SQL> show parameter undo;

NAME                                 TYPE                              VALUE
------------------------------------ --------------------------------- ---------
undo_management                      string                            MANUAL
undo_retention                       integer                           900
undo_tablespace                      string                            undo_new

SQL> create rollback segment undo_xff2 tablespace undotbs1;

Rollback segment created.

SQL> create rollback segment undo_xff3 tablespace undotbs1;

Rollback segment created.

SQL> alter rollback segment undo_xff1 online;

Rollback segment altered.

SQL> alter rollback segment undo_xff2 online;

Rollback segment altered.

SQL> alter rollback segment undo_xff3 online;

Rollback segment altered.

SQL> conn chf/xifenfei
Connected.
SQL> set transaction use rollback segment undo_xff3;

Transaction set.

SQL> delete from t_xifenfei where rownum<10;

9 rows deleted.

SQL> select XIDUSN from V$TRANSACTION;

    XIDUSN
----------
        23

SQL> select name from sys.undo$ where us#=23;

NAME
--------------------------------
UNDO_XFF3

SQL> commit;

Commit complete.

SQL> set transaction use rollback segment undo_xff1;

Transaction set.

SQL> delete from t_xifenfei where rownum<10;

9 rows deleted.

SQL> select XIDUSN from V$TRANSACTION;

    XIDUSN
----------
        21

SQL> select name from sys.undo$ where us#=21;

NAME
--------------------------------
UNDO_XFF1