Re: sign function with INTERVAL? - Mailing list pgsql-hackers

On Wed, Apr 13, 2016 at 12:35 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Jim Nasby <Jim.Nasby@BlueTreble.com> writes:
>> Actually, after looking at the code for interval_lt, all that needs to
>> happen to add this support is to expose interval_cmp_internal() as a
>> strict function. It already does exactly what you want.
>
> interval_cmp() is already SQL-accessible.

Thanks! The interval_cmp() function does not appear in the 9.5 docs.
http://www.postgresql.org/docs/9.5/static/functions-datetime.html

On Wed, Apr 13, 2016 at 11:54 AM, Gianni Ceccarelli
<dakkar@thenautilus.net> wrote:
> I'm not sure that "positive time interval" is a thing. Witness:
>
> (snip)
>
>  dakkar=> select date '2016-03-01' + interval '1 month - 30 days';
>  ┌─────────────────────┐
>  │      ?column?       │
>  ├─────────────────────┤
>  │ 2016-03-02 00:00:00 │
>  └─────────────────────┘
>  (1 row)
>
> when used this way, it looks positive, but
>
>  dakkar=> select date '2016-02-01' + interval '1 month - 30 days';
>  ┌─────────────────────┐
>  │      ?column?       │
>  ├─────────────────────┤
>  │ 2016-01-31 00:00:00 │
>  └─────────────────────┘
>  (1 row)
>
> when used this way, it looks negative.
>
> So I suspect the reason SIGN() is not defined for intervals is that
> it cannot be made to work in the general case.

I hadn't considered this case of an interval like '1 month - 30 days',
which could be either positive or negative depending on the starting
date to which it is added.

interval_cmp's handling of this case seems surprising to me. If I've
got this right, it assumes that (interval '1 month' == interval '30
days') exactly:
http://doxygen.postgresql.org/backend_2utils_2adt_2timestamp_8c_source.html#l02515

Do I have that right? I'm having trouble envisioning an application
that would ever generate intervals that contain months and days
without opposite signs, but it's useful to know that such a corner
case could exist.

Given this behavior, the only 100% reliable way to check whether an
interval is forward, backwards, or zero would be to first add, and
then subtract, the starting point:

postgres=# select interval_cmp( (date '2016-02-01' + interval '1 month
- 30 days') - date '2016-02-01', interval '0' );
 interval_cmp
--------------
           -1
(1 row)

postgres=# select interval_cmp( (date '2016-04-01' + interval '1 month
- 30 days') - date '2016-04-01', interval '0' );
 interval_cmp
--------------
            0
(1 row)