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enq: TX - Row Lock Contention Error

Oracle Database Tips by Burleson Consulting

May 27, 2015

Question:  I am getting the following wait event:

enq: TX - row lock contention

I do not understand the reason for this event.

How do I diagnose the "enq: TX - row lock contention" event?

Answer:  The "enq: TX - row lock contention" event often indicates an application level locking problem. The TX enqueue is the transaction enqueue (a.k.a. enq: TX - contention) and can also be related to buffer busy waits, in conditions where multiple transaction attempt to update the same data blocks. TX enqueue is issued when a transaction makes its first change, and released when the transaction performs a COMMIT or ROLLBACK.

The wait event "enq: TX - row lock contention" corresponds to several situations of TX enqueue. Waits for TX in mode 6 occur when a session is waiting for a row level lock that is already held by another session. This occurs when one application is updating or deleting a row that another session is also trying to update or delete. This will generate a wait event "enq: TX - row lock contention". To solve this particular instance, the session holding the lock must perform a COMMIT or ROLLBACK.

In mode 4, a TX wait can occur if there is a potential duplicate in a unique index. When two sessions try to insert the same key value the second session must wait to see if an ORA-001 should be raised. This can cause the "enq: TX - row lock contention" wait event. This wait event can be handled by having the session holding the lock perform a COMMIT or ROLLBACK.

The wait event "enq: TX - row lock contention" can also occur in mode 4 when a session is waiting on a shared bitmap index fragment. Bitmap indexes index key values and a range of ROWIDs. Each entry in a bitmap index can cover many rows in the actual table. If two sessions want to update rows covered by the same index fragment, then the second session must wait for the first session to perform either a COMMIT or a ROLLBACK by waiting for the TX lock in mode 4, otherwise it will generate the "enq: TX - row lock contention" wait event.

To troubleshoot the wait event "enq: TX - row lock contention", use the following SQL:

 For which SQL is currently waiting on:

select
    sid,
    sql_text
from
    v$session s,
    v$sql q
where
    sid in
    (select
       sid
    from
       v$session
   where
       state in ('WAITING')
   and
       wait_class != 'Idle'
   and
       event='enq: TX - row lock contention'
   and
      (q.sql_id = s.sql_id or q.sql_id = s.prev_sql_id));

The blocking session is:

select
    blocking_session,
    sid,    serial#,
    wait_class,
    seconds_in_wait
from
    v$session
where
    blocking_session is not NULL
order by
    blocking_session;

 
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