Re: Performances

Lists: pgsql-adminpgsql-novice
From: Cedric BUSCHINI <cbuschini(at)carax(dot)com>
To: Novice Postgresql-list <pgsql-novice(at)postgresql(dot)org>, pgsql-admin(at)postgresql(dot)org
Subject: Performances
Date: 2006-12-28 09:59:39
Message-ID: 4593958B.1040601@carax.com
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Hello everyone,

A quick question:
- how long does it take you to perform that query :
"SELECT COUNT(*) FROM A_TABLE;"

There are like 56113000 rows in the table.

Thanks

Cedric

From: "A(dot) Kretschmer" <andreas(dot)kretschmer(at)schollglas(dot)com>
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: Performances
Date: 2006-12-28 10:48:33
Message-ID: 20061228104833.GA29704@a-kretschmer.de
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am Thu, dem 28.12.2006, um 10:59:39 +0100 mailte Cedric BUSCHINI folgendes:
> Hello everyone,
>
> A quick question:
> - how long does it take you to perform that query :
> "SELECT COUNT(*) FROM A_TABLE;"

Depends:

test=# \timing
Timing is on.
test=# select count(*) from a_table ;
count
-------
0
(1 row)

Time: 1.700 ms
test=#

>
> There are like 56113000 rows in the table.

A query like above performs a seq. scan on the table - this needs time.
You can get an estimation for this value by asking pg_class.reltuples.

select reltuples from pg_class where relname='a_table';

But this is only a estimation, depends on actual statistics.
It is updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE
INDEX.

Andreas
--
Andreas Kretschmer
Kontakt: Heynitz: 035242/47215, D1: 0160/7141639 (mehr: -> Header)
GnuPG-ID: 0x3FFF606C, privat 0x7F4584DA http://wwwkeys.de.pgp.net

From: Bricklen Anderson <banderson(at)presinet(dot)com>
To: Cedric BUSCHINI <cbuschini(at)carax(dot)com>
Cc: Novice Postgresql-list <pgsql-novice(at)postgresql(dot)org>, pgsql-admin(at)postgresql(dot)org
Subject: Re: Performances
Date: 2006-12-28 17:17:51
Message-ID: 4593FC3F.8020207@presinet.com
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Cedric BUSCHINI wrote:
> Hello everyone,
>
> A quick question:
> - how long does it take you to perform that query :
> "SELECT COUNT(*) FROM A_TABLE;"
>

Not long :)

dev5=# SELECT COUNT(*) FROM A_TABLE;
ERROR: relation "a_table" does not exist

From: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
To: Cedric BUSCHINI <cbuschini(at)carax(dot)com>
Cc: Novice Postgresql-list <pgsql-novice(at)postgresql(dot)org>, pgsql-admin(at)postgresql(dot)org
Subject: Re: Performances
Date: 2006-12-29 03:09:35
Message-ID: 20061229030935.GC29576@alvh.no-ip.org
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Cedric BUSCHINI wrote:
> Hello everyone,
>
> A quick question:
> - how long does it take you to perform that query :
> "SELECT COUNT(*) FROM A_TABLE;"

Longer than I want to wait in any case. In most cases you can rewrite
that query to something else; what that something looks like, depends on
how exact you want the result to be. So the question to you is, what do
you want that number for?

--
Alvaro Herrera http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support