Re: Add semi-join pushdown to postgres_fdw

Hello,

While playing with this feature I found the following.

Two foreign tables:
postgres@demo_postgres_fdw(17.0)=# \det aircrafts|seats
       List of foreign tables
  Schema |   Table   |   Server
--------+-----------+-------------
  public | aircrafts | demo_server
  public | seats     | demo_server
(2 rows)


This query uses optimization:

postgres@demo_postgres_fdw(17.0)=# EXPLAIN (costs off, verbose) SELECT *
FROM aircrafts a
WHERE a.aircraft_code = '320' AND EXISTS (
   SELECT * FROM seats s WHERE s.aircraft_code = a.aircraft_code
);
                                                                                                  QUERY PLAN                                                                                       >
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------->
  Foreign Scan
    Output: a.aircraft_code, a.model, a.range
    Relations: (public.aircrafts a) SEMI JOIN (public.seats s)
    Remote SQL: SELECT r1.aircraft_code, r1.model, r1.range FROM bookings.aircrafts r1 WHERE ((r1.aircraft_code = '320')) AND EXISTS (SELECT NULL FROM bookings.seats r2 WHERE ((r2.aircraft_code =>
(4 rows)


But optimization not used for NOT EXISTS:

postgres@demo_postgres_fdw(17.0)=# EXPLAIN (costs off, verbose) SELECT *
FROM aircrafts a
WHERE a.aircraft_code = '320' AND NOT EXISTS (
   SELECT * FROM seats s WHERE s.aircraft_code = a.aircraft_code
);
                                                    QUERY PLAN
----------------------------------------------------------------------------------------------------------------
  Nested Loop Anti Join
    Output: a.aircraft_code, a.model, a.range
    ->  Foreign Scan on public.aircrafts a
          Output: a.aircraft_code, a.model, a.range
          Remote SQL: SELECT aircraft_code, model, range FROM bookings.aircrafts WHERE ((aircraft_code = '320'))
    ->  Materialize
          Output: s.aircraft_code
          ->  Foreign Scan on public.seats s
                Output: s.aircraft_code
                Remote SQL: SELECT aircraft_code FROM bookings.seats WHERE ((aircraft_code = '320'))
(10 rows)

Also, optimization not used after deleting first condition (a.aircraft_code = '320'):

postgres@demo_postgres_fdw(17.0)=# EXPLAIN (costs off, verbose) SELECT *
FROM aircrafts a
WHERE EXISTS (
   SELECT * FROM seats s WHERE s.aircraft_code = a.aircraft_code
);
                                    QUERY PLAN
--------------------------------------------------------------------------------
  Hash Join
    Output: a.aircraft_code, a.model, a.range
    Inner Unique: true
    Hash Cond: (a.aircraft_code = s.aircraft_code)
    ->  Foreign Scan on public.aircrafts a
          Output: a.aircraft_code, a.model, a.range
          Remote SQL: SELECT aircraft_code, model, range FROM bookings.aircrafts
    ->  Hash
          Output: s.aircraft_code
          ->  HashAggregate
                Output: s.aircraft_code
                Group Key: s.aircraft_code
                ->  Foreign Scan on public.seats s
                      Output: s.aircraft_code
                      Remote SQL: SELECT aircraft_code FROM bookings.seats
(15 rows)


But the worst thing is that replacing AND with OR causes breaking session and server restart:

postgres@demo_postgres_fdw(17.0)=# EXPLAIN (costs off, verbose) SELECT *
FROM aircrafts a
WHERE a.aircraft_code = '320' OR EXISTS (
   SELECT * FROM seats s WHERE s.aircraft_code = a.aircraft_code
);
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
The connection to the server was lost. Attempting reset: Failed.

-- 
Pavel Luzanov
Postgres Professional:https://postgrespro.com