Обсуждение: Performances

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

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

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

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