Re: POC: GROUP BY optimization

On Wed, 13 Jul 2022 at 15:37, David Rowley <dgrowleyml@gmail.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