DB2 has an index advisor (part of the DB2 Design Advisor) which helps to identify missing indexes to speed up a given workload. Unfortunately, it lacks pureXML support. The good news is that there is a tool on IBM alphaWorks, called the “IBM XML Index Advisor for DB2 for Linux, UNIX, and Windows”.

Give the tool a try and provide feedback to the team (I already did and, e.g., requested more examples).