Re: Teaching planner to short-circuit empty UNION/EXCEPT/INTERSECT inputs

Tom Lane <tgl@sss.pgh.pa.us>

From: Tom Lane <tgl@sss.pgh.pa.us>

To: David Rowley <dgrowleyml@gmail.com>

Cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>

Date: 2025-10-02T15:18:56Z

Lists: pgsql-hackers

David Rowley <dgrowleyml@gmail.com> writes:
> Good idea. Less code and still get to keep the one that did well in
> the benchmark. See attached.

0001's change in is_dummy_rel() seems like a complete hack, especially
since you didn't bother to change the adjacent comment.  Why is that
necessary?

v2 LGTM otherwise.

			regards, tom lane

Commits

Fix UNION planner estimate_num_groups with varno==0
- eaa159632d03 19 (unreleased) landed
Fix possible usage of incorrect UPPERREL_SETOP RelOptInfo
- fdda78e361f1 19 (unreleased) landed
Teach planner to short-circuit EXCEPT/INTERSECT with dummy inputs
- 9c9d41af4db7 19 (unreleased) landed
Fix incorrect targetlist in dummy UNIONs
- 928df067d1e6 19 (unreleased) landed
Teach UNION planner to remove dummy inputs
- 03d40e4b523b 19 (unreleased) landed