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The Computer Journal Advance Access originally published online on August 19, 2006
The Computer Journal 2007 50(1):68-80; doi:10.1093/comjnl/bxl047
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© The Author 2006. Published by Oxford University Press on behalf of The British Computer Society. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

An Adaptive Materialization Method for Globally Optimized Query Evaluation in a Hierarchical Mediator System

Kil Hong Joo1,* and Won Suk Lee2

1 Department of Computer Education, Gyeongin National University of Education San 6-8 Seoksudong Manangu Anyangsi, Gyeonggi, Korea
2 Department of Computer Science, Yonsei University 134 Shinchondong Seodaemoongu, Seoul, Korea

*Corresponding author: khjoo{at}ginue.ac.kr

Semantically related mediators form a hierarchical structure based on the definition of views in each mediator, since a view in a mediator is defined by views in other mediators or base tables in several local information servers. A global query is transformed into a set of its sub-queries; and, each sub-query is the unit of evaluation in an information server or a child mediator. Therefore, it is possible to speed up the execution of a global query if the previous results of frequently requested sub-queries are materialized in a mediator. However, the integrating schema in a mediator can be incrementally modified; and, the evaluation frequency of a global query can also be continuously varied. In order to select the optimized set of materialized sub-queries, with respect to their current evaluation frequencies, the proposed method applies a decay factor 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. Consequently, this paper proposes an algorithm for selecting the optimized set of sub-queries to be materialized in each mediator of a hierarchical mediator system adaptively such that the overall query evaluation cost of the hierarchical mediator system can be minimized at all times.

Key Words: mediator • data materialization • distributed query evaluation • heterogeneous information system • data integration • optimized query selection


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