In the SQL Server XML model, whenever you query or manipulate XML data, the data is first converted to a relational format in a process known as shredding. This process can be time consuming when manipulating large XML documents or when querying large numbers of xml data type instances. SQL Server 2008 supports indexing of xml data type columns. Indexing xml columns helps the SQL optimizer significantly improve query performance on XML data stored in the database. The performance is improved by building an index of your XML data by converting it to a relational format, a process known as preshredding. The XML index preshredding process eliminates the shredding step during a query or XML data manipulation, resulting in much faster and less resource-intensive XML query operations. New DML statements have been added to T-SQL to make XML index management relatively easy.