Strategies for query unnesting in XML databases

Authors:
Norman May;Sven Helmer;Guido Moerkotte
Affiliations:
University of Mannheim, Mannheim, Germany;University of London, U.K.;University of Mannheim, Mannheim, Germany
Venue:
ACM Transactions on Database Systems (TODS)
Year:
2006

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Abstract

Queries formulated in a nested way are very common in XQuery. Unfortunately, their evaluation is usually very inefficient when done in a straightforward fashion. We present a framework for handling nested queries that is based on unnesting the queries after having translated them into an algebra. We not only present a collection of algebraic equivalences, but also supply a strategy on how to use them effectively. The full potential of the approach is demonstrated by applying our rewrites to actual queries and showing that performance gains of several orders of magnitude are possible.