I am investigating row lock contention issue in one of our PROD systems.
i have been checking DBA_HIST_ACTIVE_SESS_HISTORY and GV$ACTIVE_SESSION_HISTORY, and observed a column BLOCKING_SESSION.
does BLOCKiNG_SESSION displays the session holding the lock or it displays the sessions waiting due to lock?
because, the description of BLOCKING_SESSION column on Oracle Docs is as follows.
Session identifier of the blocking session. Populated only when the session was waiting for enqueues or a "buffer busy" wait.
The second statement "Populated only when the session was waiting for enqueues" is confusing me.
any expert opinions on it ?
how to find out, which session is holding the locks ?
I have written a sample query to find out, but i want to know the list of sessions holding the locks.
any tweaks for my query ?
any recommendations will help me a lot 
The query i used is below.
SELECT DISTINCT A.SQL_ID,TO_CHAR(A.SAMPLE_TIME,'YYYY-MM-DD HH24:MI:SS') SAMPLE_TIME,A.BLOCKING_SESSION,D.OBJECT_NAME,S.SQL_TEXT
FROM DBA_HIST_ACTIVE_SESS_HISTORY A, GV$SQL S, DBA_OBJECTS D
WHERE A.SQL_ID=S.SQL_ID
AND BLOCKING_SESSION IS NOT NULL
AND A.USER_ID <> 0
AND A.CURRENT_OBJ# = D.OBJECT_ID
AND A.SAMPLE_TIME BETWEEN
TO_TIMESTAMP('12.05.2015 00:00:00', 'dd.mm.yyyy hh24:mi:ss') AND
TO_TIMESTAMP('12.05.2015 23:59:59', 'dd.mm.yyyy hh24:mi:ss')
AND A.EVENT = 'enq: TX - row lock contention'
ORDER BY
SAMPLE_TIME DESC