oid not "UNIQUE" for use as FOREIGN KEY?

Hi!

Imagine I have tables like those in PostgreSQL 7.1.3:

CREATE TABLE rabattgruppe (
    gruppe_oid OID,
    produktgruppe VARCHAR(256)
);

CREATE TABLE gruppe (
    obergruppe_oid OID,
    name VARCHAR(32)
);

Then I try to establish integrity check 

ALTER TABLE gruppe ADD
  FOREIGN KEY (obergruppe_oid) REFERENCES gruppe (oid);

But:

NOTICE:  ALTER TABLE ... ADD CONSTRAINT will create implicit 
trigger(s) for FOREIGN KEY check(s)
ERROR:  UNIQUE constraint matching given keys for referenced 
table "gruppe" not found

I thought the problem was I was referring to the same table,
but also this doesn't work:

ALTER TABLE rabattgruppe ADD
  FOREIGN KEY (gruppe_oid) REFERENCES gruppe (oid);

NOTICE:  ALTER TABLE ... ADD CONSTRAINT will create implicit 
trigger(s) for FOREIGN KEY check(s)
ERROR:  UNIQUE constraint matching given keys for referenced 
table "gruppe" not found

Now, what is the problem?  "oid" for sure should be an UNIQUE
field in each table, doesn't it?  Is it a bug?


Thanks,
Ernesto

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