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From: =?UTF-8?Q?Martin_Norb=C3=A4ck_Olivers?= <martin@norpan.org>
Date: Tue, 26 Nov 2024 09:13:22 +0100
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 <CALoTC6uBuw0KAC_Ys5YMRfCazXh3gdxeY9OVkpvmnexsE__FAg@mail.gmail.com>
Subject: Re: Inconsistent results for division and multiplication operations
To: szy <598546998@qq.com>
Cc: Erik Brandsberg <erik@heimdalldata.com>,
 pgsql-sql <pgsql-sql@lists.postgresql.org>
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On Mon, Nov 25, 2024 at 5:18=E2=80=AFPM szy <598546998@qq.com> wrote:

> If the number of significant digits in the input is not fixed, it becomes
> challenging to achieve consistent results by rounding.
> for example
> postgres=3D# select round(1.003/1.002*5.01,2);
>          ?column?
> --------------------------
>  5.01
> (1 row)
>
> postgres=3D# select round(1.003*5.01/1.002,2);
>       ?column?
> --------------------
>  5.02
> (1 row)
>
>
Correct. That's why you should always use numeric with the desired
precision if you want precision numbers.

for instance
select 1.003/1.002*5.01 :: numeric(10,4)
will give the same result as
select 1.003*5.01/1.002 :: numeric(10,4)

They are much slower to calculate than floating point, however, so if you
don't care about precision you can keep using just floating point.

Regards,

Martin

--=20
Martin Norb=C3=A4ck Olivers
IT-konsult, Masara AB
Telefon: +46 703 22 70 12
E-post: martin@norpan.org
K=C3=A4rrh=C3=B6ksv=C3=A4gen 4
656 72 Skattk=C3=A4rr

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<div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><div dir=3D"ltr"><br></d=
iv><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">On =
Mon, Nov 25, 2024 at 5:18=E2=80=AFPM szy &lt;<a href=3D"mailto:598546998@qq=
.com">598546998@qq.com</a>&gt; wrote:<br></div><blockquote class=3D"gmail_q=
uote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,2=
04);padding-left:1ex"><div>If the number of significant digits in the input=
 is not fixed, it becomes challenging to achieve consistent results by roun=
ding.</div><div>for example</div><div><div>postgres=3D# select round(1.003/=
1.002*5.01,2);</div><div>=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0?column?</div><d=
iv>--------------------------</div><div>=C2=A0<font color=3D"#ff0000">5.01=
=C2=A0=C2=A0</font></div><div>(1 row)</div><div><br></div><div>postgres=3D#=
 select round(1.003*5.01/1.002,2);</div><div>=C2=A0 =C2=A0 =C2=A0 ?column?<=
/div><div>--------------------</div><div>=C2=A05.02</div><div>(1 row)</div>=
</div><div><div style=3D"font-size:14px;font-family:Verdana;color:rgb(0,0,0=
)"><br></div></div></blockquote><div><br></div>Correct. That&#39;s why you =
should always use numeric with the desired precision if you want precision =
numbers.<br><br>for instance<br>select 1.003/1.002*5.01 :: numeric(10,4)<br=
>will give the same result as<br>select 1.003*5.01/1.002 :: numeric(10,4)<b=
r><br>They are much slower to calculate than floating point, however, so if=
 you don&#39;t care about precision you can keep using just floating point.=
<br><br>Regards,<div><br>Martin=C2=A0</div></div><div><br></div><span class=
=3D"gmail_signature_prefix">-- </span><br><div dir=3D"ltr" class=3D"gmail_s=
ignature"><div dir=3D"ltr">Martin Norb=C3=A4ck Olivers<div>IT-konsult, Masa=
ra AB</div><div>Telefon: +46 703 22 70 12</div><div>E-post: <a href=3D"mail=
to:martin@norpan.org" target=3D"_blank">martin@norpan.org</a></div><div>K=
=C3=A4rrh=C3=B6ksv=C3=A4gen 4</div><div>656 72 Skattk=C3=A4rr</div></div></=
div></div></div></div>

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