SELECT * FROM (
SELECT c.table_name||'.'||cc.column_name||'.'||cc.position
FROM user_constraints c, user_cons_columns cc
WHERE c.constraint_name = cc.constraint_name
AND c.constraint_type = 'R'
MINUS
SELECT i.table_name||'.'||ic.column_name||'.'||ic.column_position
FROM user_indexes i, user_ind_columns ic
WHERE i.index_name = ic.index_name
)
ORDER BY 1;