Re: [SQL] Interval subtracting

Tom Lane wrote:
> Mark Dilger <pgsql@markdilger.com> writes:
> 
>>The current code (without the patch) behaves as follows:
> 
> 
>>   select justify_days(justify_hours('1 month 95 days -36:00:00'::interval));
>>         justify_days
>>   -------------------------
>>    4 mons 4 days -12:00:00
> 
> 
> So?  If we liked the current behavior we wouldn't be discussing a patch...
> 
> My thought is that justify_hours should reduce that input to
> '1 month 93 days 12:00:00' and then justify_days would produce
> '4 months 3 days 12:00:00'.
> 
>             regards, tom lane
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 2: Don't 'kill -9' the postmaster
> 

Your proposal is that justify_hours borrows 24 hours from the days column in 
order to bring the -12 hours up to a positive 12 hours.  Should it only do that 
if the days column is a positive number?  What if it is negative?

I think we all agree on the following but nobody is explicitly saying so:
  select justify_days(justify_hours('2 days -12:00:00'::interval))        justify_days  -------------------------   1
day12:00:00
 
  select justify_days(justify_hours('-2 days -12:00:00'::interval))        justify_days  -------------------------   -2
days-12:00:00
 

Am I correct that the second case should still have negative hours?  If so, then 
justify_hours(...) needs to examine the sign of the days and months portion of 
the interval while performing its work.

mark