Assume that you run a Rebuild Index task (Maintenance Plan) in Microsoft SQL Server 2012 or SQL Server 2014 and that the index contains one or more XML, varchar(max), nvarchar(max) or varbinary(max) columns. In this situation, the task is performed as an offline operationeven if you’veselected the Keep index online while reindexing checkbox in Maintenance Plan Wizard
Note Microsoft SQL Server 2012 introduces a new feature that allows indexes containing XML, varchar(max), nvarchar(max), and varbinary(max) columns to be created, rebuilt, and dropped as an online operation.
This issue occurs because Maintenance Plan does not take account of the new feature.
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To work around this issue, follow these steps:
Open Maintenance Plan in Management tree view node in Object Explorer in SQL Server Management Studio.
Right-click Rebuild Index Task and then click Edit.
Click View T-SQL and then copy the Transact-SQL statement to a plain text file.
Modify the Transact-SQL statement to use online operation, ONLINE=OFF to ONLINE=ON.
Run the Transact-SQL statement or create a new SQL Server Agent T-SQL job for the Transact-SQL statement and then schedule it.
For more information about this new feature in SQL Server 2012, see online index create, rebuild, and drop.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.