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Local and Global Query Optimization Mechanisms for Relational Databases

In a relational database management system (DBMS), query optimization is essential since the DBMS must produce candidate internal access strategies from a high level query and select the optimal one. The goals of inplementing a good optimization mechanism are: (a) to maximize the performance of selected access strategies and (b) to minimize the optimization overhead. We have developed an optimization mechanisin, built it in a relational DBMS product at Hitachi. The mechanism can estimate precise tuple selectivity of predicates in queries for optimal access strategy selection, and employs a new method, called the cascade method, for optimization overhead reduction. This paper also proposes a global optimization mechanisn which executes optimization over multiple database queries.