Using a union in the query rewrite feature

We have the following situation: I have three tables and one dummy table:

create table cdr_type1 (a number, b number, c number); 
insert into cdr_type1 values (1,2,3);
insert into cdr_type1 values (6,7,8);

create table cdr_type2 (a number, c number, d number);
insert into cdr_type2 values (34,35,67);
insert into cdr_type2 values (39,31,32); 

create table cdr_type3  (a number, c number, e number);
insert into cdr_type3 values (900,900,900);

create table  dummy_union (n number primary key, a number, b number,
c number, d number, e number, type varchar2(10) );

CREATE MATERIALIZED VIEW cdr_type1
ON PREBUILT TABLE WITHOUT REDUCED PRECISION
ENABLE QUERY REWRITE
as select a,b,c from dummy_union where type='type1';

CREATE MATERIALIZED VIEW cdr_type2
ON PREBUILT TABLE WITHOUT REDUCED PRECISION
ENABLE QUERY REWRITE
as select a,c,d from dummy_union where type='type2'; 

CREATE MATERIALIZED VIEW cdr_type3
ON PREBUILT TABLE WITHOUT REDUCED PRECISION
ENABLE QUERY REWRITE
as select a,c,e from dummy_union where type='type3';

alter system set query_rewrite_enabled=true;
alter session set optimizer_goal = first_rows;
alter session set query_rewrite_integrity = trusted;

SQL> select a, d from dummY_union where type='type2'; 

         A          D
---------- ----------
        34         67
        39         32

SQL> select a from dummy_union where type='type3';

         A
---------- 
       900

SQL> select a from dummy_union where type='type3' or type='type2';

no rows selected