Re: Performances

["A. Kretschmer" <andreas(dot)kretschmer(at)schollglas(dot)com>]

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
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Andreas Kretschmer
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