Union rewritings for XPath fragments

In this paper, we study the problem of finding an equivalent rewriting of an XPath query using multiple views, and we show that the union operator may be required in order to find such a rewriting. In particular, focusing on the fragment of XPath containing both descendant edges and wildcard labels, we propose an algorithm that outputs a union of single-view rewritings (if there exists any) which equivalently rewrites a given query. For the same fragment of XPath, we give necessary and sufficient conditions for query containment and equivalence of unions of queries.