Explanation of the ROUND function with NUMERIC as an argument

On Sat, 2022-12-03 at 08:54 +0000, PG Doc comments form wrote:

Explanation of the manual's ROUND function
round ( numeric ) → numeric
round ( double precision ) → double precision
Rounds to nearest integer. For numeric, ties are broken by rounding away
from zero. For double precision, the tie-breaking behavior is platform
dependent, but “round to nearest even” is the most common rule.
round(42.4) → 42

Operation of the ROUND function
postgres=# SELECT round(CAST(42.5 AS numeric));
 round
-------
    43
(1 row)

I am thinking that the value of the ROUND function with NUMERIC as an
argument is rounded off, not rounded away from zero.

It is rounded away from zero, since 43 is farther away from 0 than 42.5.
This may be a language problem.

Yours,
Laurenz Albe

Laurenz Albe <laurenz.albe@cybertec.at> writes:

On Sat, 2022-12-03 at 08:54 +0000, PG Doc comments form wrote:

I am thinking that the value of the ROUND function with NUMERIC as an
argument is rounded off, not rounded away from zero.

It is rounded away from zero, since 43 is farther away from 0 than 42.5.
This may be a language problem.

Perhaps, since I don't see anything wrong with that text either.
(Of course, it's moderately likely that I wrote that text,
or at least copy-edited it at some point ;-). Don't remember.)

The point is that for 42.5, or anything-point-5, the basic
"round to nearest integer" rule is insufficient because 42 and
43 are equally near. We need a tie-breaking rule, and for numeric
that rule has historically been to round to the larger absolute
value (or "away from zero", as the text puts it to avoid two-dollar
terminology). Sadly, that is not what IEEE has established as
best practice for floating-point rounding, so round(float8)
acts differently.

regards, tom lane

Thank you for your answer.
I am convinced.
Thank you for replying to my question in poor English.

2022年12月3日(土) 23:19 Tom Lane <tgl@sss.pgh.pa.us>: