Horton

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

David Rowley <dgrowleyml@gmail.com>

View thread View on postgr.es
From: David Rowley <dgrowleyml@gmail.com>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2025-11-04T10:00:09Z
Lists: pgsql-hackers

Attachments

On Tue, 4 Nov 2025 at 22:54, David Rowley <dgrowleyml@gmail.com> wrote:
> The reason we end up with the same result_rel is that we're not
> passing all the relids in fetch_upper_rel(root, UPPERREL_SETOP,
> relids) due to having removed dummy rels. I guess the fix might be
> something like record the relids even when skipping dummy relations.
> I'll go and explore that as an option.

This seems to fix it. I'll study it more in the morning (it's late in
my time zone).

David

Commits

  1. Fix UNION planner estimate_num_groups with varno==0

  2. Fix possible usage of incorrect UPPERREL_SETOP RelOptInfo

  3. Teach planner to short-circuit EXCEPT/INTERSECT with dummy inputs

  4. Fix incorrect targetlist in dummy UNIONs

  5. Teach UNION planner to remove dummy inputs