SQL> create table tbl(
  2                   id number,
  3                   val number(3)
  4                  )
  5  /

Table created.

SQL> alter table tbl
  2    add constraint tbl_pk
  3      primary key(id)
  4  /

Table altered.

SQL> create materialized view log on tbl with(val)
  2  /

Materialized view log created.

SQL> alter table tbl
  2    modify val number(5)
  3  /

Table altered.

SQL> insert
  2    into tbl
  3    values(1,9999)
  4  /
  into tbl
       *
ERROR at line 2:
ORA-12096: error in materialized view log on "SCOTT"."TBL"
ORA-01438: value larger than specified precision allowed for this column


SQL> desc mlog$_tbl
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID                                                 NUMBER
 VAL                                                NUMBER(3)
 SNAPTIME$$                                         DATE
 DMLTYPE$$                                          VARCHAR2(1)
 OLD_NEW$$                                          VARCHAR2(1)
 CHANGE_VECTOR$$                                    RAW(255)
 XID$$                                              NUMBER

SQL>  

SQL> create table tbl(
  2                   id number,
  3                   val number(3)
  4                  )
  5  /

Table created.

SQL> alter table tbl
  2    add constraint tbl_pk
  3      primary key(id)
  4  /

Table altered.

SQL> create materialized view log on tbl with(val)
  2  /

Materialized view log created.

SQL> create materialized view tbl_mv
  2    as
  3      select  *
  4        from  tbl
  5  /

Materialized view created.

SQL> select  owner,
  2          name
  3    from  dba_dependencies
  4    where referenced_owner = 'SCOTT'
  5      and referenced_name = 'TBL'
  6  /

OWNER      NAME
---------- ----------
SCOTT      TBL_MV

SQL>