Re: Casting dates

>>>>> "JB" == Josh Berkus <josh@agliodbs.com> writes:
   JB> Joe,   >> Does anyone know if it's possible to calculate the number of   >> days (or weeks, seconds, minutes, or
what-have-you)in a given   >> interval without casting to EPOCH and performing manipulations   >> there?   >>    >>
PostgreSQLusually returns intervals created by subtraction in   >> days and smaller fragments, I would like to know if
it's  >> possible for it to return years. (Integer division of the   >> number of days by 365 would produce an
almost-correctresult,   >> but it's rather inelegant.) 
   JB> Unfortunately, you've hit one of Postgres implementation   JB> limits.  Multiplying and dividing INTERVAL
values,while   JB> covered by the SQL92 spec, has not been completely implemented   JB> in Postgres.  That is, it
shouldbe possible for you to: 
   JB> SELECT INTERVAL '3 days 5 hours' / INTERVAL '1 minute'
   JB> However, implementing this has been a challenge for all SQL-DB   JB> designers.  I don't know of any DB that
doeshandle INTERVAL   JB> division. 
   JB> It's a surmountable challenge, though, but requires somebody   JB> to take the lead in a) working out the logic,
andb) writing   JB> the code to implement it.  A) is not a trivial task, either   JB> ... while 60 minutes / 1 second
isobvious, how about 4 months   JB> / 4 days?  Months are not constant in length. 

For the purposes of this application (because it deals in relatively
short intervals, it's a calendaring application), I just divide the
day count by 365 and throw away the remainder.

For example,

select date_part('day', (timestamp 'jan 1, 1996' -                       timestamp 'jan 1, 1992'));

is 1461 days, or 1+365*4 (1992 is a leap year). Integer division of
1461/365 produces the correct year count. This doesn't work for longer
intervals:

cal=> select date_part('day', (timestamp 'jan 1, 2600'              - timestamp 'jan 1, 1000'))/365;    ?column?
------------------1601.06301369863  <-- an error of +1 year, due to accumulated leap days

Obviously, this naive method won't work for astronomical time
intervals, but for my purposes, few people have calendars that
stretch across millennia.

--Joe