Abstract join ,the optimization algorithm based on semi-join, the SDD_ 1 algorithm. Finally, this paper based on the research of the query optimization algorithms of distributed database above, faced with the shortages of the SDD_1 algorithm (The selected gene is hard to estimate, and many assistant operations and so on), puts forward a new query optimization algorithm, for a while named by the multi-join queries based on two semi-join of distributed database. This algorithm avoids the shortages of the SDD_1 algorithm , introduces the two semi-join technique to reduce entirely the joined cost between two stations ,and this bined by the algorithm of minimum spanning tree, gets the munication cost about the multi-join queries optimization and the assemble circs and the join order in the munication cost. According to the analysis of the algorithms’ examples, this algorithm has the similar query efficiency like the SDD_1 algorithm, and putation is not big .So in some special situations, this algorithm can almost replace the SDD_1 algorithm and get good query optimization effort. Key words: distributed database; query optimization; SDD_1 algorithm; two semi-join; multi-join queries 分布式数据库查询优化算法研究 目录 摘 要............................................................................................................................ I Abstract ......................................................................................................................... II 目录............................................................................................................................IV 第一章引 言..............................................................................................................1 论文背景........................................................................................................1 研究现状........................................................................................................3 本文主要研究内容......................