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rows in explain

From: "Lee Wu" <Lwu(at)mxlogic(dot)com>
To: <pgsql-admin(at)postgresql(dot)org>
Subject: rows in explain
Date: 2005-07-29 17:54:18
Message-ID: E1F0E4D89D8FA44789DFC065C8E755281A36B6@mxlhq-exch01.corp.mxlogic.com (view raw or flat)
Thread:
Lists: pgsql-admin
Hi all,

 

I want to find out how PG gets rows when it explains a query.

For example, when I issue:

 

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"?

 

Thank you!

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Subject: Re: rows in explain
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