Refer below on MOS
Troubleshooting 'enq: TX - index contention' Waits in a RAC Environment. [ID 873243.1]
http://www.confio.com/English/Tips/Index_Block_Split.php
# Show all sessions waiting for any lock:
select event,p1,p2,p3 from v$session_wait where wait_time=0 and event='enqueue';
# From 10g a different more descriptive event name exists for the more frequent enqueues and you can query the TX wait event as follows:
select sid,p1raw, p2, p3 from v$session_wait
where wait_time=0 and event='enq: TX - row lock contention';
# Show sessions waiting for a TX lock:
select * from v$lock where type='TX' and request>0;
# Show sessions holding a TX lock:
select * from v$lock where type='TX' and lmode>0;
