Обсуждение: Should "select 'nan'::float = 'nan'::float;" return false as per IEEE 754

This is how PostgreSQL currently works -

test=# select 'NaN'::float = 'NaN'::float as must_be_false; must_be_false
---------- t
(1 row)

I think that PostgreSQL's behaviour of comparing two
NaN-s as equal is wrong and Iwe should follow the IEEE 754 spec here

As per IEEE 754 a NaN behaves similar to NULL in SQL.

There is some discussion of why it is so at:


http://stackoverflow.com/questions/1565164/what-is-the-rationale-for-all-comparisons-returning-false-for-ieee754-nan-values

especially the first comment

---------
Hannu

Would this introduce problems finding rows where the stored value is NaN? You'd need to add a function or operator to
avoidthat. 

Il giorno 28/ott/2012, alle ore 20:43, Hannu Krosing ha scritto:

> This is how PostgreSQL currently works -
>
> test=# select 'NaN'::float = 'NaN'::float as must_be_false;
> must_be_false
> ----------
> t
> (1 row)
>
> I think that PostgreSQL's behaviour of comparing two
> NaN-s as equal is wrong and Iwe should follow the IEEE 754 spec here
>
> As per IEEE 754 a NaN behaves similar to NULL in SQL.
>
> There is some discussion of why it is so at:
>
>
http://stackoverflow.com/questions/1565164/what-is-the-rationale-for-all-comparisons-returning-false-for-ieee754-nan-values
>
> especially the first comment
>
> ---------
> Hannu
>
>
> --
> Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-hackers

On 28 October 2012 09:43, Hannu Krosing <hannu@2ndquadrant.com> wrote:
> This is how PostgreSQL currently works -
>
> test=# select 'NaN'::float = 'NaN'::float as must_be_false;
>  must_be_false
> ----------
>  t
> (1 row)
>
> I think that PostgreSQL's behaviour of comparing two
> NaN-s as equal is wrong and Iwe should follow the IEEE 754 spec here
>
> As per IEEE 754 a NaN behaves similar to NULL in SQL.

FWIW there is a note in the documentation about this:

"Note: IEEE754 specifies that NaN should not compare equal to any
other floating-point value (including NaN). In order to allow
floating-point values to be sorted and used in tree-based indexes,
PostgreSQL treats NaN values as equal, and greater than all non-NaN
values."

On 10/28/2012 11:05 AM, Chris Corbyn wrote:
> Would this introduce problems finding rows where the stored value is NaN? You'd need to add a function or operator to
avoidthat.
 
I guess it should behave similar to NULL-s

That is IS NOT DISTINCT FROM should still return true

test=# select NULL IS NOT DISTINCT FROM NULL as must_be_true; must_be_true
-------------- t
(1 row)

I guess making the NaN comparison IEEE compliant could introduce
some problems with indexes, so I propose that index operators would
treat NaNs like NULLs


Hannu

> Il giorno 28/ott/2012, alle ore 20:43, Hannu Krosing ha scritto:
>
>> This is how PostgreSQL currently works -
>>
>> test=# select 'NaN'::float = 'NaN'::float as must_be_false;
>> must_be_false
>> ----------
>> t
>> (1 row)
>>
>> I think that PostgreSQL's behaviour of comparing two
>> NaN-s as equal is wrong and Iwe should follow the IEEE 754 spec here
>>
>> As per IEEE 754 a NaN behaves similar to NULL in SQL.
>>
>> There is some discussion of why it is so at:
>>
>>
http://stackoverflow.com/questions/1565164/what-is-the-rationale-for-all-comparisons-returning-false-for-ieee754-nan-values
>>
>> especially the first comment
>>
>> ---------
>> Hannu
>>
>>
>> -- 
>> Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
>> To make changes to your subscription:
>> http://www.postgresql.org/mailpref/pgsql-hackers
>
>

