An Adaptive Query Optimization in a Hierarchical Mediator System

To hide the heterogeneous environment from a global user as much as possible while preserving the autonomy of each individual system to the highest degree, the study of a mediator has been actively carried out as an integration method for distributed heterogeneous information sources. To evaluate a global query on a view that integrates several distributed information sources, the query can be rewritten into a set of transformed queries based on the definition of the view. Each transformed query is targeted to one of the information sources. To speed up the execution of a global query, the previous results of frequently requested sub-queries are materialized in a mediator. This paper proposes a method of choosing the optimized set of materialized queries in each mediator such that available storage in each mediator can be highly utilized at any time. The integrating schema in a mediator can be incrementally modified and the evaluation frequency of a global query can also he continuously varied. In order to select the optimized set of materialized sub-queries with respect to their current evaluation frequencies. the proposed method for modeling the recent access behavior of each sub-query. As a result, it is possible to adjust the optimized set of materialized sub-queries adaptively according to the recent changes in the evaluation frequencies of sub-queries.