Using Namespace Prefixes in Index Path Expressions
XML namespace prefixes are permitted in all valid path range index expressions, but you must define the namespace binding in your database configuration. For example, if your path expression is /ns:a/ns:b, you must configure a namespace binding for the prefix ns.
To pre-define a namespace binding, use the Path Namespaces configuration page for your database in the Admin Interface or the XQuery function admin:database-add-path-namespace.
For details, see Defining Path Range Indexes.