XML data and query performance
XML data is more expensive to process, and might affect the performance of most SQL statements. Each row of an XML column contains an XML document, requires more processing, and requires more space in Db2.
When
you use an XQuery or XPath expression to search or extract the XML
data, you can lessen the performance impact by avoiding the descendant
or descendant-or-self axis in the expression. The XMLEXISTS predicate
is always stage 2. However, you can use the XML index to reduce the
number of rows, that is, the number of XML documents, that must be
searched at the second stage.
Creating and maintaining XML indexes might be more costly than
non-XML indexes. Write queries to use non-XML indexes to filter as
many rows as possible before the second stage.