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Subject: paper on intersection of XPath expressions
One of the problems that must be solved for any policy language that supports use of XPath expressions to identify policy vocabulary items is that the intersection of two XPath expressions is not always computable. WS-PolicyConstraints supports such expressions, and defines a very small subset of XPath for which intersection is computable, but a more general solution would be helpful; for example, is there a broader set of XPath expressions for which intersection is efficiently computable. I just noticed a paper that addresses this problem: On the Intersection of XPath Expressions, by Beda Christoph Hammerschmidt, University of Lübeck Martin Kempa, sd&m AG Volker Linnemann, University of Lübeck 9th International Database Engineering #38; Application Symposium (IDEAS'05) pp. 49-57 Part of abstract: In this paper we introduce the intersection problem for XPath and give a motivation for its relevance. We present an efficient intersection algorithm for XPath expressions without the NOT operator that is based on finite automata. For expressions that contain the NOT operator the intersection problem becomes NP-complete leading to exponential computations in general. With an average case simulation we show that the NPcompleteness is no significant limitation for most real-world database operations. Regards, Anne -- Anne H. Anderson Anne.Anderson@sun.com Sun Microsystems Labs 1-781-442-0928 Burlington, MA USA
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