Optimization for updating foreign tables in Postgres FDW

Attached is a WIP patch for the following:

/*
 * postgresPlanForeignModify
 *      Plan an insert/update/delete operation on a foreign table
 *
 * Note: currently, the plan tree generated for UPDATE/DELETE will always
 * include a ForeignScan that retrieves ctids (using SELECT FOR UPDATE)
 * and then the ModifyTable node will have to execute individual remote
 * UPDATE/DELETE commands.  If there are no local conditions or joins
 * needed, it'd be better to let the scan node do UPDATE/DELETE RETURNING
 * and then do nothing at ModifyTable.  Room for future optimization ...
 */

In the patch postgresPlanForeignModify has been modified so that if, in
addition to the above condition, the followings are satisfied, then the
ForeignScan and ModifyTable node will work that way.

 - There are no local BEFORE/AFTER triggers.
 - In UPDATE it's safe to evaluate expressions to assign to the target
columns on the remote server.

Here is a simple performance test.

On remote side:
postgres=# create table t (id serial primary key, inserted timestamp
default clock_timestamp(), data text);
CREATE TABLE
postgres=# insert into t(data) select random() from generate_series(0,
99999);
INSERT 0 100000
postgres=# vacuum t;
VACUUM

On local side:
postgres=# create foreign table ft (id integer, inserted timestamp, data
text) server myserver options (table_name 't');
CREATE FOREIGN TABLE

Unpatched:
postgres=# explain analyze verbose delete from ft where id < 10000;
                                                      QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------
 Delete on public.ft  (cost=100.00..162.32 rows=910 width=6) (actual
time=1275.255..1275.255 rows=0 loops=1)
   Remote SQL: DELETE FROM public.t WHERE ctid = $1
   ->  Foreign Scan on public.ft  (cost=100.00..162.32 rows=910 width=6)
(actual time=1.180..52.095 rows=9999 loops=1)
         Output: ctid
         Remote SQL: SELECT ctid FROM public.t WHERE ((id < 10000)) FOR
UPDATE
 Planning time: 0.112 ms
 Execution time: 1275.733 ms
(7 rows)

Patched (Note that the DELETE command has been pushed down.):
postgres=# explain analyze verbose delete from ft where id < 10000;
                                                    QUERY PLAN
-------------------------------------------------------------------------------------------------------------------
 Delete on public.ft  (cost=100.00..162.32 rows=910 width=6) (actual
time=0.006..0.006 rows=0 loops=1)
   ->  Foreign Scan on public.ft  (cost=100.00..162.32 rows=910 width=6)
(actual time=0.001..0.001 rows=0 loops=1)
         Output: ctid
         Remote SQL: DELETE FROM public.t WHERE ((id < 10000))
 Planning time: 0.101 ms
 Execution time: 8.808 ms
(6 rows)

I'll add this to the next CF.  Comments are welcome.

Thanks,

Best regards,
Etsuro Fujita