Allow CHECK constraints to be placed on foreign tables.

As with NOT NULL constraints, we consider that such constraints are merely
reports of constraints that are being enforced by the remote server (or
other underlying storage mechanism).  Their only real use is to allow
planner optimizations, for example in constraint-exclusion checks.  Thus,
the code changes here amount to little more than removal of the error that
was formerly thrown for applying CHECK to a foreign table.

(In passing, do a bit of cleanup of the ALTER FOREIGN TABLE reference page,
which had accumulated some weird decisions about ordering etc.)

Shigeru Hanada and Etsuro Fujita, reviewed by Kyotaro Horiguchi and
Ashutosh Bapat.

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -2588,6 +2588,91 @@ select c2, count(*) from "S 1"."T 1" where c2 < 500 group by 1 order by 1;
  407 |   100
 (13 rows)
 
+-- ===================================================================
+-- test check constraints
+-- ===================================================================
+-- Consistent check constraints provide consistent results
+ALTER FOREIGN TABLE ft1 ADD CONSTRAINT ft1_c2positive CHECK (c2 >= 0);
+EXPLAIN (VERBOSE, COSTS false) SELECT count(*) FROM ft1 WHERE c2 < 0;
+                            QUERY PLAN                             
+-------------------------------------------------------------------
+ Aggregate
+   Output: count(*)
+   ->  Foreign Scan on public.ft1
+         Remote SQL: SELECT NULL FROM "S 1"."T 1" WHERE ((c2 < 0))
+(4 rows)
+
+SELECT count(*) FROM ft1 WHERE c2 < 0;
+ count 
+-------
+     0
+(1 row)
+
+SET constraint_exclusion = 'on';
+EXPLAIN (VERBOSE, COSTS false) SELECT count(*) FROM ft1 WHERE c2 < 0;
+           QUERY PLAN           
+--------------------------------
+ Aggregate
+   Output: count(*)
+   ->  Result
+         One-Time Filter: false
+(4 rows)
+
+SELECT count(*) FROM ft1 WHERE c2 < 0;
+ count 
+-------
+     0
+(1 row)
+
+RESET constraint_exclusion;
+-- check constraint is enforced on the remote side, not locally
+INSERT INTO ft1(c1, c2) VALUES(1111, -2);  -- c2positive
+ERROR:  new row for relation "T 1" violates check constraint "c2positive"
+DETAIL:  Failing row contains (1111, -2, null, null, null, null, ft1       , null).
+CONTEXT:  Remote SQL command: INSERT INTO "S 1"."T 1"("C 1", c2, c3, c4, c5, c6, c7, c8) VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+UPDATE ft1 SET c2 = -c2 WHERE c1 = 1;  -- c2positive
+ERROR:  new row for relation "T 1" violates check constraint "c2positive"
+DETAIL:  Failing row contains (1, -1, 00001_trig_update, 1970-01-02 08:00:00+00, 1970-01-02 00:00:00, 1, 1         , foo).
+CONTEXT:  Remote SQL command: UPDATE "S 1"."T 1" SET c2 = $2 WHERE ctid = $1
+ALTER FOREIGN TABLE ft1 DROP CONSTRAINT ft1_c2positive;
+-- But inconsistent check constraints provide inconsistent results
+ALTER FOREIGN TABLE ft1 ADD CONSTRAINT ft1_c2negative CHECK (c2 < 0);
+EXPLAIN (VERBOSE, COSTS false) SELECT count(*) FROM ft1 WHERE c2 >= 0;
+                             QUERY PLAN                             
+--------------------------------------------------------------------
+ Aggregate
+   Output: count(*)
+   ->  Foreign Scan on public.ft1
+         Remote SQL: SELECT NULL FROM "S 1"."T 1" WHERE ((c2 >= 0))
+(4 rows)
+
+SELECT count(*) FROM ft1 WHERE c2 >= 0;
+ count 
+-------
+   821
+(1 row)
+
+SET constraint_exclusion = 'on';
+EXPLAIN (VERBOSE, COSTS false) SELECT count(*) FROM ft1 WHERE c2 >= 0;
+           QUERY PLAN           
+--------------------------------
+ Aggregate
+   Output: count(*)
+   ->  Result
+         One-Time Filter: false
+(4 rows)
+
+SELECT count(*) FROM ft1 WHERE c2 >= 0;
+ count 
+-------
+     0
+(1 row)
+
+RESET constraint_exclusion;
+-- local check constraint is not actually enforced
+INSERT INTO ft1(c1, c2) VALUES(1111, 2);
+UPDATE ft1 SET c2 = c2 + 1 WHERE c1 = 1;
+ALTER FOREIGN TABLE ft1 DROP CONSTRAINT ft1_c2negative;
 -- ===================================================================
 -- test serial columns (ie, sequence-based defaults)
 -- ===================================================================