Re: BUG #19467: Inconsistency in MOD() result involving POWER() and floating-point precision in PostgreSQL

John Naylor <johncnaylorls(at)gmail(dot)com> writes:
> On Mon, Apr 27, 2026 at 6:33 PM PG Bug reporting form
> <noreply(at)postgresql(dot)org> wrote:
>> Mathematically, this corresponds to the fractional part of 3^70.31, which
>> should be deterministic for a given evaluation strategy.
>>
>> However, different systems produce significantly different results:

> These two statements don't contradict eachother.

> Trying the expression on WolframAlpha shows 0.41 is close to the
> expected value, so I don't see a bug here.

Those other systems are probably using float8 arithmetic, which has
nowhere near enough precision to give a nonzero answer.

In Postgres, constants like "3.00" are type numeric not type float8,
so:

regression=# select pow(3.00, 70.31);
pow
---------------------------------------
3518806773889710662003177340498520.41
(1 row)

regression=# select mod(pow(3.00, 70.31), 1);
mod
------
0.41
(1 row)

You can duplicate the lower-precision answer if you want:

regression=# select pow(3.00::float8, 70.31::float8);
pow
------------------------
3.5188067738897196e+33
(1 row)

regression=# select pow(3.00::float8, 70.31::float8)::numeric;
pow
------------------------------------
3518806773889720000000000000000000
(1 row)

regression=# select mod(pow(3.00::float8, 70.31::float8)::numeric, 1);
mod
-----
0
(1 row)

regards, tom lane