enq: TX - row lock contention Wait Event , Cause, Related SQL queries, Possible Resolution

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The "enq: TX - row lock contention" wait event in Oracle occurs when multiple database sessions are contending for locks on the same row in a table. This contention can lead to performance issues, as sessions have to wait for the locks to be released before they can proceed with their operations.

 Explanation: In Oracle, locks are used to control access to data to maintain data consistency and integrity. 

The "enq: TX - row lock contention" wait event specifically indicates contention for row-level locks in a table. 

This wait event typically occurs when multiple sessions are trying to update or modify the same row concurrently, and one session is blocking others from accessing or modifying that row.

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Identifying Sessions with "enq: TX - row lock contention" Wait Event:

SELECT SID, SERIAL#, EVENT, WAIT_TIME, SECONDS_IN_WAIT
FROM V$SESSION
WHERE EVENT = 'enq: TX - row lock contention';

Identifying Tables and Locking Sessions:

SELECT OBJECT_NAME, SESSION_ID, ORACLE_USERNAME
FROM DBA_BLOCKERS; -- Sessions holding locks

SELECT OBJECT_NAME, SESSION_ID, ORACLE_USERNAME
FROM DBA_WAITERS; -- Sessions waiting for locks

Resolution: 

 Optimize SQL Statements: Review and optimize SQL statements that are causing the contention. Ensure that queries are efficient and minimize the need for row-level locks. 

 Use Indexing: Properly index tables to reduce the likelihood of row-level lock contention. Indexes can help with data access and reduce the need for full table scans and locks. Transaction 

Isolation Levels: Consider adjusting the transaction isolation level (e.g., using READ COMMITTED) to control the level of locking and contention. 

 Lock Timeout: Implement lock timeouts to prevent long waits for locks. Sessions can be configured to wait only for a certain period before giving up and retrying. 

 Batch Processing: If possible, batch similar operations together to reduce the likelihood of contention. For example, update multiple rows at once rather than one at a time. 

 Row-Level Locks: Consider using row-level locking mechanisms provided by Oracle to reduce contention. Row-level locking allows more concurrent access to different rows in the same table. 

 Concurrency Control: Implement application-level concurrency control mechanisms to coordinate access to shared data. This could involve using semaphores or other synchronization techniques. 

 Monitor Locking Activity: Continuously monitor the database for locking and contention issues using Oracle's performance monitoring tools. 

 Database Design: In some cases, you might need to reconsider your database design to minimize contention. This could involve denormalization or partitioning of data. 

 Consider Partitioning: If applicable, consider using table partitioning to distribute data across multiple partitions, reducing contention on a single table. 

 Resolving "enq: TX - row lock contention" wait events often involves a combination of optimizing SQL statements, configuring the appropriate isolation levels, and improving database and application design. It's important to analyze the specific scenarios leading to contention and apply the most suitable resolution strategies for your workload. 

 You can find the sql statements that are waiting on enq: TX – row lock contention wait event using the following script.

select v.sql_text,v.sql_fulltext,sub.* from gv$sql v,
(select sample_time,s.sql_id sql_id, session_state, blocking_session,
owner||'.'||object_name||':'||nvl(subobject_name,'-') obj_name,s.program,s.module,s.machine
from dba_hist_active_sess_history s, dba_objects o
where sample_time between
to_date('03/09/2022  10:40:06','DD/MM/YYYY HH24:MI:SS')
and
to_date('03/09/2022 16:11:05','DD/MM/YYYY HH24:MI:SS')
and event = 'enq: TX - row lock contention'
and o.data_object_id = s.current_obj#
order by 1 desc) sub where sub.sql_id=v.sql_id;

You can find historical blocking session and lock with below scripts. Change date time column which is specified with highliged color.

select v.sql_text,v.sql_fulltext,sub.* from v$sql v,
(select sample_time,s.sql_id sql_id, session_state, blocking_session,
owner||'.'||object_name||':'||nvl(subobject_name,'-') obj_name,s.program,s.module,s.machine
from dba_hist_active_sess_history s, dba_objects o
where sample_time between
to_date('25/03/2023 11:40:02','DD/MM/YYYY HH24:MI:SS')
and
to_date('25/03/2023 13:40:02','DD/MM/YYYY HH24:MI:SS')
and event = 'enq: TX - row lock contention'
and o.data_object_id = s.current_obj#
order by 1 desc) sub where sub.sql_id=v.sql_id;

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