Generated columns

This is an SQL-standard feature that allows creating columns that are
computed from expressions rather than assigned, similar to a view or
materialized view but on a column basis.

This implements one kind of generated column: stored (computed on
write).  Another kind, virtual (computed on read), is planned for the
future, and some room is left for it.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/b151f851-4019-bdb1-699e-ebab07d2f40a@2ndquadrant.com

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -6431,6 +6431,31 @@ select * from rem1;
  11 | bye remote
 (4 rows)
 
+-- ===================================================================
+-- test generated columns
+-- ===================================================================
+create table gloc1 (a int, b int);
+alter table gloc1 set (autovacuum_enabled = 'false');
+create foreign table grem1 (
+  a int,
+  b int generated always as (a * 2) stored)
+  server loopback options(table_name 'gloc1');
+insert into grem1 (a) values (1), (2);
+update grem1 set a = 22 where a = 2;
+select * from gloc1;
+ a  | b  
+----+----
+  1 |  2
+ 22 | 44
+(2 rows)
+
+select * from grem1;
+ a  | b  
+----+----
+  1 |  2
+ 22 | 44
+(2 rows)
+
 -- ===================================================================
 -- test local triggers
 -- ===================================================================