Simplifying XPath queries for optimization with regard to the elimination of intersect and except operators

XPath is widely used as an XML query language and is embedded in XQuery expressions and in XSLT stylesheets. In this paper, we propose a rule set which logically simplifies XPath queries by using a heuristic method in order to improve the processing time. Furthermore, we show how to substitute the XPath 2.0 intersect and except operators in a given XPath query with computed filter expressions. A performance evaluation comparing the execution times of the original XPath queries, which contain the intersect and except operators, and of the queries that are the result of our simplification approach shows that, depending on the used query evaluator and on the original query, performance improvements of a factor of up to 350 are possible. Additionally, we prove that XPath 1.0 is closed under complementation and first order complete.