Re: [BUGS] BUG #2846: inconsistent and confusing

Tom Lane wrote:
> Roman Kononov <kononov195-pgsql@yahoo.com> writes:
> > In float4mul() and float4div(), the computation should be double precision.
>
> Why?  It's going to have to fit in a float4 eventually anyway.

One issue is in the patch comment:

    !    *  Computations that slightly exceed FLOAT8_MAX are non-Infinity,
    !    *  but those that greatly exceed FLOAT8_MAX become Infinity.  Therefore
    !    *  it is difficult to tell if a value is really infinity or the result
    !    *  of an overflow.  The solution is to use a boolean indicating if
    !    *  the input arguments were infiity, meaning an infinite result is
    !    *  probably not the result of an overflow.  This allows various
    !    *  computations like SELECT 'Inf'::float8 + 5.

    +    *  Underflow has similar issues to overflow, i.e. if a computation is
    +    *  slighly smaller than FLOAT8_MIN, the result is non-zero, but if it is
    +    *  much smaller than FLOAT8_MIN, the value becomes zero.  However,
    +    *  unlike overflow, zero is not a special value and can be the result
    +    *  of a computation, so there is no easy way to pass a boolean
    +    *  indicating whether a zero result is reasonable or not.  It might
    +    *  be possible for multiplication and division, but because of rounding,
    +    *  such tests would probably not be reliable.

For overflow, it doesn't matter, but by using float8, you have a much
larger range until you underflow to zero.  I will make adjustments to
the patch to use this, and add comments explaining its purpose.

--
  Bruce Momjian   bruce@momjian.us
  EnterpriseDB    http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +