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From: Josh Berkus <josh@agliodbs.com>
To: pgsql-performance@postgresql.org
Subject: Re: change sample size for statistics
Date: Fri, 10 Jun 2011 12:58:19 -0700
Message-ID: <4DF2775B.7070501@agliodbs.com> (raw)
In-Reply-To: <BANLkTik4st_UXVpjJYKcLp+wAtA+wKTe=g@mail.gmail.com>
References: <BANLkTik4st_UXVpjJYKcLp+wAtA+wKTe=g@mail.gmail.com>

On 6/10/11 5:15 AM, Willy-Bas Loos wrote:
> Hi,
> 
> is there a way to change the sample size for statistics (that analyze
> gathers)?
> It is said to be 10%. i would like to raise that, because we are getting bas
> estimations for n_distinct.

It's not 10%.  We use a fixed sample size, which is configurable on the
system, table, or column basis.

Some reading (read all these pages to understand what you're doing):
http://www.postgresql.org/docs/9.0/static/planner-stats.html
http://www.postgresql.org/docs/9.0/static/runtime-config-query.html#RUNTIME-CONFIG-QUERY-OTHER
http://www.postgresql.org/docs/9.0/static/planner-stats-details.html
http://www.postgresql.org/docs/9.0/static/sql-altertable.html
(scroll down to "set storage" on that last page)

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com

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