Horton

Re: wip: functions median and percentile

Dean Rasheed <dean.a.rasheed@gmail.com>

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From: Dean Rasheed <dean.a.rasheed@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Hitoshi Harada <umi.tanuki@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Greg Stark <gsstark@mit.edu>, Pavel Stehule <pavel.stehule@gmail.com>, David Fetter <david@fetter.org>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2010-10-11T17:42:10Z
Lists: pgsql-hackers 
On 11 October 2010 18:37, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Dean Rasheed <dean.a.rasheed@gmail.com> writes:
>> The estimate of 200 x 8K is below work_mem, so it uses a hash
>> aggregate. In reality, each tuplesort allocates around 30K initially,
>> so it very quickly uses over 1GB. A better estimate for the aggregate
>> wouldn't improve this situation much.
>
> Sure it would: an estimate of 30K would keep the planner from using
> hash aggregation.
>

Not if work_mem was 10MB.

Regards,
Dean