An eddy is a query processing mechanism that can reorder queries at runtime for optimization purposes. Eddies were designed to minimize synchronization barrier overhead and take advantage of moments of symmetry for reordering. The authors use a pre-optimizer to set up joins and kept this step very simple so that the focus stayed on eddies. An eddy keeps track of which operators the input tuples have been processed by and only outputs tuples that have been seen by every eligible operator.
The "naive eddy" works well for operators with the same selectivity, whereas the "fast eddy" takes into account both the consumption and production rates, and hence both cost and selectivity, via lottery scheduling. A window scheme is added to the lottery scheduling to only take recent history into account, allowing the eddy to adapt over time to changes in performance and data.
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