Re: floating point representation

Peter Eisentraut <peter_e(at)gmx(dot)net> writes:
> The GNU C library now offers a %a (and %A) format that prints floating
> point numbers in a semi-internal form that is meant to be portable. (I
> image this was done because of C99, but I'm speculating.) It might be
> useful to offer this to preserve accurate data across dumps.

Here's what I find in the C99 draft:

a,A A double argument representing a (finite) floating-
point number is converted in the style
[-]0xh.hhhhpd, where there is one hexadecimal digit
^ == "+/-" ... tgl
(which is nonzero if the argument is a normalized
floating-point number and is otherwise unspecified)
before the decimal-point character (219) and the
number of hexadecimal digits after it is equal to
the precision; if the precision is missing and
FLT_RADIX is a power of 2, then the precision is
sufficient for an exact representation of the value;
if the precision is missing and FLT_RADIX is not a
power of 2, then the precision is sufficient to
distinguish (220) values of type double, except that
trailing zeros may be omitted; if the precision is
zero and the # flag is not specified, no decimal-
point character appears. The letters abcdef are
used for a conversion and the letters ABCDEF for A
conversion. The A conversion specifier produces a
number with X and P instead of x and p. The
exponent always contains at least one digit, and
only as many more digits as necessary to represent
the decimal exponent of 2. If the value is zero,
the exponent is zero.

A double argument representing an infinity or NaN is
converted in the style of an f or F conversion
specifier.

____________________

219Binary implementations can choose the hexadecimal digit
to the left of the decimal-point character so that
subsequent digits align to nibble (4-bit) boundaries.

220The precision p is sufficient to distinguish values of
the source type if 16p-1>bn where b is FLT_RADIX and n is
the number of base-b digits in the significand of the
source type. A smaller p might suffice depending on the
implementation's scheme for determining the digit to the
left of the decimal-point character.

7.19.6.1 Library 7.19.6.1

314 Committee Draft -- August 3, 1998 WG14/N843

So, it looks like C99-compliant libc implementations will have this,
but I'd hesitate to rely on it for pg_dump purposes; it would certainly
not be very portable for awhile yet.

Peter's idea of a SET variable to control float display format might
not be a bad idea, but what if anything should pg_dump do with it?
Maybe just crank the precision up a couple digits from the current
defaults?

regards, tom lane