| From: | Emanuel Calvo <emanuel(dot)calvo(at)2ndquadrant(dot)com> |
|---|---|
| To: | pgsql-general(at)postgresql(dot)org |
| Subject: | Re: Query planning |
| Date: | 2014-10-31 19:09:42 |
| Message-ID: | 5453DE76.6070905@2ndquadrant.com |
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| Lists: | pgsql-general |
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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