On the minimization of Xpath queries

Abstract

This chapter focuses the minimization problem for tree patterns belonging to the fragment of XPath XP{/, //, [ ], *} (i.e. the fragment containing branches, descendant edges, and the wildcard symbol) and provides some relevant contributions. XML queries are usually expressed by means of XPath expressions identifying portions of the selected documents. An XPath expression defines a way of navigating an XML tree and returns the set of nodes, which are reachable from one or more starting nodes through the paths specified by the expression. The problem of efficiently answering XPath queries is very interesting and has recently received increasing attention by the research community. In particular, an increasing effort has been devoted to define effective optimization techniques for XPath queries. One of the main issues related to the optimization of XPath queries is their minimization. The minimization of XPath queries has been studied for limited fragments of XPath, containing only the descendent, the child and the branch operators. The chapter identifies an interesting tractable case and proposes an ad hoc algorithm handling the minimization of this kind of queries in polynomial time.