An integrative approach to query optimization in native XML database management systems

Even though an effective cost-based query optimizer is of utmost importance for the efficient evaluation of XQuery expressions in native XML database systems, such a component is currently out of sight, because former approaches do not pay attention to the latest advances in the area of physical operators (e. g., Holistic Twig Joins and advanced indexes) or just focus only on some of them. To support the development of native XML query optimizers, we introduce an extensible cost-based optimization framework that integrates the cutting-edge XML query evaluation operators into a single system. Using the well-known plan generation techniques from the relational world and a novel set of plan equivalences---which allows for the generation of alternative query plans consisting of Structural Joins, Holistic Twig Joins, and numerous indexes (especially path indexes and content-and-structure indexes)---our optimizer can now benefit from the knowledge on native XML query evaluation to speed-up query execution significantly.