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Re: wip: functions median and percentile

From: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Greg Stark <gsstark(at)mit(dot)edu>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, David Fetter <david(at)fetter(dot)org>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: wip: functions median and percentile
Date: 2010-10-11 17:55:32
Message-ID: AANLkTimTv7UNeVLS_B9SfqJ0xmmNMtUW4Wn3X-C8L0yK@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-rrreviewers
On 11 October 2010 18:48, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> writes:
>> On 11 October 2010 18:37, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> Sure it would: an estimate of 30K would keep the planner from using
>>> hash aggregation.
>
>> Not if work_mem was 10MB.
>
> And?  If the memory requirement actually fits, you're in good shape.
>

Yeah but the actual memory requirement, if it uses a hash aggregate,
is over 1GB, and could easily be much higher. It's also hard to kill,
because it eats up that memory so quickly.

Regards,
Dean

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