HOW to FIND long RUNNING TRANSACTIONS in ORACLE database?

SELECT t.start_time,
  s.sid,
  s.serial#,
  s.username,
  s.status,
  s.schemaname,
  s.osuser,
  s.process,
  s.machine,
  s.terminal,
  s.program,
  s.module,
  s.type,
  TO_CHAR(s.logon_time,'DD/MON/YY HH24:MI:SS') logon_time
FROM v$transaction t,
  v$session s
WHERE s.saddr = t.ses_addr
AND s. status = 'ACTIVE'
AND s.username IS NOT NULL
ORDER BY start_time desc;