select * from v$tablespace

create undo tablespace undotbs01 datafile '/oradata/rdev/undotbs01.dbf' size 2000m autoextend on

/* Formatted on 2007/05/01 17:04 (Formatter Plus v4.8.8) */
SELECT a.usn, a.status, b.tablespace_name, b.segment_name
FROM v$rollstat a, dba_rollback_segs b
WHERE a.usn = b.segment_id

alter system set undo_tablespace=undotbs01 scope=both

SELECT a.usn, a.status, b.tablespace_name, b.segment_name
FROM v$rollstat a, dba_rollback_segs b
WHERE a.usn = b.segment_id

alter tablespace undotbs2 offline

alter tablespace undotbs2 online

alter tablespace undotbs2 offline

drop tablespace undotbs2 including contents

delete the datafile from os