Rule-Based Generation of Logical Query Plans with Controlled Complexity

Abstract

Rule--based query optimizers are recognized as particularly valuable for extensible and object--oriented database management systems by providing a high flexibility in adapting query optimization strategies to nonstandard application needs. On the other hand rule--based optimizers areproblematic with regard to run--time behavior for more complex queries as the generation of query plans based on a declarative rule base tends to be difficult to control. In this paper we show that this is not a fundamental problem of rule--based optimizers, but rather a question of careful design of the rule system. We exemplify this for one fundamental optimization problem, namely join enumeration for linear queries.There,arule--basedoptimizationstrategycanachievethetheoreticallyoptimalcomplexity. The design principles used to achieve this have been derived from and are used in the design of the VODAK query optimizer developed at GMD--IPSI