YA attempt to stabilize the results of the postgres_fdw regression test.

We've made multiple attempts to stabilize the plans shown by commit
1bc0100d2, with little success so far.  The reason for the remaining
instability seems to be that if a transaction (such as auto-analyze)
is running concurrently with the test, then get_actual_variable_range may
return a maximum value for "T 1"."C 1" that's far away from the actual max,
as a result of our having transiently inserted such a value earlier in
the test.  Because we use a non-MVCC snapshot to fetch the value (for
performance reasons), the presence of other transactions can cause that
function to return entries that are actually dead.

To fix, use a less extreme value in the earlier transient insertion, so
that whether it is visible or not won't affect the selectivity estimate.
The use of 9999 there seems to have been picked with the aid of a
dartboard anyway, rather than having a specific reason.

Discussion: https://postgr.es/m/16962.1523551784@sss.pgh.pa.us

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -5367,49 +5367,49 @@ SELECT c1,c2,c3,c4 FROM ft2 ORDER BY c1;
 (819 rows)
 
 EXPLAIN (verbose, costs off)
-INSERT INTO ft2 (c1,c2,c3) VALUES (9999,999,'foo') RETURNING tableoid::regclass;
+INSERT INTO ft2 (c1,c2,c3) VALUES (1200,999,'foo') RETURNING tableoid::regclass;
                                                                                            QUERY PLAN                                                                                            
 -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  Insert on public.ft2
    Output: (tableoid)::regclass
    Remote SQL: INSERT INTO "S 1"."T 1"("C 1", c2, c3, c4, c5, c6, c7, c8) VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
    ->  Result
-         Output: 9999, 999, NULL::integer, 'foo'::text, NULL::timestamp with time zone, NULL::timestamp without time zone, NULL::character varying, 'ft2       '::character(10), NULL::user_enum
+         Output: 1200, 999, NULL::integer, 'foo'::text, NULL::timestamp with time zone, NULL::timestamp without time zone, NULL::character varying, 'ft2       '::character(10), NULL::user_enum
 (5 rows)
 
-INSERT INTO ft2 (c1,c2,c3) VALUES (9999,999,'foo') RETURNING tableoid::regclass;
+INSERT INTO ft2 (c1,c2,c3) VALUES (1200,999,'foo') RETURNING tableoid::regclass;
  tableoid 
 ----------
  ft2
 (1 row)
 
 EXPLAIN (verbose, costs off)
-UPDATE ft2 SET c3 = 'bar' WHERE c1 = 9999 RETURNING tableoid::regclass;             -- can be pushed down
+UPDATE ft2 SET c3 = 'bar' WHERE c1 = 1200 RETURNING tableoid::regclass;             -- can be pushed down
                                      QUERY PLAN                                     
 ------------------------------------------------------------------------------------
  Update on public.ft2
    Output: (tableoid)::regclass
    ->  Foreign Update on public.ft2
-         Remote SQL: UPDATE "S 1"."T 1" SET c3 = 'bar'::text WHERE (("C 1" = 9999))
+         Remote SQL: UPDATE "S 1"."T 1" SET c3 = 'bar'::text WHERE (("C 1" = 1200))
 (4 rows)
 
-UPDATE ft2 SET c3 = 'bar' WHERE c1 = 9999 RETURNING tableoid::regclass;
+UPDATE ft2 SET c3 = 'bar' WHERE c1 = 1200 RETURNING tableoid::regclass;
  tableoid 
 ----------
  ft2
 (1 row)
 
 EXPLAIN (verbose, costs off)
-DELETE FROM ft2 WHERE c1 = 9999 RETURNING tableoid::regclass;                       -- can be pushed down
+DELETE FROM ft2 WHERE c1 = 1200 RETURNING tableoid::regclass;                       -- can be pushed down
                              QUERY PLAN                             
 --------------------------------------------------------------------
  Delete on public.ft2
    Output: (tableoid)::regclass
    ->  Foreign Delete on public.ft2
-         Remote SQL: DELETE FROM "S 1"."T 1" WHERE (("C 1" = 9999))
+         Remote SQL: DELETE FROM "S 1"."T 1" WHERE (("C 1" = 1200))
 (4 rows)
 
-DELETE FROM ft2 WHERE c1 = 9999 RETURNING tableoid::regclass;
+DELETE FROM ft2 WHERE c1 = 1200 RETURNING tableoid::regclass;
  tableoid 
 ----------
  ft2

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1099,14 +1099,14 @@ DELETE FROM ft2 USING ft1 WHERE ft1.c1 = ft2.c2 AND ft1.c1 % 10 = 2;
 DELETE FROM ft2 USING ft1 WHERE ft1.c1 = ft2.c2 AND ft1.c1 % 10 = 2;
 SELECT c1,c2,c3,c4 FROM ft2 ORDER BY c1;
 EXPLAIN (verbose, costs off)
-INSERT INTO ft2 (c1,c2,c3) VALUES (9999,999,'foo') RETURNING tableoid::regclass;
-INSERT INTO ft2 (c1,c2,c3) VALUES (9999,999,'foo') RETURNING tableoid::regclass;
+INSERT INTO ft2 (c1,c2,c3) VALUES (1200,999,'foo') RETURNING tableoid::regclass;
+INSERT INTO ft2 (c1,c2,c3) VALUES (1200,999,'foo') RETURNING tableoid::regclass;
 EXPLAIN (verbose, costs off)
-UPDATE ft2 SET c3 = 'bar' WHERE c1 = 9999 RETURNING tableoid::regclass;             -- can be pushed down
-UPDATE ft2 SET c3 = 'bar' WHERE c1 = 9999 RETURNING tableoid::regclass;
+UPDATE ft2 SET c3 = 'bar' WHERE c1 = 1200 RETURNING tableoid::regclass;             -- can be pushed down
+UPDATE ft2 SET c3 = 'bar' WHERE c1 = 1200 RETURNING tableoid::regclass;
 EXPLAIN (verbose, costs off)
-DELETE FROM ft2 WHERE c1 = 9999 RETURNING tableoid::regclass;                       -- can be pushed down
-DELETE FROM ft2 WHERE c1 = 9999 RETURNING tableoid::regclass;
+DELETE FROM ft2 WHERE c1 = 1200 RETURNING tableoid::regclass;                       -- can be pushed down
+DELETE FROM ft2 WHERE c1 = 1200 RETURNING tableoid::regclass;
 
 -- Test UPDATE/DELETE with RETURNING on a three-table join
 INSERT INTO ft2 (c1,c2,c3)