This is a Swift implementation of the Kossmann and Stocker Iterative Dynamic Programming query planning algorithm. It is generic, and can be used with any system with types conforming to two protocols:

1. a query plan provider, with methods such as accessPlans and joinPlans
2. a cost estimator which can produce a Cost type for a plan, and compare two cost values

Kossmann, Donald, and Konrad Stocker. "Iterative dynamic programming: a new class of query optimization algorithms." ACM Transactions on Database Systems (TODS) 25.1 (2000): 43-82.