Обсуждение: 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

So I would have to put in lots of rows of data in the table before using
the explain command?

-----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

---------------------------(end of broadcast)---------------------------
TIP 1: if posting/reading through Usenet, please send an appropriate      subscribe-nomail command to
majordomo@postgresql.orgso that your      message can get through to the mailing list cleanly 

Kashmira Patel wrote:

> So I would have to put in lots of rows of data in the table
> before using the explain command?

No, but PostgreSQL's query planner may take a different approach for a small table than a large one.  The statistics
usedare generated during VACUUM ANALYZE/VACUUM FULL operations and, under 8.1, are probably maintained by autovacuum,
butyou can always vacuum manually and see if that changes the query plan. 

-Owen

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
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 1: if posting/reading through Usenet, please send an appropriate
>        subscribe-nomail command to majordomo@postgresql.org so that your
>        message can get through to the mailing list cleanly

-- 
Andrew Sullivan  | ajs@crankycanuck.ca
The fact that technology doesn't work is no bar to success in the marketplace.    --Philip Greenspun

Thanks for the explanations and the links. Will try some of this out and
come back to you guys if I still don't understand :)

-Kashmira

-----Original Message-----
From: Andrew Sullivan [mailto:ajs@crankycanuck.ca]
Sent: Thursday, February 16, 2006 1:14 PM
To: Kashmira Patel (kupatel)
Cc: Andrew Sullivan; pgsql-sql@postgresql.org
Subject: Re: [SQL] Using EXPLAIN-ANALYZE

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_Explaini
> ng
> _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
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 1: if posting/reading through Usenet, please send an appropriate
>        subscribe-nomail command to majordomo@postgresql.org so that
your
>        message can get through to the mailing list cleanly

--
Andrew Sullivan  | ajs@crankycanuck.ca
The fact that technology doesn't work is no bar to success in the
marketplace.    --Philip Greenspun