On 03/05/2013 07:23 PM, Tom Lane wrote:
Maciek Sakrejda <m.sakrejda@xxxxxxxxx> 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
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