Re: BUG #5150: math bug

On Fri, Oct 30, 2009 at 1:39 AM, Gray <gray(at)ms-irk(dot)ru> wrote:
>
> The following bug has been logged online:
>
> Bug reference:      5150
> Logged by:          Gray
> Email address:      gray(at)ms-irk(dot)ru
> PostgreSQL version: 8.2.6
> Operating system:   i686-pc-linux-gnu
> Description:        math bug
> Details:
>
> select 1/3*3,(1.0/3.0)*3.0,floor((1.0/3.0)*3.0);
>
> returns
> 0, 1, 0

Well, the first answer is correct, because 1/3 is a request for
integer division, so you get 0, and 0 * 3 is still zero.

I don't believe the second answer is really what you got, because
surely if you requested floating-point division the answer would be a
floating point number, not just 1. On pg 8.3.8, I get
0.999999999999999999990, which explains why the third answer comes out
to zero.

In general, floating point arithmetic is inaccurate and sucky. That
has nothing to do with PostgreSQL; it's just life.

http://en.wikipedia.org/wiki/Floating_point#Accuracy_problems

...Robert