With the increasing need for manipulating and exchanging XML data, the topic of processing XML documents and optimizing XML queries is being studied by many researchers. There are still, however, many open issues in query optimization in the context of XML database systems. In this paper, we give an overview on the state of XML query optimization by reviewing the different optimization techniques developed by previous research and employed by existing XML database systems. We subsequently enumerate some of the still unresolved problems in the optimization process. We conclude this paper with a description of our current work in which we focus on improving the evaluation of XPath expressions by rewriting the path at the algebraic level.
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