Deadlocks Advisor

Using the Deadlock Advisor in SQL Server to Diagnose and Fix Deadlocks

The Deadlock Advisor in SQL Server is a tool that helps analyze deadlocks by displaying detailed information about the processes involved, the affected objects, and the blocking behavior. This allows DBAs and developers to diagnose the cause of the deadlock and apply fixes to prevent future occurrences.

How the Deadlock Advisor Helps Analyze Deadlocks

From the image, the Deadlock Advisor provides key details, including:

  1. Timestamp of the Deadlock
    • The exact date and time when the deadlock occurred.
  2. Database and Object Name
    • Identifies which database and table were involved in the deadlock.
    • In this case, the database is DeadlockTestDB, and the table involved is TableA.
  3. Winner and Victim Transactions
    • SQL Server automatically chooses a victim to terminate in order to resolve the deadlock.
    • The winner transaction successfully completes, while the victim transaction is rolled back.
    • This allows you to see which query was terminated and which one was able to complete.
  4. Blocking Details
    • The hostname, session ID, blocking mode, isolation level, and other details about the transaction holding the lock.
    • The blocking session ID (56) indicates which process was waiting on resources.
  5. SQL Statements Causing the Deadlock
    • The exact SQL code that caused the deadlock is shown for both the winner and the victim.
    • In this case, the deadlock is caused by two transactions updating TableA and TableB in different orders, leading to circular blocking.

How to Use Deadlock Advisor to Fix Deadlocks

  1. Identify the Conflicting Queries
    • Look at the queries in both transactions and see how they interact.
    • In the provided image, one transaction updates TableA first, while the other updates TableB first, leading to deadlock.
  2. Apply Consistent Lock Ordering
    • Ensure that all transactions access resources in the same order to prevent circular blocking.
    • In this case, both transactions should update TableA first, then TableB, or vice versa.
  3. Reduce Transaction Lock Time
    • The WAITFOR DELAY statement increases the risk of deadlocks by keeping locks open longer.
    • Avoid unnecessary delays inside transactions and commit as quickly as possible.
  4. Use Deadlock-Free Isolation Levels
    • Consider using Snapshot Isolation (RCSI) to minimize locking conflicts.
    • Example: ALTER DATABASE DeadlockTestDB SET READ_COMMITTED_SNAPSHOT ON;
    • This reduces blocking by allowing reads without locks.
  5. Use Indexing to Reduce Locking
    • Ensure that tables have proper indexing to speed up queries and avoid full table scans, which can hold locks longer.
  6. Implement Deadlock Retry Logic
    • If deadlocks occur frequently, implement retry logic in the application to retry a failed transaction after a short delay.
    • Example: BEGIN TRY -- Your transactional logic END TRY BEGIN CATCH IF ERROR_NUMBER() = 1205 -- Deadlock error BEGIN WAITFOR DELAY '00:00:03'; -- Wait 3 seconds and retry -- Retry logic here END END CATCH

Conclusion

The Deadlock Advisor is a powerful tool for diagnosing deadlocks by providing detailed insights into conflicting transactions, resource locks, and blocking behavior. By analyzing the queries and session details, you can take corrective actions such as consistent locking order, reducing transaction times, using better isolation levels, and improving indexing to prevent future deadlocks.