Adaptive query processing: why, how, when, what next?

Adaptive query processing has been the subject of a great deal of recent work, particularly in emerging data management environments such as data integration and data streams. We provide an overview of the work in this area, identifying its common themes, laying out the space of query plans, and discussing open research problems. We discuss why adaptive query processing is needed, how it is being implemented, where it is most appropriately used, and finally, what next, i.e., open research problems.