Saturday, January 24, 2009

Table-Level Locking Hints in SQL Server

I was recently asked by a friend, how can we control the level of locking in SQL Server when executing SELECT, UPDATE, INSERT and DELETE statements.

The answer to this question is by using locking hints. Locking hints direct SQL Server to the type of locks to be used. Even though the SQL Server Query Optimizer automatically determines the best locking option when executing a statement of the above kinds, there are cases where a DBA/Database Developer might needs to explicitly control the level of locking.

The available locking hints in SQL Server are the following:

HOLDLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE

NOLOCK
Applies to: SELECT

PAGLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE

READCOMMITTED
Applies to: SELECT, UPDATE, INSERT, DELETE

READPAST
Applies to: SELECT, UPDATE, DELETE

READUNCOMMITTED
Applies to: SELECT

REPEATABLEREAD
Applies to: SELECT, UPDATE, INSERT, DELETE

ROWLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE

SERIALIZABLE
Applies to: SELECT, UPDATE, INSERT, DELETE

TABLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE

TABLOCKX
Applies to: SELECT, UPDATE, INSERT, DELETE

UPDLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE

XLOCK
Applies to: SELECT, UPDATE, INSERT, DELETE

The syntax for using locking hints is very simple. You only have to add the expression with (LOCKING_HINT) just right after you are referencing a database table name (or after a table alias) in your query.

An example of using the NOLOCK locking hint within a SELECT statement is the following:

SELECT column
FROM table with (NOLOCK)

The following MSDN Library link fully describes the above locking hints.
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Reference: The SQL Server and .NET Hub (http://www.sqlnethub.com)

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