SQL Server Execution Plans - Red Gate Software

Chapter 7: Special Datatypes and Execution Plans
As you can see, it's easy to bring XML data into the database for use within your queries,
or for inclusion within your database. As discussed previously, OPENXML is a useful
tool for importing the semi-structured data within the XML documents into the wellmaintained
relational database structure. It can also allow you to pass in data for other
uses. For example, you can pass in a list of variables to be used as a join in a SELECT
statement. The main point to take away is that once the OPENXML has been formatted,
you get to use it as if it were just another table within your queries.
One caveat worth mentioning is that parsing XML uses a lot of memory. You should
plan on opening the XML, getting the data out, and then closing and de-allocating the
XML parser as soon as possible. This will reduce the amount of time that the memory is
allocated within your system.
XQuery
Along with the introduction of the XML data type in SQL Server 2005, came the introduction
of XQuery as a method for querying XML data. Effectively, the inclusion of
XQuery means a completely new query language to learn in addition to T-SQL. The XML
data type is the mechanism used to provide the XQuery functionality through the SQL
Server system. When you want to query from the XML data type, there are five basic
methods, each of which is reflected in execution plans in different ways.
• .query() – Used to query the XML data type and return the XML data type.
• .value() – Used to query the XML data type and return a non-XML scalar value.
• .nodes() – A method for pivoting XML data into rows.
• .exist() – Queries the XML data type and returns a Bool to indicate whether or not
the result set is empty, just like the EXISTS keyword in T-SQL.
• .modify() – A method for inserting, updating and deleting XML snippets within the
XML dataset.
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