Putting XML Query Algebras into Context

While the XML community appears to be converging on XQuery as a standard for querying XML documents, there is currently much less consensus about a standard algebra. In this paper, we describe the algebra we have implemented in our XML query evaluation system. Our goal was to specify an algebra that is powerful enough to handle the XQuery language, yet simple enough to be amenable to optimization and implementation. A novel aspect of our algebra is the use of the context construct. Whereas XML algebras using bindings must at some point resort to "shredding" XML into collections of tuples that may need to be recombined by subsequent operators (often, for example, in a result construction step), the context construct allows us to "carry along" the structure of the input XML in a way that is transparent to the optimizer and to later operators in the query plan. It is our hope that publishing our algebra, and the context construct in particular, will contribute to the discussion of what features and characteristics an XML algebra should possess.