Re: BUG #3387: mod on non-integer returns bad result

This has been saved for the 8.4 release:

    http://momjian.postgresql.org/cgi-bin/pgpatches_hold

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Tom Lane wrote:
> Gregory Stark <stark@enterprisedb.com> writes:
> > In the case reported div_var was getting 70/70 = 0.99999. Which is
> > really just wrong.
>
> Agreed, but I think your proposed patch is just a band-aid.  The real
> problem with div_var is that it generates inaccurate output digits at
> all --- it's assuming that computing a few guard digits before rounding
> will suffice to ensure that all the delivered digits are correct, but
> I think there will always be corner cases where it fails.
>
> I just blew the dust off my old copy of Knuth vol 2, and see that his
> algorithm for multi-precision division generates output digits that are
> correct to start with (or at least he never needs to revisit a digit
> after moving on to the next).  ISTM we should go over to an approach
> like that.  For the truncated-output case the result will be exact,
> and for the rounded-output case you generate exactly one guard digit
> to see if it's >= base/2.
>
>             regards, tom lane

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  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
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