| From: | Teodor Sigaev <teodor(at)sigaev(dot)ru> |
|---|---|
| To: | Claudio Freire <klaussfreire(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
| Cc: | PostgreSQL-Dev <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: POC: GROUP BY optimization |
| Date: | 2018-06-07 13:48:28 |
| Message-ID: | a645a819-c0ab-243a-f903-ca9e0f1f5245@sigaev.ru |
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| Thread: | |
| Lists: | pgsql-hackers |
>> I don't see why not to generate all possible orderings (keeping only the
>> cheapest path for each pathkey variant) and let the optimizer to sort it
>> out.
>
> I'm assuming costing the full N! possible orderings would be
> prohibitively expensive.
That's true, but for the first step we need to improve cost_sort and only then
try to find the best pathkeys order by optimal way.
> - If the user requested that order, we assume it "somewhat
> subjectively better" (by giving it a slightly reduced cost).
I don't think so. It's not a SQL way - DB should define the optimal way to
execute query.
--
Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru
WWW: http://www.sigaev.ru/
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