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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 15:44:08
Message-ID: 24543.1286811848@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 15:03, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Reflecting on it, I think it'd be best to allow an agg to
>> provide an estimation function that'd be told the input data type and
>> expected number of rows --- even on a per-aggregate basis, a constant
>> estimate just isn't good enough.

> How good will that estimate of the number of rows be though?

It can't possibly be any worse than a hard-wired constant ;-)

> If they're coming from a SRF it could be a huge under-estimate, and you'd
> still risk eating all the memory, if you allowed a hash aggregate.

If, for a particular aggregate, you're too chicken to ever allow hash
aggregation, you could just return a very large number from the
estimation hook function.  I doubt that's a very useful behavior in the
majority of cases though.

			regards, tom lane

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