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Re: Speeding up aggregates

From: Hannu Krosing <hannu(at)tm(dot)ee>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Speeding up aggregates
Date: 2002-12-06 21:32:06
Message-ID: 1039210325.2069.9.camel@rh72.home.ee (view raw or flat)
Thread:
Lists: pgsql-performance
Tom Lane kirjutas L, 07.12.2002 kell 01:46:
> "Josh Berkus" <josh(at)agliodbs(dot)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(at)tm(dot)ee>

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