The CREATE INDEX statement has been enhanced to support XML indexing. XML indexes are created on columns of type XML based on path expressions (xmlpattern). When creating an XML index, the following fields are required:
Index name: Specify the name of an XML index.
Table and column names: Specify which XML column is indexed.
XMLPATTERN: Specify the node you want to index.
Data type: Specify the SQL data type for the XML index.

For eg. CREATE INDEX xidx1 on PRODUCT(XMLDOC)
GENERATE KEY USING XMLPATTERN
‘/products/product/id’ as SQL DOUBLE

This will create a XML index on the XMLDOC column of the PRODUCT table.
The index xidx1 will have all the product id values indexed with their data type being DOUBLE.

You can check section 5.1.2 on http://www.redbooks.ibm.com/redbooks/pdfs/sg247298.pdf link for more information on this.