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Re: wip: functions median and percentile

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com>
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 18:05:11
Message-ID: 27242.1286820311@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-rrreviewers
Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> writes:
> On 11 October 2010 18:48, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> 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.

In that case the estimate of 30K per instance was wrong.
You still haven't explained why this is impossible to estimate,
or even particularly hard, as long as we can provide some code that
knows specifically about the behavior of this aggregate.  The amount
of space needed to sort X amount of data is not unpredictable.

			regards, tom lane

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