I agree. 64 bits testing doesn't test on equality, it just tests for a
smaller rounding error.
Jan
On 8-1-2010 21:25, Paragon Corporation wrote:
Paul,
Huh. What do you want to change = to? A real =?
If you do remember for better or worse it will have impact on -- GROUP BY,
ORDER BY
as the = operator I can only guess is baked in deeply into the PostgreSQL
implementation of these (not sure how though).
So if you ever change it to a true equality operator, I suspect these things
may become as slow as molasses, and well I see no point
of making it a 64-bit check because the only reason is to make it more of a
true equality which it isn't anyway. People should just get out of that
mindset.
Thanks,
Regina
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Paul
Ramsey
Sent: Friday, January 08, 2010 12:51 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] st_equals strangeness
Well, that brings us to the API level, no? Because operators have meanings
in index (32-bit) terms. Should all our operators have 64-bit rechecks?
Should some of them? Should '=' just get a special treatment, because we
have such an in-grained understanding of what that symbol connotes, and
other symbols be left as pure index ops?
P
On Fri, Jan 8, 2010 at 9:44 AM, Mark Cave-Ayland
<[email protected]> wrote:
Paul Ramsey wrote:
More to the point, it's a float32 box, not a double64 box. Your two
points are in fact different, waaaaaay down deep into the precision
of their 64-bit double coordinates. So deep in fact that the human
readable decimal representations don't show it (look at the hexewkb
output, and you'll see the differences, you should be able to do
that, points are small enough to eyeball in hex).
So when the 32-bit box is extracted from the 64-bit doubles, the
points are identical at that level of precision and = returns true,
while st_equals working against the doubles does not.
P
Again, another demonstration as to why BOX2DFLOAT4s should never be
used for calculations and only for internal "within" checks :(
ATB,
Mark.
--
Mark Cave-Ayland - Senior Technical Architect PostgreSQL - PostGIS
Sirius Corporation plc - control through freedom
http://www.siriusit.co.uk
t: +44 870 608 0063
Sirius Labs: http://www.siriusit.co.uk/labs
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