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Title: Structural Characterizations of the Semantics of Xpath as Navigation Tool on a Document
Authors: GYSSENS, Marc 
Paredaens, Jan
Van Gucht, Dirk
Fletcher, George H.L.
Issue Date: 2006
Publisher: ACM Press
Source: Vansummeren, Stijn (Ed.) Proceedings of the 25th ACM SIGMOD-SIGACT-SIGART Symposium on Principles of Database Systems (PODS 2006). p. 318-327.
Abstract: Given a document D in the form of an unordered labeled tree, we study the expressibility on D of various fragments of XPath, the core navigational language on XML documents. We give characterizations, in terms of the structure of D, for when a binary relation on its nodes is definable by an XPath expression in these fragments. Since each pair of nodes in such a relation represents a unique path in D, our results therefore capture the sets of paths in D definable in XPath. We refer to this perspective on the semantics of XPath as the "global view." In contrast with this global view, there is also a "local view" where one is interested in the nodes to which one can navigate starting from a particular node in the document. In this view, we characterize when a set of nodes in D can be defined as the result of applying an XPath expression to a given node of D. All these definability results, both in the global and the local view, are obtained by using a robust two-step methodology, which consists of first characterizing when two nodes cannot be distinguished by an expression in the respective fragments of XPath, and then bootstrapping these characterizations to the desired results.
Keywords: XPath, definability, expressibility
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ISBN: 1-59593-318-2
Category: C1
Type: Proceedings Paper
Appears in Collections:Research publications

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