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.

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"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

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Dhugael McLean wrote:

> 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, 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.

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