A Foundation for the Replacement of Pipelined Physical Join Operators in Adaptive Query Processing

Adaptive query processors make decisions as to the most effective evaluation strategy for a query based on feedback received while the query is being evaluated. In essence, any of the decisions made by the optimizer (e.g., on operator order or on which operators to use) may be revisited in an adaptive query processor. This paper focuses on changes to physical operators (e.g., the specific join operators used, such as hash-join or merge-join) in pipelined query evaluators. In so doing, the paper characterizes the runtime properties of pipelined operators in a way that makes explicit when specific operators may be replaced, and that allows the validity of operator replacements to be proved. This is illustrated with reference to the substitution of join operators during their evaluation.