FIX: A blocking issue occurs when you update rows in a table in SQL Server 2005

When you update rows in a table in SQL Server 2005, a blocking issue may occur. You run the sp_lock stored procedure to show the current locks. However, you notice that the blocked session IDs (SPIDs) are waiting to obtain an update lock for the same row that has already been granted an exclusive lock by another update statement. This behavior occurs even if the update statement is configured to update a different row.

This problem may occur in one of the following scenarios:

• In SQL Server 2005, you perform positioned updates in a cursor by using the SQL Native Client ODBC driver in an application. You use the SQLSetPos method and use the SQL_UPDATE option to perform the positioned updates. You set the statement attribute SQL_ATTR_CONCURRENCY to SQL_CONCUR_LOCK to update different rows of the same table in different connections at the same time.
• You use the sp_cursor stored procedure to update different rows of the same table in different connections at the same time.

This problem occurs because the positioned update in a transaction performs a table scan on all involved tables. This behavior causes many update locks to be generated on many rows in the table. Additionally, SQL Server tries to add an update lock on the row that has already been granted an exclusive lock by another transaction. Therefore, a blocking issue occurs.

The fix for this issue was first released in Cumulative Update 6. For more information about how to obtain this cumulative update package for SQL Server 2005 Service Pack 2, click the following article number to view the article in the Microsoft Knowledge Base:Note Because the builds are cumulative, each new fix release contains all the hotfixes and all the security fixes that were included with the previous SQL Server 2005 fix release. Microsoft recommends that you consider applying the most recent fix release that contains this hotfix. For more information, click the following article number to view the article in the Microsoft Knowledge Base:Microsoft SQL Server 2005 hotfixes are created for specific SQL Server service packs. You must apply a SQL Server 2005 Service Pack 2 hotfix to an installation of SQL Server 2005 Service Pack 2. By default, any hotfix that is provided in a SQL Server service pack is included in the next SQL Server service pack.

To work around this problem, use one of the following methods:

• Create an index for the table that prevents a table scan.
• If you experience the first scenario that is described in the "Symptoms" section, set the statement attribute SQL_ATTR_CONCURRENCY to SQL_CONCUR_VALUES instead of SQL_CONCUR_LOCK.
• If you experience the second scenario that is described in the "Symptoms" section, use the OPEN statement to open a Transact-SQL server cursor instead of using the sp_cursor stored procedure.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

To resolve the problem, enable trace flag 4117 after you apply this hotfix.For more information about what files are changed, and for information about any prerequisites to apply the cumulative update package that contains the hotfix that is described in this Microsoft Knowledge Base article, click the following article number to view the article in the Microsoft Knowledge Base:

For more information about the list of builds that are available after SQL Server Service Pack 2, click the following article number to view the article in the Microsoft Knowledge Base: For more information about the Incremental Servicing Model for SQL Server, click the following article number to view the article in the Microsoft Knowledge Base:For more information about how to obtain SQL Server 2005 Service Pack 2, click the following article number to view the article in the Microsoft Knowledge Base: For more information about the new features and the improvements in SQL Server 2005 Service Pack 2, visit the following Microsoft Web site: For more information about the naming schema for SQL Server updates, click the following article number to view the article in the Microsoft Knowledge Base:For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base: