Oracle DB Reaming undo / roll back after killing session

col USERNAME for a15
col MACHINE for a30
col PROGRAM for a15
SELECT t.USED_UBLK, s.username, s.machine, s.program,s.sid
FROM V$TRANSACTION t, v$session s
where t.addr = s.taddr ;--and s.sid=1515;



select
   s.sid, 
   s.username,
   r.name "RBS name", 
   t.start_time,
   t.used_ublk "Undo blocks",
   t.used_urec "Undo recs"
from
   v$session s,
   v$transaction t,
   v$rollname r
where
   t.addr = s.taddr 
and
   r.usn = t.xidusn;