Re: round returns -0

Le 2013-03-06 à 21:42, Tony Dare a écrit :

> I'm taking an standard deviation of a population and subtracting it from the average of the same population and rounding the result. Sometimes that result is negative and rounding it returns (or shows up as) a negative zero (-0) in a SELECT.
>
> basically:
> SELECT
> client_name, avg(rpt_cnt),
> stddev_pop(rpt_cnt),
> round(avg(rpt_cnt) - stddev_pop(rpt_cnt))
> from client_counts
> group by client_name
>
> and what I sometimes get is :
> client_name | a dp number | a dp number | -0
>
> In postgresql-world, is -0 = 0? Can I use that negative 0 in further calculations without fear? Is this a bug?

This is related to the recent discussion of floating point values on this mailing list. You can read more about IEEE 754 and whether 0 == -0 on Wikipedia: https://en.wikipedia.org/wiki/Signed_zero#Comparisons

According to that article, IEEE 754 specifies that 0 == -0 in Java/C/etc.

Hope that helps!
François Beausoleil