Push down more UPDATEs/DELETEs in postgres_fdw

Hi,

Attached is a WIP patch extending the postgres_fdw DML pushdown in 9.6
so that it can perform an update/delete on a join remotely.  An example
is shown below:

* without the patch:
postgres=# explain verbose delete from ft1 using ft2 where ft1.a = ft2.a;

              QUERY PLAN


--------------------------------------------------------------------------------------------------------------------------------------------------------
---------------------------
  Delete on public.ft1  (cost=100.00..102.04 rows=1 width=38)
    Remote SQL: DELETE FROM public.t1 WHERE ctid = $1
    ->  Foreign Scan  (cost=100.00..102.04 rows=1 width=38)
          Output: ft1.ctid, ft2.*
          Relations: (public.ft1) INNER JOIN (public.ft2)
          Remote SQL: SELECT r1.ctid, CASE WHEN (r2.*)::text IS NOT NULL
THEN ROW(r2.a, r2.b) END FROM (public.t1 r1 INNER JOIN public.t2 r2 ON
(((r1.a =
  r2.a)))) FOR UPDATE OF r1
          ->  Nested Loop  (cost=200.00..202.07 rows=1 width=38)
                Output: ft1.ctid, ft2.*
                Join Filter: (ft1.a = ft2.a)
                ->  Foreign Scan on public.ft1  (cost=100.00..101.03
rows=1 width=10)
                      Output: ft1.ctid, ft1.a
                      Remote SQL: SELECT a, ctid FROM public.t1 FOR UPDATE
                ->  Foreign Scan on public.ft2  (cost=100.00..101.03
rows=1 width=36)
                      Output: ft2.*, ft2.a
                      Remote SQL: SELECT a, b FROM public.t2
(15 rows)

* with the patch:
postgres=# explain verbose delete from ft1 using ft2 where ft1.a = ft2.a;
                                                          QUERY PLAN

-----------------------------------------------------------------------------------------------------------------------------
  Delete on public.ft1  (cost=100.00..102.04 rows=1 width=38)
    ->  Foreign Delete  (cost=100.00..102.04 rows=1 width=38)
          Remote SQL: DELETE FROM public.t1 r1 USING (SELECT ROW(a, b),
a FROM public.t2) ss1(c1, c2) WHERE ((r1.a = ss1.c2))
(3 rows)

The WIP patch has been created on top of the join pushdown patch [1].
So, for testing, please apply the patch in [1] first.

I'll add this to the the November commitfest.

Best regards,
Etsuro Fujita

[1]
https://www.postgresql.org/message-id/1688885b-5fb1-8bfa-b1b8-c2758dbe0b38@lab.ntt.co.jp