Torsten Grust, Jan Rittinger, Jens Teubner. eXrQuy: Order Indifference in XQuery. In Proceedings of the 23rd International Conference on Data Engineering, ICDE 2007, April 15-20, 2007, The Marmara Hotel, Istanbul, Turkey. pages 226-235, IEEE, 2007. [doi]
There are more spots than immediately obvious in XQuery expressions where order is immaterial for evaluation―this affects most notably, but not exclusively, expressions in the scope of unordered { } and the argument of fn:unordered(). Clearly, performance gains are lurking behind such expression contexts but the prevalent impact of order on the XQuery semantics reaches deep into any compliant XQuery processor, making it non-trivial to set this potential free. Here, we describe how the relational XQuery compiler Pathfinder uniformly exploits such order indifference in a purely algebraic fashion: Pathfinder-emitted plans faithfully implement the required XQuery order semantics but (locally) ignore order wherever this is admitted.