| From: | David Rowley <dgrowleyml(at)gmail(dot)com> |
|---|---|
| To: | Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> |
| Cc: | Zhihong Yu <zyu(at)yugabyte(dot)com>, "Andrey V(dot) Lepikhov" <a(dot)lepikhov(at)postgrespro(dot)ru>, Teodor Sigaev <teodor(at)sigaev(dot)ru>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: POC: GROUP BY optimization |
| Date: | 2022-09-19 21:16:09 |
| Message-ID: | CAApHDvoEF0GsqmBuCPXb6f12jPwz8EeLG-oYsOSc5zrJtKZHOg@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, 13 Jul 2022 at 15:37, David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> I'm just in this general area of the code again today and wondered
> about the header comment for the preprocess_groupclause() function.
>
> It says:
>
> * In principle it might be interesting to consider other orderings of the
> * GROUP BY elements, which could match the sort ordering of other
> * possible plans (eg an indexscan) and thereby reduce cost. We don't
> * bother with that, though. Hashed grouping will frequently win anyway.
>
> I'd say this commit makes that paragraph mostly obsolete. It's only
> true now in the sense that we don't try orders that suit some index
> that would provide pre-sorted results for a GroupAggregate path. The
> comment leads me to believe that we don't do anything at all to find a
> better order, and that's not true now.
I've just pushed a fix for this.
David
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