Re: Speeding up aggregates

Tom Lane kirjutas L, 07.12.2002 kell 01:46:
> "Josh Berkus" <josh@agliodbs.com> writes:
> > What have other Postgres users done to speed up aggregates on large
> > tables?
>
> FWIW, I've implemented hashed aggregation in CVS tip.

Great!

This should also make it easier to implement all kinds of GROUP BY
ROLLUP|CUBE|GROUPING SETS|() queries.

Do you have any near-term plans for doing them ?

> I have not had
> the time to try to benchmark it, but I'd be interested if anyone can
> run some tests on 7.4devel. Eliminating the need for a SORT step
> should help aggregations over large datasets.

Is there a variable to set that would disable one or another, like we
currently have for disabling various join strategies ?

> Note that even though there's no SORT, the sort_mem setting is used
> to determine the allowable hashtable size, so a too-small sort_mem
> might discourage the planner from selecting hashed aggregation.

Do you mean that hashed aggregation can't overflow to disk, or would it
just be too slow ?

--
Hannu Krosing <hannu@tm.ee>