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List:       postgresql-general
Subject:    Re: [GENERAL] Query planning
From:       Emanuel Calvo <emanuel.calvo () 2ndquadrant ! com>
Date:       2014-10-31 19:09:42
Message-ID: 5453DE76.6070905 () 2ndquadrant ! com
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El 31/10/14 a las 04:46, Jayadevan M escibió:
> Hi all,
>
> I am going through PostgreSQL documentation on how row estimation is
> done in PostgreSQL.
> http://www.postgresql.org/docs/current/static/row-estimation-examples.html
> There is a reference to cardinality (rel_cardinality) there. But that
> is an estimate of the number of rows in the table, right?
>
> How many unique values are there in a column (cardinality), that is
> estimated while finding out selectivity?
>
>
Did you look into pg_stats view? There is a column colled n_distinct
that indicates
that (-1 all distinct, 1 none distintct). That's what you are asking about?

-- 
--
Emanuel Calvo                 http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services



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