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E. L. Robertson, L. Saxton, D. Van Gucht, and S. Vansummeren. Structural Recursion on Ordered Trees and List-based Complex Objects - Expressiveness and PTIME Restrictions. In Proceedings of the 11th International Conference on Database Theory, ICDT 2007, volume 4353 of Lecture Notes in Computer Science, pages 344-358. Springer-Verlag, Barcelona, Spain, January 2007.
© Springer-Verlag 2007 – http://dx.doi.org/10.1007/11965893_24

Abstract

XML query languages need to provide some mechanism to inspect and manipulate nodes at all levels of an input tree. In this paper we investigate the expressive power provided in this regard by structural recursion. We show that the combination of vertical recursion down a tree combined with horizontal recursion across a list of trees gives rise to a robust class of transformations: it captures the class of all primitive recursive queries. Since queries are expected to be computable in at most polynomial time for all practical purposes, we next identify a restriction of structural recursion that captures the polynomial time queries. Although this restriction is semantical in nature, and therefore undecidable, we provide an effective syntax. We also give corresponding results for list-based complex objects.


Updated: 2017-03-27