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Message Posted: Wed, 02 Sep 2009 @ 18:59:35 GMT

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Subj:   Re: All Amps usage
From:   Victor Sokovin

I think this is as scalable as it gets in this case. For a fixed receiver, the rows are distributed across AMPs according to the sender. Assuming the sender can be anybody, you'll need the all-AMP retrieval.

I don't think the skew should be a problem in this case either.

A theoretical threat to the plan I can think of is when the optimizer estimates that the receiver part will return a significant percentage of rows it may ignore the NUSI and go for the full scan instead. But the percentage must be really high, like > 15% (or is it 10%?). Must be a really major account or a major problem with the stats. This is done because in most cases the full scan is then indeed a better option under the circumstances. It won't be sub-second but then you can't expect sub-second performance when returning 15% of a large table.

The worst case is the largest receiver. It will give the most work for the AMPs. Can you predict how big it will be in the future? If so, create the test case and run the query. This is pretty much the worst performing case. I don't think the size of the table matters very much for the test. You don't need to future-size it unless the 10-15% limit is exceeded.


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