From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Gregory Stark <stark(at)enterprisedb(dot)com>, Filip Krska <filip(dot)krska(at)comstar(dot)cz>, pgsql-bugs(at)postgresql(dot)org |
Subject: | Re: BUG #3387: mod on non-integer returns bad result |
Date: | 2007-07-17 04:54:51 |
Message-ID: | 200707170454.l6H4spR05607@momjian.us |
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Lists: | pgsql-bugs |
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Tom Lane wrote:
> Gregory Stark <stark(at)enterprisedb(dot)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
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
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