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  1. BUG in postgres mathematic

    Max Vaschenko <max@nino.ru> — 2001-01-25T10:34:02Z

    Postgres-7.0.3-2
RedHat-6.2

SELECT int8(5*27.81*100);
13904

SELECT int4(5*27.81*100);
13905

SELECT int8(27.81*100*5);
13905

-- 
С уважением, Ващенко Максим,
Нижегородские информационные сети
(8312) 30-19-05, 34-00-02, 30-09-73

With best regards, Max Vaschenko,
Nizhny Novgorod Information Networks.

  2. Re: BUG in postgres mathematic

    Robert B. Easter <reaster@comptechnews.com> — 2001-01-25T12:29:00Z

    This problem is not specific to Postgres. If you play around with a little C 
program like:

#include <stdio.h>
 
int main(int argc, char * argv[])
{
        float f = 27.81;
        int i = 5;
        int l = 100;
 
        int ii = i*f*l;
        long ll = l*f*i;
        float ff = i*f*l;
 
        printf("%i\n", ii);
        printf("%li\n", ll);
        printf("%.5f\n", ff);
        printf("%i\n", (int) ff);
}                

It prints:

13904
13904
13905.00000
13905   

There is probably a good explanation for this.  gcc 2.95 and egcs 2.91.66 do 
this.  Maybe a rounding problem.

On Thursday 25 January 2001 05:34, Max Vaschenko wrote:
> Postgres-7.0.3-2
> RedHat-6.2
>
> SELECT int8(5*27.81*100);
> 13904
>
> SELECT int4(5*27.81*100);
> 13905
>
> SELECT int8(27.81*100*5);
> 13905

-- 
-------- Robert B. Easter  reaster@comptechnews.com ---------
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  3. Re: BUG in postgres mathematic

    Tom Lane <tgl@sss.pgh.pa.us> — 2001-01-26T03:52:30Z

    "Robert B. Easter" <reaster@comptechnews.com> writes:
> This problem is not specific to Postgres.

The fact that 5*27.81*100 != 27.81*100*5 is certainly a garden-variety
floating-point roundoff error.  However, I think Max has a fair
complaint here: it seems float-to-int8 conversion is truncating, not
rounding like the other conversions to integer do.

regression=# select 4.7::float8::int4;
 ?column?
----------
        5
(1 row)

regression=# select 4.7::float8::int8;
 ?column?
----------
        4
(1 row)

Seems to me this is a bug we should fix.

			regards, tom lane

  4. Re: BUG in postgres mathematic

    Robert B. Easter <reaster@comptechnews.com> — 2001-01-26T05:49:46Z

    On Thursday 25 January 2001 22:52, Tom Lane wrote:
> "Robert B. Easter" <reaster@comptechnews.com> writes:
> > This problem is not specific to Postgres.
>
> The fact that 5*27.81*100 != 27.81*100*5 is certainly a garden-variety
> floating-point roundoff error.  However, I think Max has a fair
> complaint here: it seems float-to-int8 conversion is truncating, not
> rounding like the other conversions to integer do.
>
> regression=# select 4.7::float8::int4;
>  ?column?
> ----------
>         5
> (1 row)
>
> regression=# select 4.7::float8::int8;
>  ?column?
> ----------
>         4
> (1 row)
>
> Seems to me this is a bug we should fix.
>
> 			regards, tom lane

Yeah, I agree. It isn't right that it truncates and that is something C does 
appearently.  The fix is to pass the float through a rounding something like 
(long)(f + 0.5) or else C just truncates it off.  This must already be 
happening for the int4 conversion or C would do the same thing to it.  I 
didn't look at the Postgres sources yet, but it is probably one of those very 
easy things to fix. :)


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  5. Re: BUG in postgres mathematic

    Tom Lane <tgl@sss.pgh.pa.us> — 2001-01-26T23:07:05Z

    >> The fact that 5*27.81*100 != 27.81*100*5 is certainly a garden-variety
>> floating-point roundoff error.  However, I think Max has a fair
>> complaint here: it seems float-to-int8 conversion is truncating, not
>> rounding like the other conversions to integer do.

I have changed float8-to-int8 to start with an rint() call, the same as
float8-to-int4 and float8-to-int2.  This should give the same roundoff
behavior as the other cases, including round-to-nearest-even if your
hardware supports IEEE-compliant float math.

Curiously, this change exposed what I take to be a platform dependency
in the int8 regress test.  It was computing
int8(float8(4567890123456789::int8)) and expecting to get back exactly
4567890123456789.  However, that value is 53 bits long and so there is
no margin for error in a standard IEEE float8 value.  I find that at
least on HP hardware, rint() treats the value as inexact and rounds to
nearest even:

regression=# select round(4567890123456788::float8) - 4567890123456780::float8;
 ?column?
----------
        8
(1 row)

regression=# select round(4567890123456789::float8) - 4567890123456780::float8;
 ?column?
----------
        8
(1 row)

regression=# select round(4567890123456790::float8) - 4567890123456780::float8;
 ?column?
----------
       10
(1 row)

regression=#

Whether this is a bug in rint or spec-compliant behavior is unclear, but
I'll bet HP's hardware is not the only platform that behaves this way.
Since I'm not eager to try to develop a new set of platform-specific
int8 expected files at this late hour, I just diked out that test
instead...

			regards, tom lane

  6. Re: BUG in postgres mathematic

    Robert B. Easter <reaster@comptechnews.com> — 2001-01-27T00:31:35Z

    On Friday 26 January 2001 18:07, Tom Lane wrote:
> Curiously, this change exposed what I take to be a platform dependency
> in the int8 regress test.  It was computing
> int8(float8(4567890123456789::int8)) and expecting to get back exactly
> 4567890123456789.  However, that value is 53 bits long and so there is
> no margin for error in a standard IEEE float8 value.  I find that at
> least on HP hardware, rint() treats the value as inexact and rounds to
> nearest even:
>
> regression=# select round(4567890123456788::float8) -
> 4567890123456780::float8; ?column?
> ----------
>         8
> (1 row)
>
> regression=# select round(4567890123456789::float8) -
> 4567890123456780::float8; ?column?
> ----------
>         8
> (1 row)
>
> regression=# select round(4567890123456790::float8) -
> 4567890123456780::float8; ?column?
> ----------
>        10
> (1 row)
>
> regression=#
>
> Whether this is a bug in rint or spec-compliant behavior is unclear, but
> I'll bet HP's hardware is not the only platform that behaves this way.
> Since I'm not eager to try to develop a new set of platform-specific
> int8 expected files at this late hour, I just diked out that test
> instead...

Here is what I get on Linux (PIII):

reaster=# select round(4567890123456788::float8) - 4567890123456780::float8;
 ?column?
----------
        8
(1 row)
 
reaster=# select round(4567890123456789::float8) - 4567890123456780::float8;
 ?column?
----------
        9
(1 row)
 
reaster=# select round(4567890123456790::float8) - 4567890123456780::float8;
 ?column?
----------
       10
(1 row)  

I'm not sure what the problem is either.  The PIII has an 80-bit FPU but not 
sure that matters.  When there is no exponent, maybe only 52 bits are really 
in the mantissa.  If you try rounding numbers <= 4503599627370495 (2^52 - 1), 
maybe you'll get expected results.  The hidden bit is 0.  Could be that round 
or rint (whatever it is) always makes the hidden bit 1 when I think it should 
only be 1 when the exponent is nonzero.  I'm no float expert! :)  Feel free 
to correct me.


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