Redo postgres_fdw's planner code so it can handle parameterized paths.

I wasn't going to ship this without having at least some example of how
to do that.  This version isn't terribly bright; in particular it won't
consider any combinations of multiple join clauses.  Given the cost of
executing a remote EXPLAIN, I'm not sure we want to be very aggressive
about doing that, anyway.

In support of this, refactor generate_implied_equalities_for_indexcol
so that it can be used to extract equivalence clauses that aren't
necessarily tied to an index.

Author: tglsfdc

contrib/postgres_fdw/deparse.c

@@ -446,6 +446,27 @@ EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 WHERE c8 = 'foo';  -- can't
    Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1"
 (4 rows)
 
+-- parameterized remote path
+EXPLAIN (VERBOSE, COSTS false)
+  SELECT * FROM ft2 a, ft2 b WHERE a.c1 = 47 AND b.c1 = a.c2;
+                                                 QUERY PLAN                                                  
+-------------------------------------------------------------------------------------------------------------
+ Nested Loop
+   Output: a.c1, a.c2, a.c3, a.c4, a.c5, a.c6, a.c7, a.c8, b.c1, b.c2, b.c3, b.c4, b.c5, b.c6, b.c7, b.c8
+   ->  Foreign Scan on public.ft2 a
+         Output: a.c1, a.c2, a.c3, a.c4, a.c5, a.c6, a.c7, a.c8
+         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE (("C 1" = 47))
+   ->  Foreign Scan on public.ft2 b
+         Output: b.c1, b.c2, b.c3, b.c4, b.c5, b.c6, b.c7, b.c8
+         Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE (($1::integer = "C 1"))
+(8 rows)
+
+SELECT * FROM ft2 a, ft2 b WHERE a.c1 = 47 AND b.c1 = a.c2;
+ c1 | c2 |  c3   |              c4              |            c5            | c6 |     c7     | c8  | c1 | c2 |  c3   |              c4              |            c5            | c6 |     c7     | c8  
+----+----+-------+------------------------------+--------------------------+----+------------+-----+----+----+-------+------------------------------+--------------------------+----+------------+-----
+ 47 |  7 | 00047 | Tue Feb 17 00:00:00 1970 PST | Tue Feb 17 00:00:00 1970 | 7  | 7          | foo |  7 |  7 | 00007 | Thu Jan 08 00:00:00 1970 PST | Thu Jan 08 00:00:00 1970 | 7  | 7          | foo
+(1 row)
+
 -- ===================================================================
 -- parameterized queries
 -- ===================================================================
@@ -646,7 +667,7 @@ EXPLAIN (VERBOSE, COSTS false) EXECUTE st5('foo', 1);
  Foreign Scan on public.ft1 t1
    Output: c1, c2, c3, c4, c5, c6, c7, c8
    Filter: (t1.c8 = $1)
-   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE (("C 1" = $2::integer))
+   Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE (("C 1" = $1::integer))
 (4 rows)
 
 EXECUTE st5('foo', 1);