Query to check blocking sessions in Oracle database

 Query to check blocking sessions in Oracle database from v$lock:

SELECT DECODE(request,0,'Holder: ','Waiter: ') || sid sess,
  id1,
  id2,
  lmode,
  request,
  type
FROM v$lock
WHERE (id1, id2, type) IN
  (SELECT id1, id2, type FROM v$lock WHERE request > 0
  )
ORDER BY id1,
  request;