Re: Teaching planner to short-circuit empty UNION/EXCEPT/INTERSECT inputs - Mailing list pgsql-hackers

From Tom Lane
Subject Re: Teaching planner to short-circuit empty UNION/EXCEPT/INTERSECT inputs
Date
Msg-id 1196118.1759438964@sss.pgh.pa.us
Whole thread Raw
In response to Re: Teaching planner to short-circuit empty UNION/EXCEPT/INTERSECT inputs  (David Rowley <dgrowleyml@gmail.com>)
List 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



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