Performance of count(*)

Hi,

I just try to find out why a simple count(*) might last that long.
At first I tried explain, which rather quickly knows how many rows
to check, but the final count is two orders of magnitude slower.

My MS_SQL server using colleague can't believe that.

$ psql InfluenzaWeb -c 'explain SELECT count(*) from agiraw ;'
QUERY PLAN
-----------------------------------------------------------------------
Aggregate (cost=196969.77..196969.77 rows=1 width=0)
-> Seq Scan on agiraw (cost=0.00..185197.41 rows=4708941 width=0)
(2 rows)

real 0m0.066s
user 0m0.024s
sys 0m0.008s

$ psql InfluenzaWeb -c 'SELECT count(*) from agiraw ;'
count
---------
4708941
(1 row)

real 0m4.474s
user 0m0.036s
sys 0m0.004s

Any explanation?

Kind regards

Andreas.

--
http://fam-tille.de