On 03/05/2013 07:23 PM, Tom Lane wrote:
> Maciek Sakrejda <m.sakrejda@gmail.com> writes:
>> Thank you: I think this is what I was missing, and what wasn't clear
>> from the proposed doc patch. But then how can pg_dump assume that it's
>> always safe to set extra_float_digits = 3?
>
> It's been proven (don't have a link handy, but the paper is at least
> a dozen years old) that 3 extra digits are sufficient to accurately
> reconstruct any IEEE single or double float value, given properly
> written conversion functions in libc. So that's where that number comes
> from. Now, if either end is not using IEEE floats, you may or may not
> get equivalent results --- but it's pretty hard to make any guarantees
> at all in such a case.
There's also gdtoa, which returns the shortest decimal representation
which rounds to the same decimal number. It would print 0.1 as 0.1, but
0.1 + 0.2 as 0.30000000000000004.
--
Florian Weimer / Red Hat Product Security Team