Query Optimization Technique For Distributed Database System Based On Semi-Join Algorithm

Aimed at the needs of data management, the emergence of the distributed database system meets some system organizations, where the various databases are dispersed in different places, and the relatively centralized databases management is required. With the continuous development of science and technology, the data volumes of most industries are larger than before. The distributed query processing, which relates to the data transmission among various data grounds in the data system, will inevitably increase the query costs. Therefore, query optimization study has very realistic significance. This thesis, based on the query cost, elaborates the query optimization of distributed database systems.Based on the management system of Science and Technology Hardware City of China, this system mainly realize the information storage, query management functions for the market information, business information and transaction information. The paper studies the cost of long-distance data transmission and query optimization and other related content. Then it introduces the relevant research results and application implementation.Firstly, the paper introduces the related basic theory of distributed database system and illustrates the necessity of query optimization of distributed database systems. Secondly, we explain the process of query optimization and illustrate those query optimization which based on the direct join and semi-join strategies as well as the respective operation process, applicable occasions and transmission costs, etc. Through the comparison of the query executing costs, we conclude that the semi-join strategy can effectively improve the query efficiency during the query process, especially for the system with a large amount of data. That’s why we focus on the technology and implementation process of distributed database system which uses the semi-join strategy in this paper. Thirdly, we use the SDD-1(System for Distributed Database-1) algorithm to analyze specific database system and calculate the transmission costs. It proves that the SDD-1algorithm can effectively reduce the amount of data of intermediate relations and decrease the network communication costs. Thus we can achieve the purpose of query optimization. Finally, we put forward a new algorithm based on the traditional semi-join method. By calculating specific query costs and comparing with traditional semi-join method, we illustrate the advantages of the new algorithm.With the arrival of the big data era and the rapid development of parallel computing technology, artificial intelligence technology and cloud computing technology, database technology will be constantly improved, as well as query optimization technology.