The chapters of this book provide an excellent snapshot of current research and development activities in the area of query processing and optimization. They supply potential answers to many questions that have been raised for new types of database systems and at the same time reflect the variety of the different approaches taken. The book acts both as a reference for the state of the art in query processing for the "next generation" of database systems, and as a good starting point for anybody interested in understanding the challenging questions in the area. Furthermore, the book will help the reader to gain an in-depth understanding of why efficient query processing is needed for future database systems.