An XML query compilation processor (20) includes an execution compiler (42) that transforms an XML query into an executable XML query plan (22). A query rewrite processor (34) performs query transformations on the XML query. Said query transformations include transforming an expression input (60) received by an expression (62) conditional upon a set of items defined by an output (64) of the expression (62) being independent of grouping of items in the expression input (60).