Optimizing cursor movement in holistic twig joins

Holistic twig join algorithms represent the state of the art for evaluating path expressions in XML queries. Using inverted in-dexes on XML elements, holistic twig joins move a set of index cursors in a coordinated way to quickly ¯nd structural matches. Because each cursor move can trigger I/O, the performance of a holistic twig join is largely determined by how many cursor moves it makes, yet, surprisingly, existing join algorithms have not been optimized along these lines. In this paper, we describe TwigOptimal, a new holistic twig join algorithm with optimal cur-sor movement. We sketch the proof of TwigOptimal's optimality, and describe how TwigOptimal can use information in the return clause of XQuery to boost its performance. Finally, experimen-tal results are presented, showing TwigOptimal's superiority over existing holistic twig join algorithms

Calculus-Based Transformations of Queries over Object-Oriented Views in a Database Mediator System

The concept of object-oriented (OO) views has been a popular approach to data integration. Nevertheless, there have been few reported results on optimization of queries over integrated OO views. In our work, we have developed an OO view system for data integration based on the AMOS database mediator system. The paper describes a system architecture and implementation that takes advantage of query optimization techniques to improve the performance of queries to integrated OO views. The main features of the system are: 1) A passive mediation framework that preserves the autonomy of the data sources. 2) A selective materialization mechanism that minimizes the number of materialized view objects. 3) A predicate based mechanism to guarantee the validity of the materialized view objects as well as the completeness of queries to the view. In order to reduce the overhead of the passive view integration, we use inexpensive calculus based transformations to generate minimal query expressions before the query decomposition and the cost-based algebraic optimization take place. 1