 THEN 50 ELSE 100 END)))
+(3 rows)
+
+-- CASE cannot be pushed down because of unshippable arg clause
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM ft1 WHERE c1 > (CASE random()::integer WHEN 0 THEN 1 WHEN 2 THEN 50 ELSE 100 END);
+                                       QUERY PLAN                                        
+-----------------------------------------------------------------------------------------
+ Foreign Scan on public.ft1
+   Output: c1, c2, c3, c4, c5, c6, c7, c8
+   Filter: (ft1.c1 > CASE (random())::integer WHEN 0 THEN 1 WHEN 2 THEN 50 ELSE 100 END)
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
+(4 rows)
+
+-- these are shippable
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM ft1 WHERE CASE c6 WHEN 'foo' THEN true ELSE c3 < 'bar' END;
+                                                                    QUERY PLAN                                                                    
+--------------------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan on public.ft1
+   Output: c1, c2, c3, c4, c5, c6, c7, c8
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((CASE c6 WHEN 'foo'::text THEN true ELSE (c3 < 'bar'::text) END))
+(3 rows)
+
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM ft1 WHERE CASE c3 WHEN c6 THEN true ELSE c3 < 'bar' END;
+                                                               QUERY PLAN                                                                
+-----------------------------------------------------------------------------------------------------------------------------------------
+ Foreign Scan on public.ft1
+   Output: c1, c2, c3, c4, c5, c6, c7, c8
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE ((CASE c3 WHEN c6 THEN true ELSE (c3 < 'bar'::text) END))
+(3 rows)
+
+-- but this is not because of collation
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT * FROM ft1 WHERE CASE c3 COLLATE "C" WHEN c6 THEN true ELSE c3 < 'bar' END;
+                                     QUERY PLAN                                      
+-------------------------------------------------------------------------------------
+ Foreign Scan on public.ft1
+   Output: c1, c2, c3, c4, c5, c6, c7, c8
+   Filter: CASE (ft1.c3)::text WHEN ft1.c6 THEN true ELSE (ft1.c3 < 'bar'::text) END
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
+(4 rows)
+
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 -- JOIN queries
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