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Re: rows in explain

From: Michael Fuhr <mike(at)fuhr(dot)org>
To: Lee Wu <Lwu(at)mxlogic(dot)com>
Cc: pgsql-admin(at)postgresql(dot)org
Subject: Re: rows in explain
Date: 2005-07-29 18:45:15
Message-ID: 20050729184515.GA98486@winnie.fuhr.org (view raw or flat)
Thread:
Lists: pgsql-admin
On Fri, Jul 29, 2005 at 11:54:18AM -0600, Lee Wu wrote:
> explain select count(*) from pg_class where relname='pg_class';
>                           QUERY PLAN
> --------------------------------------------------------------
>  Aggregate  (cost=4.55..4.55 rows=1 width=0)
>    ->  Seq Scan on pg_class  (cost=0.00..4.55 rows=1 width=0)
>          Filter: (relname = 'pg_class'::name)
> (3 rows)
> 
> Where and how PG get "rows=1"?

The planner uses statistics to estimate the number of rows that a
query will return.  Here are a couple of links that provide more
information:

http://www.postgresql.org/docs/8.0/static/planner-stats.html
http://www.postgresql.org/docs/8.0/static/view-pg-stats.html
http://www.postgresql.org/docs/8.0/static/sql-analyze.html
http://developer.postgresql.org/docs/postgres/planner-stats-details.html

-- 
Michael Fuhr
http://www.fuhr.org/~mfuhr/

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