How to identify and troubleshoot deadlocks in Oracle?
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Identifying and troubleshooting deadlocks in an Oracle database involves several steps.

Here's a general approach to help you with the process:

1. Identify a Deadlock: Deadlocks can manifest as ORA-00060: deadlock detected while waiting for resource errors. The database will detect the deadlock and automatically roll back one of the transactions involved. Monitoring database alert logs and error logs can help identify deadlock occurrences.

2. Gather Deadlock Information: When a deadlock occurs, Oracle captures information about the involved sessions and resources. You can retrieve this information from the database using tools such as Enterprise Manager, AWR reports, or by querying the DBA_HIST_ACTIVE_SESS_HISTORY or V$SESSION views. Look for information like session IDs, SQL statements, and resource IDs involved in the deadlock.

    SELECT
    sid1 || ',' || serial# AS session1,
    sid2 || ',' || serial# AS session2,
    owner || '.' || object_name AS object,
    object_type,
    mode_held AS lock_held,
    mode_requested AS lock_requested
FROM
    v$lock
JOIN
    v$session ON v$lock.sid1 = v$session.sid
JOIN
    all_objects ON v$lock.id1 = object_id
WHERE
    (mode_held != 'None' OR mode_requested != 'None')
    AND v$lock.block = 1;
This query retrieves information from the v$lock, v$session, and all_objects views to identify locks involved in a deadlock. The columns selected provide details about the sessions, the locked object, and the types of locks held and requested.

When executing this query, the result will display the following information for each deadlock:
session1: The first session ID and serial number involved in the deadlock.
session2: The second session ID and serial number involved in the deadlock.
object: The name of the object (table, index, etc.) involved in the deadlock.
object_type: The type of object (table, index, etc.).
lock_held: The type of lock held by the session.
lock_requested: The type of lock requested by the session.
By analyzing the results, you can identify the sessions, objects, and lock types involved in the deadlock scenario.

3. Identify the Deadlock Cause: Analyze the SQL statements and transactional behavior of the involved sessions to determine the cause of the deadlock. Look for scenarios where multiple sessions acquire locks on resources in conflicting order or where circular dependencies exist.

4. Resolve Deadlock: Based on the cause identified, you can take specific actions to resolve the deadlock:
  • Reroute Transactions: Adjust the transaction logic to avoid conflicting lock acquisitions, such as by modifying the order in which resources are accessed or implementing row-level locking instead of table-level locking.
  • Increase Resource Allocation: If the deadlock is due to insufficient resources (e.g., memory, disk space), consider increasing the allocated resources to accommodate concurrent transactions.
  • Use Lock Timeout or Deadlock Detection Mechanisms: Configure lock timeout values or enable Oracle's built-in deadlock detection mechanisms to automatically detect and resolve deadlocks by rolling back one of the involved transactions.
  • Optimize SQL Statements: Identify SQL statements that frequently contribute to deadlocks and optimize them for better performance and efficiency.

5. Review Database Design and Application Logic:

Deadlocks can sometimes indicate issues with the database design or application logic. Evaluate the design of your database schema, transaction management practices, and application code to identify any structural or logical improvements that can help prevent future deadlocks.

Remember that the specific resolution steps may vary depending on the nature of the deadlock and your specific Oracle environment. It's recommended to consult Oracle documentation, seek assistance from Oracle experts or DBAs, or utilize Oracle's support resources for more detailed guidance on resolving deadlocks in your specific scenario.

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