Re: BUG #14046: Bad mathematical rules for 0 cast

On Tue, Mar 29, 2016 at 11:55 PM, Jarosław Stokłosa <
jaroslaw(dot)stoklosa(at)nomino(dot)pl> wrote:

>
> W dniu 25/03/2016 o 14:32, John McKown pisze:
>
> On Fri, Mar 25, 2016 at 3:56 AM, < <jaroslaw(dot)stoklosa(at)nomino(dot)pl>
> jaroslaw(dot)stoklosa(at)nomino(dot)pl> wrote:
>
>> The following bug has been logged on the website:
>>
>> Bug reference: 14046
>> Logged by: Jarosław Stokłosa
>> Email address: jaroslaw(dot)stoklosa(at)nomino(dot)pl
>> PostgreSQL version: 9.4.5
>> Operating system: x86_64-redhat-linux-gnu
>> Description:
>>
>> Below query should return 2 times TRUE.
>>
>> SELECT cast('+0' as FLOAT) = cast('-0' as FLOAT), cast('+0' as
>> FLOAT)::TEXT
>> = cast('-0' as FLOAT)::TEXT
>>
>> +0 and -0 should be casted to +0 or 0.
>>
>
​How would you propose to implement this "or"?​

> ​I humbly disagree with you. PostgreSQL use IEEE 754 for floating point
> (mainly because that is what most - not all - computers today use). IEEE
> 754 distinguishes +0 (or just 0) from -0. cast('-0' as FLOAT) results in an
> IEEE 754 negative zero. Which is not identically equal to a positive zero.
> The case to TEXT should, and does, preserve this non-identical difference. ​
>
>
> I'm disagee with you. PostgreSQL don't distinguish +0 from -0 because
> cast('+0' as FLOAT) = cast('-0' as FLOAT) equals to TRUE. This is math
> rules. So case to float then to text shoud, and doesn't, preserve the
> equality. IEEE754 describes storage numbers in computer's memory but
> doesn't turn off math comparation rules.
>

Agreed. It is the cast to text turns off math comparison rules.

David J.