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Journal of Inforamtion Science and Engineering, Vol.7 No.2, pp.279-296 (June 1991)
Optimizing Multidatabase Queries by
Query Transformation*

Arbee L. P. Chen
Department of Computer Science
National Tsing Hua University
Hsinchu, Taiwan 30043, Republic of China

In a multidatabase system, schemas of the component databases are usually integrated into a global schema to provide users with transparencies on data distribution and schema differences. To process queries against the global schema, query modification, which maps the query into one against local schemas, is necessary. In this paper, an approach which transforms and decomposes the modified query into subqueries to optimize distributed query processing is presented. Each subquery references data contained at a local site and, therefore, can be locally processed. The partial results produced from these subquery executions are then collected to compute the query answer. This approach reduces query response time due to parallel processing. Moreover, queries which can actually be processed at a single site can be identified.

Keywords: multidatabase systems, schema integration, data fragmentation, query modification, query transformation, query optimization, localized processing

Received October 22, 1990; revised March 5, 1991.
Communicated by R. C. T. Lee.
*This work was supported in part by the Republic of China, National Science Council under Contract No. NSC 80-0408-E-007-01.