Design And Realization Of DDBS Of The Urban Electromagnetic Environment

This paper designed and realized a distributed database system of the urban electromagnetic environment and the distributed query processor in the system.The design of the system used Client/Server Mode and adopted three-layer structure: client application, Middle-ware and database server. Visual basic 6.0 is picked as the development tool and SQL Server 2000 is picked as database plane in the system, language of SQL is picked as databace interface language, ADO(ActiveX Data Objects)is picked as object model of operating databace. The progrem includes six modules: system logon, data acquisition, data backup, data retrieval, data maintenance, data statistics and analysis. The database consists two parts: one is the physical database of urban electromagnetic environment, the other is data dictionary. Aimed at the transparence, optimization and correctness of the distributed query in the data management of urban electromagnetic environment designed and realized a distributed query processor and it was divided into global and local in the design. First, the query problems are converted into relational algebra expression, and then analyzed it to query tree, the optimum local query sentence could be got after optimization process, hence realized the transparence of the distributed query, the query optimization aimed at the shortest query time and adopted algebra optimization and semi-join optimization method, after optimization process, the response time descended a order of magnitude, and the validity of the query was ensured meanwhile. This paper discussed the structure, thrad control, communication system, query optimization and so on separately, take a typical query in the urban electromagnetic environment management for an example, the paper expatiated the whole perform process of the distributed query, and compared the time cost between before and after the query optimization.By commissioning, the system satisfied the design requirement. The query processor in this paper offsets the shortages of distributed query and optimization function in SQL Server 2000, it can be used in the distributed database systems which have the similar network structure with the urban electromagnetic environment inspect system.