Re: [SQL] Interval subtracting

"Milen A. Radev" <milen@radev.net> writes:
> Milorad Poluga написа:
>>> 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?
        regards, tom lane