Horton

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

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

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <dgrowleyml@gmail.com>
Cc: PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2025-10-02T21:02:44Z
Lists: pgsql-hackers 
David Rowley <dgrowleyml@gmail.com> writes:
> On Fri, 3 Oct 2025 at 04:18, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 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?

> build_setop_child_paths() wraps the child inputs in SubqueryScanPaths,
> so we need to see through those.

Ah.

> An alternative way would be to propagate those during build_setop_child_paths()

That answer works for me.  I was expecting you to just document the
need for the extra check in is_dummy_rel ;-) ... but this way is
perhaps better.

			regards, tom lane

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