Isolating Order Semantics in Order-Sensitive XQuery-to-SQL Translation

Order is essential for XML query processing. Efficient XML processing with order consideration over relational storage is non-trivial, especially for complex nested XQuery expressions. The order semantics may impede efficient query rewriting for nested query blocks. We propose a general order-sensitive XQuery processing approach involving three steps. First an algorithm is proposed for inferencing about and then isolating the order semantics in XQuery expressions specified over virtual XML views. This turns an ordered XQuery plan into an unordered one decorated with minimized order context annotations. Then without loss of semantics, logical optimization via XQuery rewriting can be easily applied to this transformed query plan. As last step, the translation of the optimized logical plan into SQL now correctly incorporates the order context annotations to assure the original order semantics. Our experiments illustrate the performance gains achievable by our order handling strategy.