SkinnerDB: regret-bounded query evaluation via reinforcement learning

Robust query optimization becomes illusory in the presence of correlated predicates or user-defined functions. Occasionally, the query optimizer will choose join orders whose execution time is by many orders of magnitude higher than necessary. We present SkinnerDB, a novel database management system that is designed from the ground up for reliable optimization and robust performance.

SkinnerDB implements several adaptive query processing strategies based on reinforcement learning. We divide the execution of a query into small time periods in which different join orders are executed. Thereby, we converge to optimal join orders with regret bounds, meaning that the expected difference between actual execution time and time for an optimal join order is bounded. To the best of our knowledge, our execution strategies are the first to provide comparable formal guarantees. SkinnerDB can be used as a layer on top of any existing database management system. We use optimizer hints to force existing systems to try out different join orders, carefully restricting execution time per join order and data batch via timeouts. We choose timeouts according to an iterative scheme that balances execution time over different timeouts to guarantee bounded regret. Alternatively, SkinnerDB can be used as a standalone, featuring an execution engine that is tailored to the requirements of join order learning. In particular, we use a specialized multi-way join algorithm and a concise tuple representation to facilitate fast switches between join orders. In our demonstration, we let participants experiment with different query types and databases. We visualize the learning process and compare against baselines.