Update is very slow on a bigger table

hi,

i'm running an update procedure on a table with approximately 100,000
records. The problem is the procedure is running endlessly even when the
update is very simple like in the following example:

update mytable
set client_id = 1
where mytable_id = 1

mytable_id is the primary key of mytable.

the process under which the update is running reports among the usual
process data "UPDATE waiting".

I'm using PostgreSQL 7.3.4 on SuSE 9.0. From what I have seen it doesn't
look like that the host computer has problem with the resources.

Any idea why is this happening?

thanks,

dimitar