| 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 17:42:10 |
| Message-ID: | AANLkTintCTk8r0jRM0rvY8h4FO=QHVNhXArKE6CL6AqQ@mail.gmail.com |
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| Lists: | pgsql-hackers pgsql-rrreviewers |
On 11 October 2010 18:37, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> 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.
>
Not if work_mem was 10MB.
Regards,
Dean
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