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help / color / mirror / Atom feedFrom: Tom Lane <[email protected]>
To: John Naylor <[email protected]>
Cc: [email protected]
Cc: [email protected]
Subject: Re: BUG #19467: Inconsistency in MOD() result involving POWER() and floating-point precision in PostgreSQL
Date: Mon, 27 Apr 2026 10:08:37 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <CANWCAZaQfqAaATv4=tkRvV4mNW+6-XGcVTAfZw7qNcMBAuEN2Q@mail.gmail.com>
References: <[email protected]>
<CANWCAZaQfqAaATv4=tkRvV4mNW+6-XGcVTAfZw7qNcMBAuEN2Q@mail.gmail.com>
John Naylor <[email protected]> writes:
> On Mon, Apr 27, 2026 at 6:33 PM PG Bug reporting form
> <[email protected]> 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
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To: [email protected]
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Subject: Re: BUG #19467: Inconsistency in MOD() result involving POWER() and floating-point precision in PostgreSQL
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