Balder ten Cate
Carsten Lutz
ABSTRACT
Query containment has been studied extensively for fragments of XPath 1.0. For instance, the problem is known to be ExpTime-complete for CoreXPath, the navigational core of XPath 1.0. Much less is known about query containment in (fragments of) the richer language XPath 2.0. In this paper, we consider extensions of CoreXPath with the following operators, which are all part of XPath 2.0 (except the last): path intersection, path equality, path complementation, for-loops, and transitive closure. For each combination of these operators, we determine the complexity of query containment, both with and without DTDs. It turns out to range from ExpTime (for extensions with path equality) and 2-ExpTime (for extensions with path intersection) to non-elementary (for extensions with path complementation or for-loops). In almost all cases, adding transitive closure on top has no further impact on the complexity. We also investigate the effect of dropping the upward and/or sibling axes, and show that this sometimes leads to a reduction in complexity.Since the languages we study include negation and conjunction infilters, our complexity results can equivalently be stated in terms ofsatisfiability.We also analyze the above languages in terms of succinctness.
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Index Terms
- The complexity of query containment in expressive fragments of XPath 2.0
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