III this paper we address the problem of controlling the execution of a query optimizer. We describe a control for the optimization process that is based on planning. The controller described here is a goal-directed planner that intermingles planning with the execution of query transformations, and uses execution results to direct further planning of optimizer processing. We describe this control in the context of the Epoq extensible architecture. Epoq iz an approach to extensible query optimization that integrates specialized rewrite strategies through its extensible control mechanism. This paper describes our planning-based approach to extensible control and illustrates it with a simple example.