Allow foreign tables to participate in inheritance.

Foreign tables can now be inheritance children, or parents.  Much of the
system was already ready for this, but we had to fix a few things of
course, mostly in the area of planner and executor handling of row locks.

As side effects of this, allow foreign tables to have NOT VALID CHECK
constraints (and hence to accept ALTER ... VALIDATE CONSTRAINT), and to
accept ALTER SET STORAGE and ALTER SET WITH/WITHOUT OIDS.  Continuing to
disallow these things would've required bizarre and inconsistent special
cases in inheritance behavior.  Since foreign tables don't enforce CHECK
constraints anyway, a NOT VALID one is a complete no-op, but that doesn't
mean we shouldn't allow it.  And it's possible that some FDWs might have
use for SET STORAGE or SET WITH OIDS, though doubtless they will be no-ops
for most.

An additional change in support of this is that when a ModifyTable node
has multiple target tables, they will all now be explicitly identified
in EXPLAIN output, for example:

 Update on pt1  (cost=0.00..321.05 rows=3541 width=46)
   Update on pt1
   Foreign Update on ft1
   Foreign Update on ft2
   Update on child3
   ->  Seq Scan on pt1  (cost=0.00..0.00 rows=1 width=46)
   ->  Foreign Scan on ft1  (cost=100.00..148.03 rows=1170 width=46)
   ->  Foreign Scan on ft2  (cost=100.00..148.03 rows=1170 width=46)
   ->  Seq Scan on child3  (cost=0.00..25.00 rows=1200 width=46)

This was done mainly to provide an unambiguous place to attach "Remote SQL"
fields, but it is useful for inherited updates even when no foreign tables
are involved.

Shigeru Hanada and Etsuro Fujita, reviewed by Ashutosh Bapat and Kyotaro
Horiguchi, some additional hacking by me

Author: tglsfdc

contrib/file_fdw/input/file_fdw.source

@@ -133,7 +133,7 @@ SELECT tableoid::regclass, b FROM agg_csv;
 INSERT INTO agg_csv VALUES(1,2.0);
 UPDATE agg_csv SET a = 1;
 DELETE FROM agg_csv WHERE a = 100;
--- but this should be ignored
+-- but this should be allowed
 SELECT * FROM agg_csv FOR UPDATE;
 
 -- constraint exclusion tests
@@ -148,6 +148,20 @@ EXPLAIN (VERBOSE, COSTS FALSE) SELECT * FROM agg_csv WHERE a < 0;
 SELECT * FROM agg_csv WHERE a < 0;
 RESET constraint_exclusion;
 
+-- table inheritance tests
+CREATE TABLE agg (a int2, b float4);
+ALTER FOREIGN TABLE agg_csv INHERIT agg;
+SELECT tableoid::regclass, * FROM agg;
+SELECT tableoid::regclass, * FROM agg_csv;
+SELECT tableoid::regclass, * FROM ONLY agg;
+-- updates aren't supported
+UPDATE agg SET a = 1;
+DELETE FROM agg WHERE a = 100;
+-- but this should be allowed
+SELECT tableoid::regclass, * FROM agg FOR UPDATE;
+ALTER FOREIGN TABLE agg_csv NO INHERIT agg;
+DROP TABLE agg;
+
 -- privilege tests
 SET ROLE file_fdw_superuser;
 SELECT * FROM agg_text ORDER BY a;

contrib/file_fdw/output/file_fdw.source

@@ -212,7 +212,7 @@ UPDATE agg_csv SET a = 1;
 ERROR:  cannot update foreign table "agg_csv"
 DELETE FROM agg_csv WHERE a = 100;
 ERROR:  cannot delete from foreign table "agg_csv"
--- but this should be ignored
+-- but this should be allowed
 SELECT * FROM agg_csv FOR UPDATE;
   a  |    b    
 -----+---------
@@ -249,6 +249,46 @@ SELECT * FROM agg_csv WHERE a < 0;
 (0 rows)
 
 RESET constraint_exclusion;
+-- table inheritance tests
+CREATE TABLE agg (a int2, b float4);
+ALTER FOREIGN TABLE agg_csv INHERIT agg;
+SELECT tableoid::regclass, * FROM agg;
+ tableoid |  a  |    b    
+----------+-----+---------
+ agg_csv  | 100 |  99.097
+ agg_csv  |   0 | 0.09561
+ agg_csv  |  42 |  324.78
+(3 rows)
+
+SELECT tableoid::regclass, * FROM agg_csv;
+ tableoid |  a  |    b    
+----------+-----+---------
+ agg_csv  | 100 |  99.097
+ agg_csv  |   0 | 0.09561
+ agg_csv  |  42 |  324.78
+(3 rows)
+
+SELECT tableoid::regclass, * FROM ONLY agg;
+ tableoid | a | b 
+----------+---+---
+(0 rows)
+
+-- updates aren't supported
+UPDATE agg SET a = 1;
+ERROR:  cannot update foreign table "agg_csv"
+DELETE FROM agg WHERE a = 100;
+ERROR:  cannot delete from foreign table "agg_csv"
+-- but this should be allowed
+SELECT tableoid::regclass, * FROM agg FOR UPDATE;
+ tableoid |  a  |    b    
+----------+-----+---------
+ agg_csv  | 100 |  99.097
+ agg_csv  |   0 | 0.09561
+ agg_csv  |  42 |  324.78
+(3 rows)
+
+ALTER FOREIGN TABLE agg_csv NO INHERIT agg;
+DROP TABLE agg;
 -- privilege tests
 SET ROLE file_fdw_superuser;
 SELECT * FROM agg_text ORDER BY a;