查询undo的占用

select s.username,s.SID, u.name, r.RSSIZE, r.WRITES, r.HWMSIZE
  from v$transaction t, v$rollstat r, v$rollname u, v$session s
 where s.taddr = t.addr
   and t.xidusn = r.usn
   and r.usn = u.usn
 order by r.rssize desc;