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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. The authors 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, they use inexpensive calculus based transformations to generate minimal query expressions before the query decomposition and the cost-based algebraic optimization take place.