| From: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
|---|---|
| To: | John R Pierce <pierce(at)hogranch(dot)com>, pgsql-general(at)postgresql(dot)org |
| Subject: | Re: How does Postgres estimate the memory needed for sorting/aggregating |
| Date: | 2017-01-25 22:06:44 |
| Message-ID: | 2dfed0df-73bc-e3fd-3055-ea9ce6ca5005@2ndquadrant.com |
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| Lists: | pgsql-general |
On 01/25/2017 10:47 PM, John R Pierce wrote:
> On 1/25/2017 12:59 PM, Thomas Kellerer wrote:
>> So here is my question: how does Postgres estimate/know the memory
>> needed for the aggregation? Or does it dynamically resize the memory
>> if the initial assumption was wrong?
>
> my understanding is it fits as much as it can into a work_mem sized
> allocation, and if thats not enough uses temporary files and multiple
> passes.
>
That only works for hash joins, not for hash aggregates. Hash aggregate
is about the only operation in PostgreSQL that can cause OOM because of
under-estimation.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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