Mermaid is a testbed system which provides integrated access to multiple databases. Two query optimization algorithms have been developed for Mermaid. The semijoin algorithm tends to reduce the data transmission cost, while the replicate algorithm reduces the processing cost. An algorithm that integrates the features of these two algorithms to optimize the processing cost as well as the transmission cost is presented. A dynamic network environment is considered where processing speeds at each site and transmission speeds at each link can be variable. Moreover, distributed processing of aggregates is considered based on the functional dependency among the fragment attribute, the aggregate attribute, and the group-by attribute. Semantic information is utilized to obtain efficient query processing.
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