Combining Joins and Semijoins in Distributed Query Processing.
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A new approach to distributed query processing is proposed. In the conventional approach, a query is processed by the reducing phase and the joining phase. In the reducing phase, the relations referenced in the query are reduced using semijoins, and all the relations are joined at the result site in the joining phase. In the proposed approach, an arbitrary interleaving of joins and semijoins is allowed toward the generation of the query result at the result site. The proposed approach considers the local processing cost as well as the data transmission cost while the conventional approach mostly considers the data transmission cost only. The effectiveness of the query processing strategies generated according to the proposed approach is discussed for both the restricted class of join queries and the general join queries.