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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 17:37:11
Message-ID: 26554.1286818631@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:
> 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.

			regards, tom lane

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