Simple off the shelf abstractions for XML schema

Although the advent of XML Schema [25] has rendered DTDs obsolete, research on practical XML optimization is mostly biased towards DTDs and tends to largely ignore XSDs (some notable exceptions non-withstanding). One of the underlying reasons is most probably the perceived simplicity of DTDs versus the alleged impenetrability of XML Schema. Indeed, optimization w.r.t. DTDs has a local flavor and usually reduces to reasoning about the accustomed formalism of regular expressions. XSDs, on the other hand, even when sufficiently stripped down, are related to the less pervious class of unranked regular tree automata [6, 19, 20, 21]. Recent results on the structural expressiveness of XSDs [19], however, show that XSDs are in fact much closer to DTDs than to tree automata, leveraging the possibility to directly extend techniques for DTD-based XML optimization to the realm of XML Schema. The goal of the present paper is to present the results in [19] in an easy and accessible way. At the same time, we discuss possible applications, related research, and future research directions. Throughout the paper, we try to restrict notation to a minimum. We refer to [19] for further details.