| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> |
| Cc: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: limits of max, min optimization |
| Date: | 2022-07-18 14:29:07 |
| Message-ID: | 696707.1658154547@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> writes:
> On 2022-Jul-18, Pavel Stehule wrote:
>> I am trying to fix one slow query, and found that optimization of min, max
>> functions is possible only when there is no JOIN in the query.
> See preprocess_minmax_aggregates() in
> src/backend/optimizer/plan/planagg.c
> Maybe it is possible to hack that code so that this case can be handled
> better.
The comments show this was already thought about:
* We also restrict the query to reference exactly one table, since join
* conditions can't be handled reasonably. (We could perhaps handle a
* query containing cartesian-product joins, but it hardly seems worth the
* trouble.)
regards, tom lane
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