Re: Add semi-join pushdown to postgres_fdw

Hello,

While playing with this feature I found the following.

Two foreign tables:
postgres(at)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(at)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(at)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(at)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(at)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