Adding a New Composite Constraint to an Existing Table.

Hi,

I am using postgresql 7.1. I was unable to add a new PRIMARY KEY
constraint to an existing table.

i have table,
CREATE TABLE xx
(
id int,
name text,
amount int
);

I have say some 10 records in this table, which are unique and not
null (primary). Fine! Now, i am willing to have primary key constraint
to all the 3 fields.

I tried,

bhuvan=> ALTER TABLE xx ADD primary key (id, name, amount);
ERROR: ALTER TABLE / ADD CONSTRAINT is not implemented for that
constraint type.

Where i am wrong? How else, can i get that primary key constraint
WITHOUT RECREATING that table?

Hope to get some lights.

Regards,
Bhuvaneswar.