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XPath is a simple language for navigating an XML-tree and returning a set ofanswer nodes. The focus in this paper is on the complexity of the containmentproblem for various fragments of XPath. We restrict attention to the mostcommon XPath expressions which navigate along the child and/or descendant axis.In addition to basic expressions using only node tests and simple predicates,we also consider disjunction and variables (ranging over nodes). Further, weinvestigate the containment problem relative to a given DTD. With respect tovariables we study two semantics, (1) the original semantics of XPath, wherethe values of variables are given by an outer context, and (2) an existentialsemantics introduced by Deutsch and Tannen, in which the values of variablesare existentially quantified. In this framework, we establish an exactclassification of the complexity of the containment problem for many XPathfragments.

On the complexity of XPath containment in the presence of disjunction, DTDs, and variables