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: Mingli Zhang <zmlpostgres@gmail.com>
Cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>, Tom Lane <tgl@sss.pgh.pa.us>
Date: 2025-10-02T22:24:45Z
Lists: pgsql-hackers 
On Fri, 3 Oct 2025 at 02:55, Mingli Zhang <zmlpostgres@gmail.com> wrote:
> It seems that the optimization for `UNION ALL` is already implemented in the patch: it removes empty sub-paths and preserves the remaining ones.
> Should we add a test case to formally validate this behavior like Union cases?

If I were to do that, I'd have to come up with something that's
flatten_simple_union_all() proof. Maybe something like varying types
in the targetlist. I think it's probably not really worthwhile since
it's not testing any new code that is not already being tested by the
tests that I did add.

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