| From: | Andrew Borodin <borodin(at)octonica(dot)com> |
|---|---|
| To: | Andrew Borodin <amborodin(at)acm(dot)org> |
| Cc: | Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Optimizing numeric SUM() aggregate |
| Date: | 2016-07-27 06:33:03 |
| Message-ID: | CAJEAwVE-sDR_ZuQ8uKY2HW8L3TSsuEoO7qoKwcjyeR50BXfvEw@mail.gmail.com |
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| Lists: | pgsql-hackers |
>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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