DBMS_SESSION.UNIQUE_SESSION_ID

select
   *
from
   gv$session
where
   sid     = to_number(substr(dbms_session.unique_session_id, 1, 4), 'XXXX') and 
   serial# = to_number(substr(dbms_session.unique_session_id, 5, 4), 'XXXX') and
   inst_id = to_number(substr(dbms_session.unique_session_id, 9, 4), 'XXXX');