How to Check Blocking Sessions in Oracle: A Deep Dive

So, you’ve got performance woes in your Oracle database? Chances are, you’re dealing with blocking sessions. Understanding and identifying these bottlenecks is crucial for maintaining a responsive and efficient database environment. Let’s cut straight to the chase:

The most effective way to check blocking sessions in Oracle is by querying the data dictionary views V$LOCK, V$SESSION, and V$SQL. Join these views to pinpoint which sessions are holding locks and which sessions are waiting on those locks. The BLOCKING_SESSION column in V$SESSION is your golden ticket, directly indicating the session ID that is doing the blocking.

Unveiling the Blocking Landscape: Queries That Matter

While the above explanation gives you the core concept, you need practical queries to get the job done. Here are a few to start with:

Simple Blocking Session Identification

This query reveals the sid (Session ID) and serial# of the blocking sessions:

SELECT blocking_session,        sid,        serial# FROM   v$session WHERE  blocking_session IS NOT NULL; 

This provides a basic list of sessions causing the blockages. However, for detailed analysis, we need more information.

Detailed Blocking Session Information

This query provides comprehensive information about both the blocking and blocked sessions, including the SQL they are executing:

SELECT     a.sid AS blocking_session,     b.sid AS blocked_session,     a.serial# AS blocking_serial,     b.serial# AS blocked_serial,     a.username AS blocking_user,     b.username AS blocked_user,     a.osuser AS blocking_osuser,     b.osuser AS blocked_osuser,     a.program AS blocking_program,     b.program AS blocked_program,     a.machine AS blocking_machine,     b.machine AS blocked_machine,     sql_blocking.sql_text AS blocking_sql,     sql_blocked.sql_text AS blocked_sql FROM     v$session a,     v$session b,     v$sql sql_blocking,     v$sql sql_blocked WHERE     a.sid = b.blocking_session     AND a.sql_id = sql_blocking.sql_id     AND b.sql_id = sql_blocked.sql_id; 

This query leverages V$SESSION and V$SQL to provide a clear picture of the SQL statements involved in the blocking scenario. It highlights the users, programs, and machines associated with each session, enabling targeted troubleshooting.

Using DBMS_MONITOR.SESSION_TRACE_ENABLE for In-Depth Analysis

For complex blocking scenarios, you can enable SQL tracing on the blocking and blocked sessions using the DBMS_MONITOR package:

1. Identify the SIDs: Use the above queries to determine the sid and serial# of the blocking and blocked sessions.

2. Enable Tracing: Execute the following procedures (replace sid and serial# with the actual values):

   EXEC DBMS_MONITOR.SESSION_TRACE_ENABLE(session_id => &blocking_sid, serial_num => &blocking_serial, binds => TRUE, waits => TRUE, plan_line_count => 'ALL_EXECUTIONS'); EXEC DBMS_MONITOR.SESSION_TRACE_ENABLE(session_id => &blocked_sid, serial_num => &blocked_serial, binds => TRUE, waits => TRUE, plan_line_count => 'ALL_EXECUTIONS'); 

3. Reproduce the Issue: Allow the blocking situation to occur again.

4. Disable Tracing: After the issue is reproduced, disable the traces:

   EXEC DBMS_MONITOR.SESSION_TRACE_DISABLE(session_id => &blocking_sid, serial_num => &blocking_serial); EXEC DBMS_MONITOR.SESSION_TRACE_DISABLE(session_id => &blocked_sid, serial_num => &blocked_serial); 

5. Analyze the Trace Files: Use TKPROF or SQL Developer to analyze the generated trace files. These files contain detailed information about the execution of the SQL statements, including wait events and bind variables, giving insight into the root cause of the blocking.

Addressing Application Code

It’s good to note that issues are very often caused by application code not optimized for concurrent use. Review the application code to see what transaction is holding a long lock.

Frequently Asked Questions (FAQs)

Here are some common questions I often get asked about blocking sessions in Oracle.

1. What is a blocking session in Oracle?

A blocking session, sometimes called a blocker, is a database session that holds a lock on a resource (e.g., a table, a row) preventing other sessions (blocked sessions) from accessing or modifying that resource. This leads to performance degradation as blocked sessions must wait until the blocking session releases the lock.

