| From: | Robert Haas <robertmhaas(at)gmail(dot)com> |
|---|---|
| To: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
| Cc: | Jon Nelson <jnelson+pgsql(at)jamponi(dot)net>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: postmaster consuming /lots/ of memory with hash aggregate. why? |
| Date: | 2010-11-24 03:11:18 |
| Message-ID: | AANLkTiktPOkSTXk7frMVjOdUd8ojGhdUT0aWB4PH9ak5@mail.gmail.com |
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| Lists: | pgsql-performance |
On Fri, Nov 12, 2010 at 11:12 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
> if I remember well, you can set a number of group by ALTER TABLE ALTER
> COLUMN SET n_distinct = ..
>
> maybe you use it.
I'm not sure where the number 40,000 is coming from either, but I
think Pavel's suggestion is a good one. If you're grouping on a
column with N distinct values, then it stands to reason there will be
N groups, and the planner is known to estimate n_distinct on large
tables, even with very high statistics targets, which is why 9.0
allows a manual override.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
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