Rounding strategy

On Mon, 2021-04-19 at 08:00 +0000, PG Doc comments form wrote:

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/functions-math.html
Description:

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.

+1

Yours,
Laurenz Albe

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.

Yours,
Laurenz Albe

Laurenz Albe <laurenz.albe@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.

regards, tom lane

On Thu, 2021-04-22 at 13:50 -0400, Tom Lane wrote:

Laurenz Albe <laurenz.albe@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

On Fri, 2021-04-23 at 09:40 +0200, Laurenz Albe wrote:

Makes sense. Here is an updated patch.

I see that you have already committed your proposed version.
Thanks!

Yours,
Laurenz Albe

Thanks a lot!

Cheers, Rudolph