From: Andrew Sullivan <ajs@crankycanuck.ca>
To: Kashmira Patel (kupatel) <kupatel@cisco.com>
Cc: Andrew Sullivan <ajs@crankycanuck.ca>
Cc: pgsql-sql@postgresql.org
Subject: Re: Using EXPLAIN-ANALYZE
Date: Thu, 16 Feb 2006 16:14:00 -0500
Message-ID: <20060216211400.GB9872@phlogiston.dyndns.org> (raw)
In-Reply-To: <18AE59788A3FC640A367E5652E664D8001928860@xmb-sjc-237.amer.cisco.com>
References: <18AE59788A3FC640A367E5652E664D8001928860@xmb-sjc-237.amer.cisco.com>
On Thu, Feb 16, 2006 at 01:08:40PM -0800, Kashmira Patel (kupatel) wrote:
> So I would have to put in lots of rows of data in the table before using
> the explain command?
Well, no, but you won't get useful information without it.
PostgreSQL has a cost-based optimizer. The query plan is affected
by the nature of your data. That's what the ANALYZE command (on its
own, or with VACUUM) is for.
A
>
> -----Original Message-----
> From: pgsql-sql-owner@postgresql.org
> [mailto:pgsql-sql-owner@postgresql.org] On Behalf Of Andrew Sullivan
> Sent: Thursday, February 16, 2006 12:39 PM
> To: pgsql-sql@postgresql.org
> Subject: Re: [SQL] Using EXPLAIN-ANALYZE
>
> On Thu, Feb 16, 2006 at 12:07:10PM -0800, Kashmira Patel (kupatel)
> wrote:
> > Hi all,
> > I am pretty new to using Postrgres, and have been looking at the
> > messages in this forum for a while. I have noticed that the use of
> > EXPLAIN-ANALYZE is recommended quite a lot. I read the Postgres docs,
> > but am not quite sure I understand how this works. Is there some
> > tutorial or any other documentation how this can be used?
>
> Well, here's the short version:
>
> EXPLAIN [query] tells you what the planner _thinks_ it should do.
>
> EXPLAIN ANALYZE [query] tells you what the planner thinks it should do,
> and also executes the query and reports back how long every step took,
> how many rows were returned, &c. (For this reason, you want to wrap it
> in BEGIN;...;ROLLBACK; if it changes data.)
>
> For more, see the EXPLAIN EXPLAINED tutorial on techdocs.postgresql.org
> (<http://techdocs.postgresql.org/oscon2005/robert.treat/OSCON_Explaining
> _Explain_Public.sxi>)
>
> A
>
> --
> Andrew Sullivan | ajs@crankycanuck.ca
> In the future this spectacle of the middle classes shocking the avant-
> garde will probably become the textbook definition of Postmodernism.
> --Brad Holland
>
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--
Andrew Sullivan | ajs@crankycanuck.ca
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To: pgsql-sql@postgresql.org
Cc: ajs@crankycanuck.ca, kupatel@cisco.com
Subject: Re: Using EXPLAIN-ANALYZE
In-Reply-To: <20060216211400.GB9872@phlogiston.dyndns.org>
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