1. A method comprising: analyzing a query execution plan that specifies tasks to be performed by a plurality of execution units; wherein the plurality of execution units include a first execution unit and a second execution unit; wherein the query execution plan is to service a request for data; wherein analyzing the query execution plan includes: detecting that a portion of the query execution plan specifies first tasks to be performed by the first execution unit; detecting that another portion of the query execution plan specifies second tasks to be performed by the second execution unit; and detecting that, as a result of performing the first tasks, the first execution unit will generate XML data that the second execution unit needs to perform the second tasks; generating information to send to said first execution unit to cause said first execution unit to perform said first tasks; wherein, in the absence of any annotation, said information would cause said first execution unit to generate the XML data in a first form that cannot be used by said second execution unit; wherein, in said first form, the XML data generated by the first execution unit includes one or more references to execution unit-specific data that is accessible by the first execution unit but that is not accessible by the second execution unit; and annotating said information with an annotation that causes the XML data generated by said first execution unit in the first form to be transformed to a second form for use by said second execution unit in performing said second tasks; wherein, in said second form, the XML data does not include any references to execution unit-specific data; and wherein the method is performed by one or more computing devices.