пробвай това

CREATE OR REPLACE FUNCTION f_uk(c1 NUMBER, c2 NUMBER) RETURN VARCHAR2
DETERMINISTIC AS
BEGIN
IF c2 IS NOT NULL
THEN
RETURN c1 || '-' || c2;
ELSE
RETURN NULL;
END IF;
END f_uk;

create table TEST2
(
ID NUMBER not null,
ID2 NUMBER
);

create unique index test_uk on TEST2(substr(f_uk(ID,ID2),1,100));


при мен работи(Oralce 9.2.0.1)

Успех!!