with t as (
    select * from (
        select 
               sum(ss.value) / 1024 / 1024 as undo_size_mb,
               s.inst_ID,
               s.sid
        from  gv$sesstat ss
          join gv$session s on s.sid = ss.sid
          join gv$statname stat on stat.statistic# = ss.statistic#
        where stat.name = 'undo change vector size'
        and s.type <> 'BACKGROUND'
        and s.username IS NOT NULL
        group by s.inst_id,s.sid, s.username,s.client_info
    )
    where undo_size_mb>0
    order by undo_size_mb desc
)
select t.*,s.* 
from gv$session s 
join t on s.sid=t.sid and t.inst_id=s.inst_id
order by 1 desc