[BUG?] postgres_fdw incorrectly updates remote table if it has inherited children.

Hello,

I noticed the following scenario under the development of truncate
support on FDW.

In case when 'ftable' maps a remote table that has inherited children,...

postgres=# create table rtable_parent (id int, label text, x text);
CREATE TABLE
postgres=# create table rtable_child () inherits (rtable_parent);
CREATE TABLE
postgres=# insert into rtable_parent (select x, 'parent', md5(x::text)
from generate_series(1,10) x);
INSERT 0 10
postgres=# insert into rtable_child (select x, 'child', md5(x::text)
from generate_series(6,15) x);
INSERT 0 10
postgres=# create foreign table ftable (id int, label text, x text)
server loopback options (table_name 'rtable_parent');
CREATE FOREIGN TABLE

The 'ftable' shows the results from both of the parent and children.
postgres=# select * from ftable;
id | label | x
----+--------+----------------------------------
1 | parent | c4ca4238a0b923820dcc509a6f75849b
2 | parent | c81e728d9d4c2f636f067f89cc14862c
3 | parent | eccbc87e4b5ce2fe28308fd9f2a7baf3
4 | parent | a87ff679a2f3e71d9181a67b7542122c
5 | parent | e4da3b7fbbce2345d7772b0674a318d5
6 | parent | 1679091c5a880faf6fb5e6087eb1b2dc
7 | parent | 8f14e45fceea167a5a36dedd4bea2543
8 | parent | c9f0f895fb98ab9159f51fd0297e236d
9 | parent | 45c48cce2e2d7fbdea1afc51c7c6ad26
10 | parent | d3d9446802a44259755d38e6d163e820
6 | child | 1679091c5a880faf6fb5e6087eb1b2dc
7 | child | 8f14e45fceea167a5a36dedd4bea2543
8 | child | c9f0f895fb98ab9159f51fd0297e236d
9 | child | 45c48cce2e2d7fbdea1afc51c7c6ad26
10 | child | d3d9446802a44259755d38e6d163e820
11 | child | 6512bd43d9caa6e02c990b0a82652dca
12 | child | c20ad4d76fe97759aa27a0c99bff6710
13 | child | c51ce410c124a10e0db5e4b97fc2af39
14 | child | aab3238922bcc25a6f606eb525ffdc56
15 | child | 9bf31c7ff062936a96d3c8bd1f8f2ff3
(20 rows)

When we try to update the foreign-table without DirectUpdate mode,
remote query tries to update the rows specified by "ctid" system column.
However, it was not a unique key in this case.

postgres=# explain update ftable set x = 'updated' where id > 10 and
pg_backend_pid() > 0;
QUERY PLAN
-----------------------------------------------------------------------------
Update on ftable (cost=100.00..133.80 rows=414 width=74)
-> Result (cost=100.00..133.80 rows=414 width=74)
One-Time Filter: (pg_backend_pid() > 0)
-> Foreign Scan on ftable (cost=100.00..133.80 rows=414 width=42)
(4 rows)

[*] Note that pg_backend_pid() prevent direct update.

postgres=# update ftable set x = 'updated' where id > 10 and
pg_backend_pid() > 0;
UPDATE 5
postgres=# select ctid,* from ftable;
ctid | id | label | x
--------+----+--------+----------------------------------
(0,1) | 1 | parent | c4ca4238a0b923820dcc509a6f75849b
(0,2) | 2 | parent | c81e728d9d4c2f636f067f89cc14862c
(0,3) | 3 | parent | eccbc87e4b5ce2fe28308fd9f2a7baf3
(0,4) | 4 | parent | a87ff679a2f3e71d9181a67b7542122c
(0,5) | 5 | parent | e4da3b7fbbce2345d7772b0674a318d5
(0,11) | 6 | parent | updated
(0,12) | 7 | parent | updated
(0,13) | 8 | parent | updated
(0,14) | 9 | parent | updated
(0,15) | 10 | parent | updated
(0,1) | 6 | child | 1679091c5a880faf6fb5e6087eb1b2dc
(0,2) | 7 | child | 8f14e45fceea167a5a36dedd4bea2543
(0,3) | 8 | child | c9f0f895fb98ab9159f51fd0297e236d
(0,4) | 9 | child | 45c48cce2e2d7fbdea1afc51c7c6ad26
(0,5) | 10 | child | d3d9446802a44259755d38e6d163e820
(0,11) | 11 | child | updated
(0,12) | 12 | child | updated
(0,13) | 13 | child | updated
(0,14) | 14 | child | updated
(0,15) | 15 | child | updated
(20 rows)

The WHERE-clause (id > 10) should affect only child table.
However, it updated the rows in the parent table with same ctid.

How about your thought?
Probably, we need to fetch a pair of tableoid and ctid to identify
the remote table exactly, if not direct-update cases.

Best regards,
--
HeteroDB, Inc / The PG-Strom Project
KaiGai Kohei <kaigai(at)heterodb(dot)com>