Towards Cost-based Query Optimization in Native XML Database Management Systems

In the last few years, XML became a de-facto standard for the exchange of structured and semi-structured data. The database research community took this development into account by proposing native XML database management systems for efficient and transactional management of XML documents. One of the most important factors for success of such systems is a powerful query optimizer. Many researchers proposed sophisticated Structural Join and Holistic Twig Join algorithms as well as several index structures supporting the evaluation of twig query patterns. Even though almost all XML query evaluation approaches proposed so far use some of these methods, we believe that they provide no sufficient input for real-world cost-based query optimization scenarios, because they only cover a small part of the overall query evaluation process. To provide adequate input for a cost-based XML query optimizer, we propose the XML Query Graph Model as a new internal representation enabling a smooth transition between XQuery language level and physical algebra operators. Furthermore, we introduce a set of rewrite rules for improving the execution of twig queries, e. g., by fusing two adjacent binary join operators to a complex n-way join operator. By presenting further rewrite rules, we make the most of existing joins and indexes—even before query transformation. Using these concepts, we are ready to sketch its integration into our upcoming costbased XML query optimizer.