10.15

Suppose a stream can deliver tuples out of order with respect to tuple timestamps. What extra information should the stream provide, so a stream query processing system can decide when all tuples in a window have been seen?

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Such streams should contain punctuations, that is, metadata tuples that state that all future tuples will have a timestamp greater than some value. Such punctuations are emitted periodically and can be used by window operators to decide when an aggregate result, such as aggregates for an hourly window, is complete and can be output.