Re: Optimizing numeric SUM() aggregate

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От Andrew Borodin
Тема Re: Optimizing numeric SUM() aggregate
Дата
Msg-id CAJEAwVE-sDR_ZuQ8uKY2HW8L3TSsuEoO7qoKwcjyeR50BXfvEw@mail.gmail.com
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Ответ на Re: Optimizing numeric SUM() aggregate  (Andrew Borodin <borodin@octonica.com>)
Ответы Re: Optimizing numeric SUM() aggregate  (Dean Rasheed <dean.a.rasheed@gmail.com>)
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>I think we could do carry every 0x7FFFFFF / 10000 accumulation, couldn't we?

I feel that I have to elaborate a bit. Probably my calculations are wrong.

Lets assume we already have accumulated INT_MAX of 9999 digits in
previous-place accumulator. That's almost overflow, but that's not
overflow. Carring that accumulator to currents gives us INT_MAX/10000
carried sum.
So in current-place accumulator we can accumulate: ( INT_MAX - INT_MAX
/ 10000 ) / 9999, where 9999 is max value dropped in current-place
accumulator on each addition.
That is INT_MAX * 9999 / 99990000 or simply INT_MAX / 10000.

If we use unsigned 32-bit integer that is 429496. Which is 43 times
less frequent carring.

As a bonus, we get rid of 9999 const in the code (:

Please correct me if I'm wrong.


Best regards, Andrey Borodin, Octonica & Ural Federal University.



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