Re: AGG_PLAIN thinks sorts are free

Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
> On Thu, Jul 18, 2013 at 8:04 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> DISTINCT (and also ORDER BY) properties of aggregates are implemented
>> at runtime; the planner doesn't really do anything about them, except
>> suppress the choice it might otherwise make of using hashed aggregation.

> Couldn't a hash aggregate be superior to a sort one (for the distinct,
> not the order by)?

If it worked at all, it might be superior, but since it doesn't, it ain't.

This isn't really a matter of lack of round tuits, but a deliberate
judgment that it's probably not worth the trouble. Per the comment in
choose_hashed_grouping:

/*
* Executor doesn't support hashed aggregation with DISTINCT or ORDER BY
* aggregates. (Doing so would imply storing *all* the input values in
* the hash table, and/or running many sorts in parallel, either of which
* seems like a certain loser.)
*/

regards, tom lane