Who is using UNDO tablespace??

select s.sid, 
       s.username,
       sum(ss.value) / 1024 / 1024 as undo_size_mb
from  v$sesstat ss
  join v$session s on s.sid = ss.sid
  join v$statname stat on stat.statistic# = ss.statistic#
where stat.name = 'undo change vector size'
and s.type <> 'BACKGROUND'
and s.username IS NOT NULL
group by s.sid, s.username order by undo_size_mb desc