| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Kirill Reshke <reshkekirill(at)gmail(dot)com> |
| Cc: | pingw33n(at)gmail(dot)com, pgsql-bugs(at)lists(dot)postgresql(dot)org |
| Subject: | Re: BUG #18751: Sub-optimal UNION ALL plan |
| Date: | 2024-12-23 15:18:56 |
| Message-ID: | 1347038.1734967136@sss.pgh.pa.us |
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| Lists: | pgsql-bugs |
Kirill Reshke <reshkekirill(at)gmail(dot)com> writes:
> I reproduced this on REL_16_STABLE, HEAD & REL_13_STABLE, so this is
> not really a bug, just a missing optimization?
Yeah. I believe what is happening is that the addition of the WHERE
clause forces the second sub-SELECT to be planned as an independent
query. And that level of planning has no idea that it might be
useful to produce a result ordered by "t", so it doesn't generate
a sub-plan that can do that. Then the best that the outer level
can do is sort after-the-fact.
You could work around this by writing the second sub-SELECT like
(select * from t2 where ... order by t)
regards, tom lane
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