Primary keys and NOT NULL

Hi -

I noticed that all of my primary keys are specified as not null and I did
not specify not null when I built the tables.  Does Postgres automatically
make all primary keys not null?  This is a problem for me because I have a
few tables that have multiple column primary keys where one of the columns
may be null.  Is there any way for me to change the not null specification?
or is that a requirement for primary keys in Postgres?  This was not the
case in Oracle....which is where I'm trying to port my data from.
--
Susan Lane
DPN, Incorporated
4631 Spring Mountain Road
Las Vegas, NV 89102
Email  suel@dpn.com
Ph. (702) 873-3282
Fax (702) 873-3913
http://www.dpn.com