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Rewrite Rules for Quantified Subqueries in a Federated Database


Graham J. L. Kemp, Peter M. D. Gray, and Andreas R. Sjöstedt

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Return to Query Processing, Optimization and Constraints


Abstract

Transforming queries for effcient execution is particularly important in federated database systems since a more effcient execution plan can require many fewer data requests to be sent to the component databases. Also, it is important to do as much as possible of the selection and processing close to where the data are stored, making best use of facilities provided by the federation's component database management systems. In this paper we address the problem of processing complex queries including quantifers, which have to be executed against different databases in an expanding heterogeneous federation. This is done by transforming queries within a mediator for global query improvement, and within wrappers to make best use of the query processing capabilities of external databases. Our approach is based on pattern matching and query rewriting. We introduce a high level language for expressing rewrite rules declaratively, and demonstrate the use and fexibility of such rules in improving query performance for existentially quantifed subqueries. Extensions to this language that allow generic rewrite rules to be expressed are also presented. The value of performing final transformations within a wrapper for a given remote database is shown in several examples that use AMOS II | an SQL3-like system.


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