An XML-algebra for efficient set-at-a-time execution

The importance of XML query optimization is growing due to the rising number of XML-intensive data mining tasks. Earlier work on algebras for XML query focused mostly on rule-based optimization and used node-at-a-time execution model. Heavy query workloads in modern applications require cost-based optimization which is naturally supported by the set-at-a-time execution model. This paper introduces an algebra with only set-at-a-time operations, and discusses expression reduction methods and lazy evaluation techniques based on the algebra. Our experiments demonstrate that, for queries with complex conditional and quantified expressions, the proposed algebra results in plans with much better performance than those produced by the state-of-the-art algebras. For relatively simple queries, the proposed methods are expected to yield plans with comparable performance.

Transformation Based XML Query Optimization

This chapter addresses the fundamental issues of XML query optimization – equivalences and transformation rules. Equivalences are typically used as the basis to pursue transformation-based query optimization in traditional database systems. In this chapter, we develop the conception of equivalences in the context of XML database and XML query optimization. We then propose to derive deterministic transformation rules by incorporating general and XML-specific optimization heuristics to achieve high optimization efficiency for XML queries. We hope the work presented in this chapter will set a preliminary foundation or framework for further in-depth investigation for transformation-based XML query optimization.