Thread: BUG #18906: EXCEPT with identical left/right queries isn't eliminated

The following bug has been logged on the website:

Bug reference:      18906
Logged by:          jinhui lai
Email address:      jh.lai@qq.com
PostgreSQL version: 17.4
Operating system:   ubuntu 22.04
Description:

Dear PostgreSQL Developer:
q1 EXCEPT q1 should always return an empty set and consume minimal execution
time. It should not be executed, but the empty set is returned quickly and
directly.  However, PostgreSQL actually executed such an unnecessary query.
I think such queries should be optimized.
Best regards,
1. How to repeat?
docker pull postgres:latest
docker run --name postgres -e POSTGRES_PASSWORD=1213 -d -p 5432:5432
postgres
docker exec -it postgres psql -U postgres -c "CREATE DATABASE testdb;"
docker exec -it postgres pgbench -U postgres -i -s 10 testdb
docker exec -it  postgres psql -U postgres
psql (17.4 (Debian 17.4-1.pgdg120+2))
Type "help" for help.
\timing on
\c testdb
SELECT * FROM pgbench_accounts a CROSS JOIN pgbench_tellers WHERE a.aid
>50000
EXCEPT
SELECT * FROM pgbench_accounts a CROSS JOIN pgbench_tellers WHERE a.aid >
50000;
 aid | bid | abalance | filler | tid | bid | tbalance | filler
-----+-----+----------+--------+-----+-----+----------+--------
(0 rows)
Time: 231290.916 ms (03:51.291)
2. What do i expect to see?
SELECT * FROM pgbench_accounts a CROSS JOIN pgbench_tellers WHERE a.aid
>50000
EXCEPT
SELECT * FROM pgbench_accounts a CROSS JOIN pgbench_tellers WHERE a.aid >
50000;
 aid | bid | abalance | filler | tid | bid | tbalance | filler
-----+-----+----------+--------+-----+-----+----------+--------
(0 rows)
Time: 0.xxx ms (few time)
3. What do i  see instead?
SELECT * FROM pgbench_accounts a CROSS JOIN pgbench_tellers WHERE a.aid
>50000
EXCEPT
SELECT * FROM pgbench_accounts a CROSS JOIN pgbench_tellers WHERE a.aid >
50000;
 aid | bid | abalance | filler | tid | bid | tbalance | filler
-----+-----+----------+--------+-----+-----+----------+--------
(0 rows)
Time: 231290.916 ms (03:51.291)

On Sunday, April 27, 2025, PG Bug reporting form <noreply@postgresql.org> wrote:

The following bug has been logged on the website:

Bug reference:      18906
Logged by:          jinhui lai
Email address:      jh.lai@qq.com
PostgreSQL version: 17.4
Operating system:   ubuntu 22.04
Description: