| 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 |
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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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