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Message Posted: Tue, 21 Jun 2011 @ 11:34:24 GMT

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Subj:   Re: Distinct vs. group by in TD 13 onwards
From:   Walter, Todd

100% is the intent. By the time it gets to plan generation there is only one representation internally so the decisions are all made from that one form. Then Opt decides which algorithm to use based on what it knows about demographics, especially information about how unique the source relation already is.

Side effect is that while you could force one algorithm or another before, you no longer can. So it becomes much more important to have Stats on the columns that go into the list that is being grouped/distinct-ed.

Also, there is a run-time component that detects cases where the estimates did not accurately portray the demographics and adjust the algorithm on the fly - eg turn off local aggregate cache if it is not doing any good.

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