From: Tom Lane <tgl@sss.pgh.pa.us>
To: John Naylor <johncnaylorls@gmail.com>
cc: fmusqlgen@163.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #19467: Inconsistency in MOD() result involving POWER() and floating-point precision in PostgreSQL
In-reply-to: <CANWCAZaQfqAaATv4=tkRvV4mNW+6-XGcVTAfZw7qNcMBAuEN2Q@mail.gmail.com>
References: <19467-b75f37ccc0b69261@postgresql.org> <CANWCAZaQfqAaATv4=tkRvV4mNW+6-XGcVTAfZw7qNcMBAuEN2Q@mail.gmail.com>
Comments: In-reply-to John Naylor <johncnaylorls@gmail.com>
	message dated "Mon, 27 Apr 2026 19:20:21 +0700"
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Date: Mon, 27 Apr 2026 10:08:37 -0400
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John Naylor <johncnaylorls@gmail.com> writes:
> On Mon, Apr 27, 2026 at 6:33 PM PG Bug reporting form
> <noreply@postgresql.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