Re: [HACKERS] Floating point comparison inconsistencies of thegeometric types

On Tue, Jan 31, 2017 at 1:06 PM, Emre Hasegeli <emre@hasegeli.com> wrote:
> This is what he wrote:
>
>> As I understand it, the key problem is that tests like "is point on line"
>> would basically never succeed except in the most trivial cases, because of
>> roundoff error.  That's not very nice, and it might cascade to larger
>> problems like object-containment tests failing unexpectedly.  We would
>> need to go through all the geometric operations and figure out where that
>> kind of gotcha is significant and what we can do about it.  Seems like a
>> fair amount of work :-(.  If somebody's willing to do that kind of
>> investigation, then sure, but I don't think just blindly removing these
>> macros is going to lead to anything good.
>
> I re-implemented "is point on line" operator on my patch so that it
> would, at least, work on the most trivial cases.

Right.  So you and he do not agree on the goal.  He wants it to work
in MORE than the most trivial cases, and you wrote it so that it will
work in AT LEAST the most trivial cases.  Those are not the same.

> I listed the problems I am trying to solve in here:
>
> https://www.postgresql.org/message-id/CAE2gYzzNufOZqh4HO3Od8urzamNSeX-OXJxfNkRcU3ex9RD8jw%40mail.gmail.com

Those problems boil down to "let's get rid of the tolerance
completely", and there's no consensus on that being a good thing to
do.  In particular, I can't remember anybody but you being
unequivocally in favor of it.

> We couldn't agree on how we should treat on floating point numbers.  I
> think we should threat them just like the "float" datatype.

Got it, but if other people don't agree then this is going nowhere.

Personally, I've got my doubts about how sane it is to make anything
work here with the built-in floating point types.  The CPU is going to
do some kind of black magic, which may differ between machines, and
good luck getting consistent semantics that work everywhere.  For
example it seems entirely plausible (with some implementation) that
you might find that (1, 2) is on the line from (0, 0) to (2, 4) but
that (1, 3) is not on the line from (0, 0) to (2, 6) because the slope
1/2 can be represented exactly in base 2 and the slope 1/3 cannot.  If
you try to fix that kind of thing by introducing a tolerance, then you
might get it wrong when the line goes from (0, 0) to (1000000000,
2000000001).  Now, if we represented values using home-grown
arbitrary-precision arithmetic - like numeric does - then it might be
possible to get such cases right, especially if you supported exact
rather than approximate representations of fractional quantities.
Otherwise, I think there are inevitably going to be artifacts, and
tinkering with this is just changing which artifacts we get without
really solving the problem.  But I am notorious for my low opinion of
floating-point arithmetic; maybe somebody who understands it better or
likes it more can come up with something that's more clearly an
improvement.  Regardless, we have to have an agreement in order to
commit anything here, and I don't see one.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company