Обсуждение: Float/Double cast to int

Feng Tian <ftian@vitessedata.com> writes:
> Here is a query, server was built witch GCC on Linux, AMD64.

> ftian=# select 1.5::int, 1.5::double precision::int, 314.5::int,
> 314.5::double precision::int;
>  int4 | int4 | int4 | int4
> ------+------+------+------
>     2 |    2 |  315 |  314
> (1 row)

> I believe this is because rint is broken -- can some expert on IEEE754
> please help confirm that this is a bug?

rint() is doing what the IEEE spec says, ie round to nearest even.
Your third expression is doing numeric-to-int, and that code doesn't
obey the IEEE spec.  We've had discussions (not too long ago) about
making these behaviors more consistent, but people seem to be too
afraid of backwards-compatibility problems if we change it.
        regards, tom lane

On Fri, May 22, 2015 at 4:33 AM, Feng Tian <ftian@vitessedata.com> wrote:
> Ah, thanks!  I did not realize numeric comes into play.   But, this is even
> more interesting -- I would expect numeric is more consistent than
> float/double when dealing with stuff like rounding.
>
> I missed the not too long ago discussion, :-)   Regardless of the mechanisms
> underneath, it would be quite hard to explain this behavior to customer.
> Maybe it is time to be brave, and be compatible with reality instead of
> backward?

Here is some food for thought:
- The original thread of pgsql-bug where this has been discussed first

http://www.postgresql.org/message-id/flat/20150320194337.2573.72944@wrigleys.postgresql.org#20150320194337.2573.72944@wrigleys.postgresql.org
- Another thread to document this behavior more appropriately (patch pending)
http://www.postgresql.org/message-id/CAB7nPqRVCbvYAWL++vCfyzOUujEay21bcLbBk_Mb=+2JX7+e5A@mail.gmail.com
Regards,
--
Michael