Optimizing distributed XML queries through localization and pruning

Abstract

Distributing data collections by fragmenting them is an effective way of improving the scalability of relational database systems. The unique characteristics of XML data present challenges that require different distribution techniques to achieve scalability. In this paper, we propose solutions to two of the problems encountered in distributed query processing and optimization on XML data, namely localization and pruning. Localization takes a fragmentation-unaware query plan and converts it to a distributed query plan that can be executed at the individual sites that hold XML data fragments in a distributed system. We then show how the resulting distributed query plan can be pruned so that only those sites are accessed that can contribute to the query result. We demonstrate that our techniques can be integrated into a real-life XML database system and that they significantly improve the performance of distributed query execution.