select 
      (select username from v$session where sid=a.sid) blocker,
       a.sid,
      ' is blocking ',
       (select username from v$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
/
