The type int8 and the use of indexes

Hi,

I realized that PostgreSQL (7.0.3) optimizer doesn´t use indexes in a query
like "select * from my table where pk=1", where the field pk is of
type int8.
I changed the type to int4, and now it uses the index (a lot faster). I also
tryed changing the type to numeric, with the same results.
Why does it happen? Is there something special with type int8 ???

Thanks,
Nelson