How to Recognise Different Kinds of Tree Patterns From Quite a Long Way Away

Tree patterns are one of the main abstractions used to access XML data. Tree patterns are used, for instance, to define XML indexes, and support a number of efficient evaluation algorithms. Unfortunately deciding whether a particular query, or query fragment, is a tree pattern is undecidable for most XML Query languages. In this paper, we identify a subset of XQuery for which the problem is decidable. We then develop a sound and complete algorithm to recognize the corresponding tree patterns for that XQuery subset. The algorithm relies on a normal form along with a set of rewriting rules that we show to be strongly normalizing. The rules have been implemented and result in a normal form which is suitable for compiling tree patterns into an appropriate XML algebra.