Published on

July 7, 2009

Understanding Deadlocks in SQL Server

Deadlocks are a common issue in SQL Server that can cause transactions to fail and impact the performance of your database. In this article, we will explore the steps taken to identify and resolve deadlocks in SQL Server.

Step 1: Enable Deadlock Trace Flag

The first step in resolving deadlocks is to enable the deadlock trace flag on the SQL Server. This can be done using the following command:

DBCC traceon(1204,-1)

By enabling this trace flag, deadlock details will be captured in the SQL Server error log.

Step 2: Gather Deadlock Details

Once the trace flag is enabled, you can gather the deadlock details from the SQL error log using the command ‘xp_readerrorlog’.

xp_readerrorlog

This will provide you with information about the deadlocks that have occurred, including the involved processes and statements.

Step 3: Identify Deadlock Statements

From the captured deadlock details, you can identify the statements that are causing the deadlocks. In the example, the deadlocks were occurring due to UPDATE statements in a stored procedure.

Step 4: Review Stored Procedure Code

Next, you should review the code of the stored procedure that is causing the deadlocks. In the example, the stored procedure was calling multiple other procedures, making it difficult to identify the deadlock statements.

Step 5: Use Page Numbers to Identify Deadlock Statements

If it is difficult to identify the deadlock statements directly from the stored procedure code, you can use the page numbers from the deadlock details to pinpoint the statements. By using the undocumented DBCC command ‘DBCC Page’, you can retrieve the table name associated with the deadlock.

DBCC page(database_id, file_id, page_id, 0)

This command will provide you with the object name associated with the page, helping you identify the deadlock statements.

Step 6: Review Dependent Stored Procedures

Once you have identified the deadlock statements, you should review the code of the dependent stored procedures on the table. In the example, an update procedure was found to be causing the deadlocks.

Step 7: Check Indexes

It is important to check the indexes on the table to ensure optimal performance. In the example, it was found that the table did not have an index on the ID column, causing the update to perform a table scan and resulting in deadlocks.

Step 8: Create Indexes

To resolve the deadlocks, you can create the necessary indexes on the table. In the example, a clustered index was created on the ID column using the following command:

CREATE CLUSTERED INDEX IX_Clustered_index_column ON dbo.TABLE(ID) ON [PRIMARY]

After creating the index, the deadlocks were resolved as the intent exclusive lock was no longer requested on all the pages of the update statement.

Conclusion

Deadlocks can occur in SQL Server when multiple transactions are trying to access the same resources simultaneously. By following the steps outlined in this article, you can identify and resolve deadlocks in your SQL Server database. It is important to review your stored procedure code, check indexes, and create the necessary indexes to optimize performance and prevent deadlocks.

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