2. What are common causes of blocking sessions?

Blocking can arise from various scenarios, including:

• Long-running transactions: Transactions that hold locks for extended periods.
• Lack of index usage: Full table scans can exacerbate locking issues.
• Application design flaws: Inefficient data access patterns.
• Deadlocks: Circular dependencies where two or more sessions are waiting for each other to release resources.
• DML operations: Uncommitted INSERT, UPDATE, or DELETE statements.

3. How can I identify the objects being locked?

The V$LOCKED_OBJECT view provides information about the objects (tables, indexes, etc.) that are currently locked. Joining this view with V$SESSION and V$LOCK helps correlate the locked object with the blocking session.

SELECT     lo.object_id,     o.object_name,     s.sid,     s.serial#,     l.type FROM     v$locked_object lo,     dba_objects o,     v$session s,     v$lock l WHERE     lo.object_id = o.object_id     AND lo.session_id = s.sid     AND l.sid = s.sid     AND l.id1 = o.object_id; 

4. What are common wait events associated with blocking sessions?

Common wait events indicating blocking include:

• enq: TX - row lock contention: Indicates a session is waiting for a row lock held by another session.
• enq: TM - contention: Indicates a session is waiting for a table lock.
• library cache lock: Occurs when a session is waiting for a lock on a library cache object (e.g., stored procedure).

5. How can I kill a blocking session?

Killing a blocking session should be a last resort, as it can lead to data inconsistency or application errors. However, if necessary, you can use the ALTER SYSTEM KILL SESSION command:

ALTER SYSTEM KILL SESSION 'sid,serial#'; 

Replace sid and serial# with the actual session ID and serial number. Important: Consider the implications before killing a session, and communicate with application owners if possible.

6. What is the difference between V$LOCK and V$LOCKED_OBJECT?

• V$LOCK: Provides information about locks held by sessions, including the lock type, mode, and session ID.
• V$LOCKED_OBJECT: Focuses on locked objects, linking objects to the sessions holding locks on them.

7. How can I prevent blocking sessions in my application?

Preventing blocking involves a multi-faceted approach:

• Optimize SQL queries: Ensure efficient query execution plans with proper indexing.
• Keep transactions short: Reduce the duration for which locks are held.
• Use optimistic locking: Avoid pessimistic locking (exclusive locks) when possible.
• Proper error handling: Ensure transactions are rolled back in case of errors.
• Avoid lock escalation: Lock escalation occurs when Oracle automatically escalates row-level locks to table-level locks. Review table DDL to avoid LOCK TABLE statements.

8. What is the role of transaction isolation levels in blocking?

Transaction isolation levels determine the degree to which transactions are isolated from each other. Higher isolation levels (e.g., serializable) provide stronger isolation but can increase the likelihood of blocking. Lower isolation levels (e.g., read committed) reduce blocking but may expose the application to phenomena like non-repeatable reads. Choosing the appropriate isolation level is a trade-off between data consistency and concurrency.

9. How can I monitor blocking sessions over time?

To track blocking sessions historically, you can create a table to store data collected from the queries outlined above. Schedule a job using DBMS_SCHEDULER to periodically (e.g., every minute or five minutes) execute the queries and insert the results into the historical table. This allows you to analyze trends and identify recurring blocking patterns.

10. What is a deadlock and how does it differ from a regular blocking situation?

A deadlock is a specific type of blocking scenario where two or more sessions are waiting for each other to release resources, creating a circular dependency. In a regular blocking situation, one session is blocking another, but there’s no circular dependency. Oracle automatically detects and resolves deadlocks by rolling back one of the involved transactions.

11. How can I identify deadlocks in Oracle?

Oracle provides a wait event called deadlock. When a deadlock is detected, one of the sessions involved will experience this wait event, and Oracle will automatically roll back that session’s transaction to break the deadlock. You can identify deadlocks by monitoring for this wait event in V$SESSION_WAIT.

12. Are there any third-party tools that can help with blocking session analysis?

Yes, several third-party tools offer advanced monitoring and analysis capabilities for Oracle databases, including features specifically designed to identify and diagnose blocking sessions. Examples include Oracle Enterprise Manager (OEM), Toad for Oracle, and DBVisualizer. These tools often provide graphical interfaces and automated analysis features that can simplify the troubleshooting process.

By mastering these techniques and understanding the underlying concepts, you’ll be well-equipped to tackle blocking sessions and keep your Oracle database running smoothly. Good luck!