When an application is built, an underlying data model is chosen to make that application effective. Frequently, other applications need the same data, only modeled differently. The naive solution of copying the underlying data and modeling is costly in terms of storage and makes data maintenance and evolution impossible. View mechanisms are a technique to model data differently for various applications without affecting the underlying format and structure of the data. The technique enables applications to customize shared data objects without affecting other applications that use the same objects. The growing data-manipulation needs of companies cannot be met by existing legacy systems that contain valuable data. Thus view mechanisms are becoming increasingly important as a way to model and use legacy data in new applications.Materialized views are views that have been computed and stored in databases. Because they reduce the need to recompute the view and/or data being queried, they speed up the querying of large amounts of data. Further, because they provide a systematic way to describe how to recompute the data, maintenance and evolution can be automated. Materialized views are especially useful in data warehousing, query optimization, integrity constraint maintenance, online analytical processing, and applications such as billing, banking, and retailing. This comprehensive volume, with a foreword by Jeff Ullman of Stanford University, will serve as a reference for students and commercial users, and encourage further use and development of materialized views.