Обсуждение: select count(*) from a large table is very slow

Make a summary table and select from there instead.

Ken

On Fri, May 18, 2007 at 06:15:25AM -0700, Jessica Richard wrote:
> I have a ~30 milliow row table, select count(*) from this table is very very very slow...
>
> How dow I get it run faster?
>
> Thanks,
> Jessica
>
>
> ---------------------------------
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On 5/18/07, Jessica Richard <rjessil@yahoo.com> wrote:
> I have a ~30 milliow row table, select count(*) from this table is very very
> very slow...

Yes, see the topic "Much Ado about COUNT(*)"

> How dow I get it run faster?

Unfortunately, aside from reducing the size of the table, you can't.

--
Jonah H. Harris, Software Architect | phone: 732.331.1324
EnterpriseDB Corporation            | fax: 732.331.1301
33 Wood Ave S, 3rd Floor            | jharris@enterprisedb.com
Iselin, New Jersey 08830            | http://www.enterprisedb.com/

On 5/18/07, Kenneth Marshall <ktm@rice.edu> wrote:
> Make a summary table and select from there instead.

Yes, a summary table updated by triggers is the only way to have
correct numbers... but you also pay the overhead of maintaining those
numbers on every INSERT/UPDATE statement.  If you just need
*reasonable* guesstimates, use reltuples in pg_class after an analyze.

--
Jonah H. Harris, Software Architect | phone: 732.331.1324
EnterpriseDB Corporation            | fax: 732.331.1301
33 Wood Ave S, 3rd Floor            | jharris@enterprisedb.com
Iselin, New Jersey 08830            | http://www.enterprisedb.com/

2007/5/18, Jonah H. Harris <jonah.harris@gmail.com>:
> Yes, a summary table updated by triggers is the only way to have
> correct numbers... but you also pay the overhead of maintaining those
> numbers on every INSERT/UPDATE statement.  If you just need
> *reasonable* guesstimates, use reltuples in pg_class after an analyze.

Oops... INSERT or DELETE... :)

And remember that if using TRUNCATE, it will not run your trigger and
you will need to reset the counter manually.

Regards...
--
Daniel Cristian Cruz

On 5/18/07, Daniel Cristian Cruz <danielcristian@gmail.com> wrote:
> Oops... INSERT or DELETE... :)

Heh, yeah... doh!

> And remember that if using TRUNCATE, it will not run your trigger and
> you will need to reset the counter manually.

Correct.

--
Jonah H. Harris, Software Architect | phone: 732.331.1324
EnterpriseDB Corporation            | fax: 732.331.1301
33 Wood Ave S, 3rd Floor            | jharris@enterprisedb.com
Iselin, New Jersey 08830            | http://www.enterprisedb.com/