Sequence

Hello.
A table has two primary keys: CREATE TABLE example ( pk1 integer, pk2
integer, PRIMARY KEY (pk1, pk2) ). To add a new record I use command:
INSERT INTO example (pk1, pk2) VALUES (0, 0). Before adding the new
record I have to find out the last value of pk2. How can I use something
like this: INSERT INTO example (pk1, pk2) VALUES (0, nextval('pk2'))?
If a table just has one primary key I can use sequence (CREATE
SEQUENCE). What about two primary keys?