Monday, March 21, 2011

Using the NOLOCK Table Hint

Each time you write and execute a T-SQL Query in SQL Server, the Database Engine selects the most optimized query plan for executing the query.

The SQL Server Database Engine component that undertakes this task is the Query Optimizer. Given the T-SQL query itself as well as the underlying database(s) design, the Query Optimizer selects the best execution plan for the query and then it executes the query.

Now, in SQL Server you can make use of several query hints, such as locking hints. Though, as SQL Server Query Optimizer typically selects the best execution plan for a query, it is not recommended for inexperienced developers and administrators to make use of them.

Nevertheless, today I will talk about a special table hint, that is the NOLOCK hint. This hint is equivalent to the READUNCOMMITTED hint.

A typical example of using the NOLOCK hint is the following:


SELECT *
FROM TABLE_NAME WITH (NOLOCK)
GO

Similar examples:

SELECT *
FROM TABLE_NAME A WITH (NOLOCK)
GO

SELECT *
FROM TABLE_NAME (NOLOCK)
GO

SELECT *
FROM TABLE_NAME A (NOLOCK)
GO

For environments where many concurrent queries are executed against the same database objects (i.e. Web Applications), the NOLOCK hint can become quite handy. By using this hint, the transaction isolation level for the SELECT statement is READ UNCOMMITTED. Of course, this means that the query may see inconsistent data, that is data not yet committed, etc. So, if this is not a problem for you, it might solve many concurrency issues.

Though note that in general is not a good idea to apply the above practice as a rule.

I hope you found this post useful!

Until next time!
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Artemakis Artemiou [MVP]

Author & Editor

Artemakis Artemiou is a Senior SQL Server Architect, Software Developer and Microsoft Data Platform MVP. He is also an author, regular blogger, president of Cyprus.NET User Group and CY Country leader of INETA-EU. He is also the creator of DBA Security Advisor and In-Memory OLTP Simulator. Artemakis is a frequent guest author of worldwide well-respected online journals where he writes articles focusing on many SQL Server topics.

Reference: The SQL Server and .NET Hub (http://www.sqlnethub.com)

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