script to get sessions that consume undo

SELECT   r.name rbs,
           NVL (s.username, 'None') oracle_user,
           s.osuser client_user,
           p.username unix_user,
           TO_CHAR (s.sid) || ',' || TO_CHAR (s.serial#) AS sid_serial,
           p.spid unix_pid,
           TO_CHAR (s.logon_time, 'mm/dd/yy hh24:mi:ss') AS login_time,
           TO_CHAR (SYSDATE - (s.last_call_et) / 86400, 'mm/dd/yy hh24:mi:ss')
              AS last_txn,
           t.used_ublk * TO_NUMBER (x.VALUE) / 1024 AS undo_kb
    FROM   v$process p,
           v$rollname r,
           v$session s,
           v$transaction t,
           v$parameter x
   WHERE       s.taddr = t.addr
           AND s.paddr = p.addr(+)
           AND r.usn = t.xidusn(+)
           AND x.name = 'db_block_size'
ORDER BY   r.name;