Create materialized view with specific column sizes

Question

Hi all,
I'm trying to create a materialized view with a specific a column size. Something like
create materialized view test_mv
refresh force on demand
as
select id,
       cast(my_compound_field as nvarchar2(50))
from ( select id,
              field1 || field2 my_compound_field
       from   my_table);But Oracle seems to ignore the cast and takes the maximum size it finds for field1 || field2 in the select query. The resulting table has a column nvarchar2(44) instead of nvarchar2(50).
This can give a problem when the view is refreshed... there could be new data that exceeds the current size, i.e. where length(field1 || field2) > 44.
How can I override the column size of a field in a materialized view?
Edit: Some additional info to clarify my case:
field1 and field2 are defined as nvarchar2(25). field1 || field2 can theoretically have a length of 50, but there is currently no data in my table that results in that length, the max is 44. I am afraid that there will be data in the future that exceeds 44, resulting in an error when the MV is refreshed!
Edited by: Pleiadian on Jan 25, 2011 2:06 PM

Answer 1

Cannot reproduce what you are saying is happening.
SQL> create table t (a nvarchar2(50), b nvarchar2(50));
Table created.
SQL> create materialized view tmv as
  2  select a, b, a || b c from t;
Materialized view created.
SQL> desc tmv
Name                                      Null?    Type
A                                                  NVARCHAR2(50)
B                                                  NVARCHAR2(50)
C                                                  NVARCHAR2(100)
SQL> drop materialized view tmv;
Materialized view dropped.
SQL> create materialized view tmv as
  2  select a, b, substr(a || b, 1, 10) c from t;
Materialized view created.
SQL> desc tmv
Name                                      Null?    Type
A                                                  NVARCHAR2(50)
B                                                  NVARCHAR2(50)
C                                                  NVARCHAR2(10)
SQL> select * from v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.1.0.7.0 - 64bit Production
PL/SQL Release 11.1.0.7.0 - Production
CORE    11.1.0.7.0      Production
TNS for Linux: Version 11.1.0.7.0 - Production
NLSRTL Version 11.1.0.7.0 - Production
SQL>Edited by: 3360 on Jan 25, 2011 8:10 AM
And with data
SQL> insert into t values ('3123423423143hhshgvcdcvw', 'ydgeew  gdfwe   dfefde  wfjjjjjjj');
1 row created.
SQL> commit;
Commit complete.
SQL> desc tmv
Name                                      Null?    Type
A                                                  NVARCHAR2(50)
B                                                  NVARCHAR2(50)
C                                                  NVARCHAR2(10)
SQL> select * from tmv;
A
B                                                  C
3123423423143hhshgvcdcvw
ydgeew  gdfwe   dfefde  wfjjjjjjj                      3123423423