Reducing SQL Server Locks
By : Brad McGehee
Apr 13, 2006
http://www.sql-server-performance.com/tips/reducing_locks_p1.aspx
if your users are complaining that they have to wait for their transactions to complete, you may want to find out if object locking on the server is contributing to this problem. To do this, use the SQL Server Locks Object: Average Wait Time (ms). You can use this counter to measure the average wait time of a variety of locks, including database, extent, Key, Page, RID, and table.
If you identify one or more types of locks causing transaction delays, then you will want to investigate further to see if you can identify what specific transactions are causing the locking. The Profiler is as a good tool for this detailed analysis. [7.0, 2000, 2005] Updated 9-18-2006
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Use sp_who and sp_who2 (the sp_who2 stored procedure is not documented in the SQL Server Books Online, but offers more detail than sp_who) to identify which processes may be blocking other processes. While blocking can also be identified using Enterprise Manager or Management Studio, these two commands work much faster and more efficiently than these GUI-based front-ends. [6.5, 7.0, 2000, 2005] Updated 9-18-2006
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On tables that change little, if at all, such as lookup tables, consider altering the default lock level for these tables. By default, SQL Server uses row level locking for all tables, unless the SQL Query Optimizer determines that a more appropriate locking level, such as page or table locks, is more appropriate. For most lookup tables that aren’t huge, SQL Server will automatically use row level locking. Because row locking has to be done at the row level, SQL Server needs to work harder to maintain row locks that it does for either page or table locks. Since lookup tables aren’t being changed by users, it would be more efficient to use a table lock instead of many individual row locks. How do you accomplish this?
You can override how SQL Server performs locking on a table by using the SP_INDEXOPTION command. Below is an example of code you can run to tell SQL Server to use page locking, not row locks, for a specific table:
SP_INDEXOPTION ‘table_name’, ‘AllowRowLocks’, FALSE
GO
SP_INDEXOPTION ‘table_name’, ‘AllowPageLocks’, FALSE
GO
This code turns off both row and page locking for the table, thus only table locking is available. [7.0, 2000, 2005] Updated 9-18-2006
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Keep all Transact-SQL transactions as short as possible. This helps to reduce the number of locks (of all types), helping to speed up the overall performance of your SQL Server applications. If practical, you may want to break down long transactions into groups of smaller transactions. In addition, only include those Transact-SQL commands within a transaction that are necessary for the transaction. Leave all other code outside of the transaction. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
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An often overlooked cause of locking is an I/O bottleneck. Whenever your server experiences an I/O bottleneck, the longer it takes user’s transactions to complete. And the longer they take to complete, the longer locks must be held, which can lead to other transactions having to wait for previous locks to be released.
If your server is experiencing excessive locking problems, be sure to check if you are also running into an I/O bottleneck. If you do find that you have an I/O bottleneck, then resolving it will help to resolve your locking problem, speeding up the performance of your server. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
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To help reduce the amount of time tables are locked, which hurts concurrency and performance, avoid interleaving reads and database changes within the same transaction. Instead, try to do all your reads first, then perform all of the database changes (UPDATES, INSERTS, DELETES) near the end of the transaction. This helps to minimize the amount of time that exclusive locks are held. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
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Any conditional logic, variable assignment, and other related preliminary setup should be done outside of transactions, not inside them. Don’t ever pause a transaction to wait for user input. User input should always be done outside of a transaction. Otherwise, you will be contributing to locking, hurting SQL Server’s overall performance. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
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Encapsulate all transactions within stored procedures, including both the BEGIN TRANSACTION and COMMIT TRANSACTION statements in the procedure. This provides two benefits that help to reduce blocking locks.
First, it limits the client application and SQL Server to communications before and after the transaction, thus forcing any messages between the client and the server to occur at a time other than when the transaction is running (reducing transaction time).
Second, It prevents the user from leaving an open transaction (holding locks open) because the stored procedure forces any transactions that it starts to complete or abort. [6.5, 7.0, 2000, 2005] Updated 8-21-2005
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If you have a client application that needs to “check-out” data for awhile, then perhaps update it later, or maybe not, you don’t want the records locked during the entire time the record is being viewed. Assuming “viewing” the data is much more common that “updating” the data, then one way to handle this particular circumstance is to have the application select the record (not using UPDATE, which will put a share lock on the record) and send it to the client.
If the user just “views” the record and never updates it, then nothing has to be done. But if the user decides to update the record, then the application can perform an UPDATE by adding a WHERE clause that checks to see whether the values in the current data are the same as those that were retrieved.
Similarly, you can check a timestamp column in the record, if it exists. If the data is the same, then the UPDATE can be made. If the record has changed, then the application must include code to notify the user so he or she can decide how to proceed. While this requires extra coding, it reduces locking and can increase overall application performance. [6.5, 7.0, 2000, 2005] Updated 10-16-2005