1. A method comprising the computer-implemented steps of: detecting that a portion of a plan to service a request for data will cause a first execution unit that will perform said portion to generate XML data for use by a second execution unit; generating information to send to said first execution unit to cause said first execution unit to perform said portion of said plan; and annotating said information with an annotation that causes XML data generated by said first execution unit to be transformed to a canonical form for use by said second execution unit.