Concepts of indexing XML data in SQL Server 2005

• SQL Server 2005 supports four different types of XML indexes.
• The primary XML index on an XML column is a clustered index on an internal table known as the node table. It cannot be used directly from the T-SQL statements. The primary XML index is a B+tree.
• The primary XML index contains one row for each node in the XML instance.
• SQL Server 2005 executes a SQL query that contains an XML data type method.
• When an SQL-with-XML query is executed against a table containing an XML data type column, the query must process every XML instance in every row. At the top level, there are two ways that such a query can be executed
• Select the rows in the base table (that is, the relational table that contains the XML data type column) that qualify first, and then process each XML instance using XQuery. This is known as top-down query processing.
• Process all XML instances using the XQuery first, and then join the rows that qualify to the base table. This is known as bottom-up query processing. 

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