[BUG?] postgres_fdw incorrectly updates remote table if it hasinherited children. - Mailing list pgsql-hackers

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@heterodb.com>