Memory consumption problem

Hi,

I have a problem with a simple request :

insert into mytable
select a.id, a.desc, 'mystring'
from mytable a;

This request runs for about two hours, and then OOMKiller kills the
postmaster !

Mytable has about 1,7 million tuples, it has a few indexes.
Postgresql version is 8.1, running on Debian 3.1, with 512kb of memory
and 1,3gb of swap space.
This request seems to end up eating all memory and swap...

Is this something that should occur or is there a problem on my server ?

Thanks,

Franck