CREATE UNDO TABLESPACE <tablespace_name>
DATAFILE '<path_and_file_name>'
SIZE <integer><K | M | G | T>
AUTOEXTEND <ON | OFF>
RETENTION <GUARANTEE | NOGUARANTEE>;
 
 
-- The following statement illustrates using the UNDO TABLESPACE 
-- clause in a CREATE DATABASE statement. The undo tablespace is 
-- named undotbs_01 and one datafile, /u01/oracle/rbdb1/undo0101.dbf, 
-- is allocated for it.
 
CREATE DATABASE rbdb1
     CONTROLFILE REUSE
     .
     .
     UNDO TABLESPACE undotbs_01 DATAFILE '/u01/oracle/rbdb1/undo0101.dbf';
 
 
-- The CREATE UNDO TABLESPACE statement is the same as the 
-- CREATE TABLESPACE statement, but the UNDO keyword is specified. 
-- The database determines most of the attributes of the undo 
-- tablespace, but you can specify the DATAFILE clause.
-- This example creates the undotbs_02 undo tablespace:
 
CREATE UNDO TABLESPACE undotbs_02
     DATAFILE '/u01/oracle/rbdb1/undo0201.dbf' SIZE 2M 
     REUSE AUTOEXTEND ON;
 
 
 
CREATE UNDO TABLESPACE undotbs02
     DATAFILE '/u01/oradata/undotbs02.dbf
     SIZE 50000M REUSE AUTOEXTEND ON
     RETENTION NOGUARANTEE;
 
desc dba_undo_extents
 
SELECT segment_name, tablespace_name, status, SUM(bytes)
FROM dba_undo_extents
GROUP BY segment_name, tablespace_name, status;
 
ALTER SYSTEM SET undo_tablespace='UNDOTBS2' scope=BOTH;