Intensional Query Optimization

Abstract

We have introduced a new query optimization framework called intensional query optimization (IQO), which enables existing optimization techniques to be applied to queries that use views. In particular, we consider that view definitions may employ unions. Advanced database technologies and applications--such as federation and mediation over heterogeneous database sources--lead to such complex view definitions, and to the need to handle complex, expensive queries. Query rewriting techniques have been proposed which exploit semantic query caches, materialized views, and semantic knowledge about the database domain to optimize query evaluation. These can augment syntactic optimization to reduce evaluation costs further. Such techniques include semantic query caching, query folding, and semantic query optimization. However, most proposed rewrite techniques ignore views in queries; that is, the views are considered as other tables. The IQO framework enables rewrites to be applied to various expansions of the query, even when no such rewrite is applicable directly to the query itself. With IQO, we optimize the query tree, not just the query. The IQO framework introduces the notion of a discounted query, which is a query with some of its expansions "separated out", so the query can be recast into pieces that can be optimized. For this approach to be effective, the sum of the costs of evaluating each piece must be less than the cost of evaluating the query itself. This includes the discounted query. We develop an evaluation plan for discounted queries that is generally more efficient than the evaluation of the queries themselves. (Also cross-referenced as UMIACS-TR-96-72)