On 10/28/2012 11:21 AM, Thomas Munro wrote:
> On 28 October 2012 09:43, Hannu Krosing <hannu@2ndquadrant.com> wrote:
>> This is how PostgreSQL currently works -
>>
>> test=# select 'NaN'::float = 'NaN'::float as must_be_false;
>>   must_be_false
>> ----------
>>   t
>> (1 row)
>>
>> I think that PostgreSQL's behaviour of comparing two
>> NaN-s as equal is wrong and Iwe should follow the IEEE 754 spec here
>>
>> As per IEEE 754 a NaN behaves similar to NULL in SQL.
> FWIW there is a note in the documentation about this:
>
> "Note: IEEE754 specifies that NaN should not compare equal to any
> other floating-point value (including NaN). In order to allow
> floating-point values to be sorted and used in tree-based indexes,
> PostgreSQL treats NaN values as equal, and greater than all non-NaN
> values."
I wonder how hard it would be to start treating NaNs as NULLs
so you could say ORDER BY fvalue NULLS AND NANS LAST :)

Hannu
>

On 28 October 2012 10:37, Hannu Krosing <hannu@2ndquadrant.com> wrote:
> On 10/28/2012 11:21 AM, Thomas Munro wrote:
>>
>> On 28 October 2012 09:43, Hannu Krosing <hannu@2ndquadrant.com> wrote:
>>>
>>> This is how PostgreSQL currently works -
>>>
>>> test=# select 'NaN'::float = 'NaN'::float as must_be_false;
>>>   must_be_false
>>> ----------
>>>   t
>>> (1 row)
>>>
>>> I think that PostgreSQL's behaviour of comparing two
>>> NaN-s as equal is wrong and Iwe should follow the IEEE 754 spec here
>>>
>>> As per IEEE 754 a NaN behaves similar to NULL in SQL.
>>
>> FWIW there is a note in the documentation about this:
>>
>> "Note: IEEE754 specifies that NaN should not compare equal to any
>> other floating-point value (including NaN). In order to allow
>> floating-point values to be sorted and used in tree-based indexes,
>> PostgreSQL treats NaN values as equal, and greater than all non-NaN
>> values."
>
> I wonder how hard it would be to start treating NaNs as NULLs
> so you could say ORDER BY fvalue NULLS AND NANS LAST :)

If you want to treat NaN like NULL, there are some other situations to
think about:

hack=> create table foo (x float);
CREATE TABLE
hack=> insert into foo values ('NaN'::float), (42);
INSERT 0 2
hack=> select sum(x) from foo;sum
-----NaN
(1 row)

That is the same result as evaluating 'NaN'::float + 42.

hack=> delete from foo;
DELETE 2
hack=> insert into foo values (null), (42);
INSERT 0 2
hack=> select sum(x) from foo;sum
----- 42
(1 row)

That is not the same results as evaluating NULL + 42 (the SUM
aggregate skips NULLs).

The trouble is, NULL is not a value within the type T (float in this
case), it's more like a nullable value has type NULL | T.  The algebra(set of operators including SUM, +, < etc) for
thealgebraic type
 
NULL | T doesn't behave the same way as the algebra for T, whereas NaN
is one of the values that IEEE float can hold, so it's part of the T
in this case.  In other words, for NaNs, if you have one set of
operators used by aggregates, ORDER BY, btrees and another set of
operators <, >, = for use in user expressions, that would be (at least
subtly) different than what happens today with NULL.

Well, I'm not that sure of what I'm saying.  But I have thought about
it for a while, as I have been (very slowly) working on an extension
which provides IEEE 754-2008 decimal number types DECIMAL32,
DECIMAL64, DECIMAL 128 (they have the same NaN, -0, +0, -Inf, +Inf
shenanigans, and some more fun problems as well, like non-normal
numbers, so that you can represent 1.002 and 1.0020 as distinct bit
patterns, and then have to argue with yourself about what it all
means).

On 28 October 2012 09:43, Hannu Krosing <hannu@2ndquadrant.com> wrote:
> test=# select 'NaN'::float = 'NaN'::float as must_be_false;
>  must_be_false
> ----------
>  t
> (1 row)
>
> I think that PostgreSQL's behaviour of comparing two
> NaN-s as equal is wrong and Iwe should follow the IEEE 754 spec here

The reason that PostgreSQL does this is that in order for float to be
a btree indexable type, its semantics must follow the reflexive law.
This and other requirements of btree operator classes are described
under src/backend/access/nbtree/README.

-- 
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services