SQL> create table tbl(col varchar2(8 byte));

Table created.

SQL> create materialized view tbl_mv as select * from tbl;

Materialized view created.

SQL> select column_name,data_length,char_length,char_used from user_tab_columns where table_name = 'TBL_MV';

COLUMN_NAM DATA_LENGTH CHAR_LENGTH C
---------- ----------- ----------- -
COL                  8           8 B

SQL> drop materialized view tbl_mv;

Materialized view dropped.


SQL> create materialized view tbl_mv as select cast(col as varchar2(8 char)) col from tbl;

Materialized view created.

SQL> select column_name,data_length,char_length,char_used from user_tab_columns where table_name = 'TBL_MV';

COLUMN_NAM DATA_LENGTH CHAR_LENGTH C
---------- ----------- ----------- -
COL                 32           8 C

SQL>

SQL> WITH T AS (SELECT  UNISTR('abc\00e5\00f1\00f6') STR FROM DUAL)
  2  SELECT  LENGTHB(STR) L1,
  3          LENGTHB(CONVERT(STR,'WE8MSWIN1252')) L2
  4    FROM  T
  5  /

        L1         L2
---------- ----------
        12          6

SQL>