BEFORE UPDATE trigger on postgres_fdw table not work

Hi,

I noticed returning a modified record in a row-level BEFORE UPDATE trigger
on postgres_fdw foreign tables do not work. Attached patch fixes this issue.

Below are scenarios similar to postgres_fdw test to reproduce the issue.

postgres=# CREATE SERVER loopback FOREIGN DATA WRAPPER postgres_fdw OPTIONS (dbname 'postgres',port '5432');
postgres=# CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
postgres=# create table loc1 (f1 serial, f2 text);
postgres=# create foreign table rem1 (f1 serial, f2 text)
postgres-#   server loopback options(table_name 'loc1');

postgres=# CREATE FUNCTION trig_row_before_insupdate() RETURNS TRIGGER AS $$
postgres$#   BEGIN
postgres$#     NEW.f2 := NEW.f2 || ' triggered !';
postgres$#     RETURN NEW;
postgres$#   END
postgres$# $$ language plpgsql;

postgres=# CREATE TRIGGER trig_row_before_insupd BEFORE INSERT OR UPDATE ON rem1
postgres-# FOR EACH ROW EXECUTE PROCEDURE trig_row_before_insupdate();

-- insert trigger is OK
postgres=# INSERT INTO rem1 values(1, 'insert');
postgres=# SELECT * FROM rem1;
  f1 |         f2
----+--------------------
   1 | insert triggered !
(1 row)

-- update trigger is OK if we update f2
postgres=# UPDATE rem1 set f2 = 'update';
postgres=# SELECT * FROM rem1;
  f1 |         f2
----+--------------------
   1 | update triggered !


Without attached patch:

postgres=# UPDATE rem1 set f1 = 10;
postgres=# SELECT * FROM rem1;
  f1 |         f2
----+--------------------
  10 | update triggered !
(1 row)

f2 should be updated by trigger, but not.
This is because current fdw code adds only columns to RemoteSQL that were
explicitly targets of the UPDATE as follows.

postgres=# EXPLAIN (verbose, costs off)
UPDATE rem1 set f1 = 10;
                              QUERY PLAN
---------------------------------------------------------------------
  Update on public.rem1
    Remote SQL: UPDATE public.loc1 SET f1 = $2 WHERE ctid = $1  <--- not set f2
    ->  Foreign Scan on public.rem1
          Output: 10, f2, ctid, rem1.*
          Remote SQL: SELECT f1, f2, ctid FROM public.loc1 FOR UPDATE
(5 rows)

With attached patch, f2 is updated by a trigger and "f2 = $3" is added to remote SQL
as follows.

postgres=# UPDATE rem1 set f1 = 10;
postgres=# select * from rem1;
  f1 |               f2
----+--------------------------------
  10 | update triggered ! triggered !
(1 row)

postgres=# EXPLAIN (verbose, costs off)
postgres-# UPDATE rem1 set f1 = 10;
                               QUERY PLAN
-----------------------------------------------------------------------
  Update on public.rem1
    Remote SQL: UPDATE public.loc1 SET f1 = $2, f2 = $3 WHERE ctid = $1
    ->  Foreign Scan on public.rem1
          Output: 10, f2, ctid, rem1.*
          Remote SQL: SELECT f1, f2, ctid FROM public.loc1 FOR UPDATE
(5 rows)

My patch adds all columns to a target list of remote update query
as in INSERT case if a before update trigger exists.

I tried to add only columns modified in trigger to the target list of
a remote update query, but I cannot find simple way to do that because
update query is built during planning phase at postgresPlanForeignModify
while it is difficult to decide which columns are modified by a trigger
until query execution.

Regards,

-- 
Shohei Mochizuki
TOSHIBA CORPORATION