Query Optimization For OLAP-XML Federations

The changing data requirements of today’s dynamic business envi-ronments are not handled well by current OLAP systems. Physi-cally integrating unexpected data into such systems is a long and time-consuming process making logical integration, i.e., federa-tion, the better choice in many situations. The increasing use of Extended Markup Language (XML), e.g. in business-to-business (B2B) applications, suggests that the required data will often be available as XML data. This means that logical federations of OLAP and XML databases will be very attractive in many cases. However, for such OLAP-XML federations to be useful, effective optimization techniques for such systems are needed. In this paper we present novel techniques for query optimization in federations of OLAP and XML databases. The techniques in-clude so-called inlining of XML data in OLAP predicates, caching, and pre-fetching. Experiments show that the proposed optimiza-tion techniques improve query execution times significantly. Fur-thermore, the performance of the optimized federation queries is comparable to the performance achieved with physical integration of the data, showing that our federated OLAP approach is indeed a feasible alternative to physical integration