BUG #3260: Subtracting intervals
The following bug has been logged online:
Bug reference: 3260
Logged by: Dhugael McLean
Email address: box@yourtechonline.com
PostgreSQL version: 8.1.8
Operating system: FreeBSD
Description: Subtracting intervals
Details:
select '1 day'::interval - '55 minutes'::interval;
?column?
-----------------
1 day -00:55:00
If the interval periods are both minutes (hours - hours, days - days, etc),
this works fine. Days - minutes seems to fail. This should output 23:05:00.
In case this was some weird casting error, I checked:
select interval '1 day' - interval '55 minutes';
and that failed as well.
"Dhugael McLean" <box@yourtechonline.com> writes:
select '1 day'::interval - '55 minutes'::interval;
?column?
-----------------
1 day -00:55:00
If the interval periods are both minutes (hours - hours, days - days, etc),
this works fine. Days - minutes seems to fail. This should output 23:05:00.
No, this result is correct IMHO. Days and minutes are not interconvertible,
because there are not always 24 hours in a day. As an example using
EST5EDT zone (current US DST law):
regression=# select '2007-03-11'::timestamptz;
timestamptz
------------------------
2007-03-11 00:00:00-05
(1 row)
regression=# select '2007-03-11'::timestamptz + '1 day'::interval;
?column?
------------------------
2007-03-12 00:00:00-04
(1 row)
regression=# select ('2007-03-11'::timestamptz + '1 day'::interval) - '55 minutes'::interval;
?column?
------------------------
2007-03-11 23:05:00-04
(1 row)
regression=# select '2007-03-11'::timestamptz + ('1 day'::interval - '55 minutes'::interval);
?column?
------------------------
2007-03-11 23:05:00-04
(1 row)
regression=# select '2007-03-11'::timestamptz + '23:05:00'::interval;
?column?
------------------------
2007-03-12 00:05:00-04
(1 row)
Postgres gets the fourth case right, but would fail if we adopted
your approach, as shown by the fifth case.
regards, tom lane
Dhugael McLean wrote:
select '1 day'::interval - '55 minutes'::interval;
?column?
-----------------
1 day -00:55:00If the interval periods are both minutes (hours - hours, days - days, etc),
this works fine. Days - minutes seems to fail. This should output 23:05:00.
No, that answer would be wrong because not all days are 24 hours long
(think DST). You can use justify_hours() if you want to make that
assumption:
alvherre=# select justify_hours('1 day'::interval - '55 minutes'::interval);
justify_hours
---------------
23:05:00
(1 fila)
--
Alvaro Herrera http://www.CommandPrompt.com/
The PostgreSQL Company - Command Prompt, Inc.