performance question

Can somebody explain to me:

> radius=# explain select count (radiuspk) from radius ;
> NOTICE: QUERY PLAN:
>
> Aggregate (cost=12839.79..12839.79 rows=1 width=8)
> -> Seq Scan on radius (cost=0.00..11843.43 rows=398543 width=8)
>
> EXPLAIN

This query answers me *instantly* after hitting return

> radius=# select count (radiuspk) from radius ;
> count
> --------
> 398543
> (1 row)

This query takes about 3 seconds. But the query plan *already* knows the
number of rows ("rows=398543"). So why does it take 3 seconds. Is my
assumption correct that the optimiser still can be optimized a little? :-)

Reinoud (not that this is a real problem, just wondering)