enq: TM - contention Wait Event , Cause, Related SQL queries, Possible Resolution

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The "enq: TM - contention" wait event in Oracle indicates contention for a table-level lock. This contention occurs when multiple sessions are attempting to acquire conflicting table-level locks concurrently, leading to performance issues as sessions have to wait for the locks to be released before they can proceed with their operations. 

 While there are similarities in the resolution strategies for "enq: TX - index contention" and "enq: TM - contention," they do involve different types of contention (index-level vs. table-level). To clarify, here are specific resolution strategies for "enq: TM - contention" related to table-level lock contention: 

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

The "enq: TM - contention" wait event specifically indicates contention for table-level locks. 

This wait event usually occurs when multiple sessions are trying to perform operations on the same table that require exclusive locks, such as DDL operations (e.g., table truncation, dropping), table-level locks, or transactions that request conflicting locks.

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Queries: Identifying Sessions with "enq: TM - contention" Wait Event:

 

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

Identifying Tables and Locking Sessions:

 

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

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

Resolution: 

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

 DDL Operations: Limit the use of DDL operations during peak usage times to reduce contention for table locks. Schedule maintenance activities during off-peak hours. 

 Lock Timeout: Implement lock timeouts to prevent long waits for table-level 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, perform bulk inserts or updates in a single transaction. 

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

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

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

 Database Design: Review your database design to minimize contention. This could involve denormalization, partitioning, or splitting data into smaller tables. 

 Adjust Locking Strategy: Review and adjust the locking strategy used by your application. Consider using different isolation levels to control locking behavior. 

 Upgrade or Patch: Sometimes, Oracle releases patches or updates that address locking and contention issues. Ensure you are using a supported version and apply relevant patches. 

 Resolving "enq: TM - contention" wait events often requires 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.

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