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Re: WIP: rewrite numeric division

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: "Michael Paesold" <mpaesold(at)gmx(dot)at>, "Bruce Momjian" <bruce(at)momjian(dot)us>, pgsql-patches(at)postgresql(dot)org
Subject: Re: WIP: rewrite numeric division
Date: 2007-07-17 16:00:04
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Lists: pgsql-patches
Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> Yeah.  I was basically waiting to see if anyone could come up with a
>> faster solution.  Since no one seems to have an idea how to do it
>> better, I'm inclined to apply the patch for 8.3.

> My only reservation is that I don't have the numeric methods background to
> tell if it's really necessary. My fix does resolve the only actual documented
> inaccuracy in the existing method.

Well, this doesn't take a lot of numerical methods background: the
fundamental problem is that the existing code generates an *approximate*
answer, whereas people who are doing div and mod on large integers tend
to expect an *exact* answer.  Approximate doesn't cut it --- there will
always be cases where an off-by-one-in-the-last-internal-place error can
carry far enough to the left to be visible to the user.

			regards, tom lane

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