R: round(x) function - Mailing list pgsql-general

I think this is not consistent with documentation that says there is just one round function, with one argument of
numerictype. It seems there is at least 2 different round functions with two different behaviours. One for
float4/float8that "round to nearest even"  and another one for numeric type that "add 0.5 and truncate" 

From the manual:
 Example 10-4. Rounding Function Argument Type Resolution

There is only one round function that takes two arguments; it takes a first argument of type numeric and a second
argumentof type integer. So the following query automatically converts the first argument of type integer to numeric: 

SELECT round(4, 4);

 round
--------
 4.0000
(1 row)
That query is actually transformed by the parser to:

SELECT round(CAST (4 AS numeric), 4);




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-----Messaggio originale-----
Da: Justin Graf [mailto:justin@magwerks.com]
Inviato: venerdì 26 marzo 2010 18.44
A: Tom Lane
Cc: Gaietti, Mauro (SELEX GALILEO Guest, Italy); pgsql-general@postgresql.org
Oggetto: Re: [GENERAL] round(x) function

On 3/26/2010 12:12 PM, Tom Lane wrote:
> "Gaietti, Mauro \(SELEX GALILEO Guest, Italy\)"<mauro.gaietti@guests.selexgalileo.com>  writes:
>
>> This query:
>> select round(0.5), round(0.5::integer), round(0.5::bigint), round(
>> 0.5::float ), round( 0.5::double precision ),round(cast(0.5 as double
>> precision )),round(cast(0.5::double precision as numeric )); has strange
>> result:
>> 1 1 1 0 0 0 1
>>
>> Is this correct?
>>
> On most machines the float4/float8 operations follow the IEEE-754 spec's
> "round to nearest even" rule for such cases.  I think all the other ones
> are "add 0.5 and truncate" implementations.  It's unlikely we'll mess
> with either; and *highly* unlikely that we would change the float
> behavior since that's not even under our direct control (the hardware or
> libm is what's doing that).  There's some case to be made for making
> numeric and integer math do it the IEEE way, but I think that would
> displease at least as many people as it pleased ...
>
>             regards, tom lane
>
>
This topic keeps coming up every few months about rounding which way is
correct.  I would be in favor of adding a option to round() function to
specify method.

Leave the defaults as they are for backwards capability, meaning if no
option is passed to Round() it follows current methods.  I agree
changing how it works now would annoy many.

The option would be very simple something like this
     Round( 0.5, RoundToEven) = 0
    Round( -0.5, RoundToEven) = 0
     Round(0.5, RoundUp) = 1
     Round(-0.5, RoundUp) = 0
     Round(0.5, RoundDown) = 0
     Round(-0.5, RoundDown) = -1
     Round(0.5, RoundToZero) = 0
      Round(-0.5, RoundToZero) = 0

There are so many methods of rounding to choose from
http://en.wikipedia.org/wiki/Rounding


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