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Re: Hash aggregates blowing out memory

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Mike Harding <mvh(at)ix(dot)netcom(dot)com>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: Hash aggregates blowing out memory
Date: 2005-02-25 21:55:42
Message-ID: 9266.1109368542@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-general
Mike Harding <mvh(at)ix(dot)netcom(dot)com> writes:
> I've been having problems where a HashAggregate is used because of a bad
> estimate of the distinct number of elements involved.

If you're desperate, there's always enable_hashagg.  Or reduce sort_mem
enough so that even the misestimate looks like it will exceed sort_mem.

In the long run it would be nice if HashAgg could spill to disk.  We
were expecting to see a contribution of code along that line last year
(from the CMU/Berkeley database class) but it never showed up.  The
performance implications might be a bit grim anyway :-(

			regards, tom lane

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