Find Blocking sessions

select
   (select username from v$session where sid=a.sid) blocker,
   a.sid,(select serial#  from v$session where sid=a.sid) serial#,
   ' is blocking ',
   (select username from v$session where sid=b.sid) blockee,
   b.sid
from
   v$lock a,
   v$lock b
where
   a.block = 1
and
   b.request > 0
and
   a.id1 = b.id1
and
   a.id2 = b.id2;