Re: [SQL] Interval subtracting

Mark Dilger wrote:
> Tom Lane wrote:
> 
>> "Milen A. Radev" <milen@radev.net> writes:
>>
>>> Milorad Poluga напи�а:
>>>
>>>>> SELECT '10 years 1 mons 1 days'::interval - '9 years 10 mons 15 
>>>>> days'::interval
>>>>> ?column?        --------------- 3 mons -14 days
>>>>> Why not '2 mons  16 days' ? 
>>
>>
>>
>>> Please read the last paragraph in section 8.5.1.4 of the manual
>>> (http://www.postgresql.org/docs/8.1/static/datatype-datetime.html#AEN4775) 
>>>
>>> . It mentions the functions named "justify_days" and "justify_hours"
>>> that could do what you need.
>>
>>
>>
>> justify_days doesn't currently do anything with this result --- it
>> thinks its charter is only to reduce day components that are >= 30 days.
>> However, I think a good case could be made that it should normalize
>> negative days too; that is, the invariant on its result should be
>> 0 <= days < 30, not merely days < 30.  Similarly for justify_hours.
>> Comments anyone?  Patch anyone?
> 
> 
> Sure, if nobody objects to this change I can write the patch.
> 
> mark

I've modified the code and it now behaves as follows:
  select justify_days('3 months -12 days'::interval);    justify_days  ----------------   2 mons 18 days
  select justify_days('3 months -33 days'::interval);   justify_days  ---------------   1 mon 27 days
  select justify_hours('3 months -33 days -12 hours'::interval);         justify_hours  ---------------------------   3
mons-34 days +12:00:00
 
  select justify_days(justify_hours('3 months -33 days -12 hours'::interval));        justify_days
------------------------  1 mon 26 days 12:00:00
 
  select justify_hours('-73 hours'::interval);     justify_hours  -------------------   -4 days +23:00:00
  select justify_days('-62 days'::interval);     justify_days  ------------------   -3 mons +28 days


I find the last two results somewhat peculiar, as the new functionality pushes 
the negative values upwards (from hours to days, days to months).  Changing '-73 
hours' to '-3 days -1 hour' might be more intuitive?  The '-4 days +23 hours' is 
however consistent with the behavior in the other cases.

Thoughts?  I will package this up into a patch fairly soon.

mark