Обсуждение: to_char(numeric type, text) rounding instead of truncating

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/15/functions-formatting.html
Description:

I've encountered an issue with to_char(numeric type, text)
Apparently, it uses rounding instead of truncation which leads to
unpexpected results. This is not reflected in the documentation.
My specific example:
select to_Char(1235::real/(2::real+1235::real)*100, '99%')
The expected output was 99%. The output was ##% - since the number was
rounded to 100, there wasn't enough digits in the format.
If I add a digit after the decimal point
select to_char(1235::real/(2::real+1235::real)*100,'90D0%')
The output becomes 99.8%

It would be nice to have this behavior explained in the documentation so
people get more predictable results.

On Fri, 2023-07-21 at 04:56 +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/15/functions-formatting.html
> Description:
>
> I've encountered an issue with to_char(numeric type, text)
> Apparently, it uses rounding instead of truncation which leads to
> unpexpected results. This is not reflected in the documentation.
> My specific example:
> select to_Char(1235::real/(2::real+1235::real)*100, '99%')
> The expected output was 99%. The output was ##% - since the number was
> rounded to 100, there wasn't enough digits in the format.
> If I add a digit after the decimal point
> select to_char(1235::real/(2::real+1235::real)*100,'90D0%')
> The output becomes 99.8%
>
> It would be nice to have this behavior explained in the documentation so
> people get more predictable results.

+1

How about the following:

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 5a47ce4343..9421ace77e 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -8505,6 +8505,14 @@ SELECT regexp_match('abc01234xyz', '(?:(.*?)(\d+)(.*)){1,1}');
       </para>
      </listitem>

+     <listitem>
+      <para>
+       If the format provides for fewer fractional digits than the number being
+       formatted has, <function>to_char()</function> will round the number to
+       the specified number of fractional digits.
+      </para>
+     </listitem>
+
      <listitem>
       <para>
        The pattern characters <literal>S</literal>, <literal>L</literal>, <literal>D</literal>,


Yours,
Laurenz Albe

On Wed, Jul 26, 2023 at 07:53:36AM +0200, Laurenz Albe wrote:
> On Fri, 2023-07-21 at 04:56 +0000, PG Doc comments form wrote:
> > The following documentation comment has been logged on the website:
> > 
> > Page: https://www.postgresql.org/docs/15/functions-formatting.html
> > Description:
> > 
> > I've encountered an issue with to_char(numeric type, text)
> > Apparently, it uses rounding instead of truncation which leads to
> > unpexpected results. This is not reflected in the documentation.
> > My specific example:
> > select to_Char(1235::real/(2::real+1235::real)*100, '99%')
> > The expected output was 99%. The output was ##% - since the number was
> > rounded to 100, there wasn't enough digits in the format.
> > If I add a digit after the decimal point
> > select to_char(1235::real/(2::real+1235::real)*100,'90D0%')
> > The output becomes 99.8%
> > 
> > It would be nice to have this behavior explained in the documentation so
> > people get more predictable results.
> 
> +1
> 
> How about the following:

I slightly modified your patch and applied it back to PG 11 since all
supported Postgres versions have the same behavior.  Thanks.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.

On Wed, 2023-09-06 at 16:53 -0400, Bruce Momjian wrote:
> I slightly modified your patch and applied it back to PG 11 since all
> supported Postgres versions have the same behavior.  Thanks.

Thanks for picking it up.

Yours,
Laurenz Albe