Metadata:
CREATE TABLE T1 (N INTEGER);
CREATE TABLE T2 (N INTEGER);
CREATE VIEW V (N1, N2, N3) AS
    select t1.n, t2.n, 3
        from t1
        full join t2
            on (t1.n = t2.n)
;

Data:
insert into t1 values (1);
insert into t1 values (2);
insert into t2 values (2);

This query:
select rdb$relation_id, v.rdb$db_key, v.*
    from rdb$database
    full outer join v
        on (1 = 0)
    ;

Should return:
RDB$RELATION_ID DB_KEY N1 N2 N3
=============== ================================ ============ ============ ============
         <null> 82000000010000008000000002000000 2 2 3
         <null> 80000000010000000000000000000000 1 <null> 3
            133 <null> <null> <null> <null>

And not:
RDB$RELATION_ID DB_KEY N1 N2 N3
=============== ================================ ============ ============ ============
         <null> 81000000010000008000000002000000 2 2 3
         <null> 81000000020000008000000001000000 1 <null> 3
            131 81000000010000008000000003000000 <null> <null> 3