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Re: bad plan: 8.4.8, hashagg, work_mem=1MB.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Jon Nelson <jnelson+pgsql(at)jamponi(dot)net>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: bad plan: 8.4.8, hashagg, work_mem=1MB.
Date: 2011-06-20 16:08:19
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Jon Nelson <jnelson+pgsql(at)jamponi(dot)net> writes:
> I ran a query recently where the result was very large. The outer-most
> part of the query looked like this:

>  HashAggregate  (cost=56886512.96..56886514.96 rows=200 width=30)
>    ->  Result  (cost=0.00..50842760.97 rows=2417500797 width=30)

> The row count for 'Result' is in the right ballpark, but why does
> HashAggregate think that it can turn 2 *billion* rows of strings (an
> average of 30 bytes long) into only 200?

200 is the default assumption about number of groups when it's unable to
make any statistics-based estimate.  You haven't shown us any details so
it's hard to say more than that.

			regards, tom lane

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