On Thu, 2021-04-22 at 13:50 -0400, Tom Lane wrote:
> Laurenz Albe <laurenz.a...@cybertec.at> writes:
> > On Mon, 2021-04-19 at 08:00 +0000, PG Doc comments form wrote:
> > > The 'round' mathematical function documentation doesn't specify its
> > > rounding
> > > strategy. If this would be documented then users can rely on its rounding
> > > behavior. For double precision the rounding probably depends on your
> > > platform. For numeric the function seems to round to nearest with ties
> > > going
> > > away from zero (sometimes referred to as "round half up). 6.5 -> 7.0 and
> > > -6.5 -> -7.0.
> >
> > Here is a proposed patch.
>
> LGTM, although I think we can be a bit more specific about the float
> case. I suggest
>
> ... by rounding away from 0. For <type>double precision</type>,
> the tie-breaking behavior is platform dependent, but
> <quote>round to nearest even</quote> is the most common rule.
Makes sense. Here is an updated patch.
Yours,
Laurenz Albe
From 7881d8a73f6cf93c21a9d1405736c6ce6cff5429 Mon Sep 17 00:00:00 2001
From: Laurenz Albe <laurenz.a...@cybertec.at>
Date: Fri, 23 Apr 2021 09:38:27 +0200
Subject: [PATCH] Document tie breaking behavior of round()
---
doc/src/sgml/func.sgml | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 67d802b02c..9ed39dabfd 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -1568,7 +1568,11 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
<returnvalue>double precision</returnvalue>
</para>
<para>
- Rounds to nearest integer
+ Rounds to nearest integer. For <type>numeric</type>, ties are
+ broken by rounding away from 0. For <type>double precision</type>,
+ the tie breaking behavior is architecture dependent, but
+ <quote>round to nearest even</quote> in accordance with IEEE-754
+ is the most common rule.
</para>
<para>
<literal>round(42.4)</literal>
@@ -1583,7 +1587,7 @@ repeat('Pg', 4) <returnvalue>PgPgPgPg</returnvalue>
</para>
<para>
Rounds <parameter>v</parameter> to <parameter>s</parameter> decimal
- places
+ places. Ties are broken by rounding away from 0.
</para>
<para>
<literal>round(42.4382, 2)</literal>
--
2.26.3
