Tom Lane wrote:
>Andreas Pflug <pgadmin@pse-consulting.de> writes:
>
>
>>Well, to me it's not well-known that floating-point addition is not
>>associative, do I need to re-learn my math?
>>
>>
>
>regression=# select (1.0::float8 + (-1.0::float8)) + 1.0e-20::float8;
> ?column?
>----------
> 1e-20
>(1 row)
>
>regression=# select 1.0::float8 + ((-1.0::float8) + 1.0e-20::float8);
> ?column?
>----------
> 0
>(1 row)
>
>
>
Hi Tom,
I already suspected an example like this. Obviously in a pure math
world, the second example is wrong, caused by implicite rounding.
Fortunately, if omitting the float8 casts numeric is used, delivering
ultimate precision.
Just for curiousity: on MSSQL2000, the first will deliver
9.99999999999995E-21, and if the type decimal(30,25) is used both give
0.00000000000. Even better, CAST( CAST(1E-20 AS DECIMAL(30,25) AS
FLOAT) is 0.0 :->
Oracle 9.2 will calculate correctly with float down to 1.0e-40.
Regards,
Andreas
