Horton

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

David Rowley <dgrowleyml@gmail.com>

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From: David Rowley <dgrowleyml@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2025-10-02T12:09:27Z
Lists: pgsql-hackers

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On Thu, 2 Oct 2025 at 16:21, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I'm wondering if it could be shortened a great deal by
> handling left-input-dummy and EXCEPT-ALL-with-right-input-dummy
> but leaving the EXCEPT-with-right-input-dummy case unimproved.

Good idea. Less code and still get to keep the one that did well in
the benchmark. See attached.

I ended up splitting the patch in two. 0001 for UNION only, then 0002
for the INTERSECT and EXCEPT.

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