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Re: BUG #4872: Geometric function problem

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Nick Roosevelt" <nroose(at)thepinc(dot)com>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #4872: Geometric function problem
Date: 2009-06-23 15:08:38
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-bugs
"Nick Roosevelt" <nroose(at)thepinc(dot)com> writes:
> I am getting bad results for distance between point and lseg.  As you can
> see below, the first result is correct, and the second is clearly not.

Hmm ... what it looks like to me is that there's an ancient thinko
in dist_ps_internal().  It's trying to calculate the slope of the
perpendicular to the given line segment, and it gets it wrong.
The segment's own slope would be deltaY / deltaX, so the slope
of the perpendicular should be the negative inverse of that, ie
-deltaX / deltaY, but what it was actually calculating was
-deltaY / deltaX.  So it was getting the wrong answers for any
situation where the given line segment's slope wasn't +1/-1
(or 0 or infinite, which are correctly special-cased).

Depressingly, fixing this changes none of the regression test outputs;
apparently all the test cases involving distances were one of the
special cases.  But it's really amazing no one complained of this
before ...

			regards, tom lane

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