Обсуждение: sign function with INTERVAL?

Hi all,
Is there a good reason why the SIGN() function does not work with the
INTERVAL type? (It is only defined for numeric types.)
(http://www.postgresql.org/docs/9.5/static/functions-math.html)

select sign(-3); -- okay
select sign(interval '4 years'); -- ERROR:  function sign(interval)
does not exist

I'm trying to find a straightforward and reliable way to differentiate
positive, negative, and zero time intervals while handling NULL in the
same way as the SIGN() function.

What I have come up with is this rather inelegant and error-prone case
statement:

case when x is null then null x>interval '0' then +1 when x<interval
'0' then -1 when x=interval '0' then 0 end

Here's an example:

with t as (select interval '4 years 2 months' as x, interval '-1
minute 2 seconds' as y, interval '0' as z, null::interval as w)
select case when x is null then null when x>interval '0' then +1 when
x<interval '0' then -1 when x=interval '0' then 0 end,
       case when y is null then null when y>interval '0' then +1 when
y<interval '0' then -1 when y=interval '0' then 0 end,
       case when z is null then null when z>interval '0' then +1 when
z<interval '0' then -1 when z=interval '0' then 0 end,
       case when w is null then null when w>interval '0' then +1 when
w<interval '0' then -1 when w=interval '0' then 0 end
from t

Is there a more obvious way to do sign(interval)? Would it be
technically difficult to make it "just work"?

Thanks,
Dan

On 2016-04-13 Daniel Lenski <dlenski@gmail.com> wrote:
> Hi all,
> I'm trying to find a straightforward and reliable way to differentiate
> positive, negative, and zero time intervals while handling NULL in the
> same way as the SIGN() function.

I'm not sure that "positive time interval" is a thing. Witness:

 dakkar=> select interval '1 month - 30 days' > interval '0';
 ┌──────────┐
 │ ?column? │
 ├──────────┤
 │ f        │
 └──────────┘
 (1 row)

not positive? maybe it's negative?

 dakkar=> select interval '1 month - 30 days' < interval '0';
 ┌──────────┐
 │ ?column? │
 ├──────────┤
 │ f        │
 └──────────┘
 (1 row)

no, not negative either. Why? Well…

 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.

--
    Dakkar - <Mobilis in mobile>
    GPG public key fingerprint = A071 E618 DD2C 5901 9574
                                 6FE2 40EA 9883 7519 3F88
                        key id = 0x75193F88

Thrashing is just virtual crashing.

On 4/13/16 1:36 PM, Daniel Lenski wrote:
> Hi all,
> Is there a good reason why the SIGN() function does not work with the
> INTERVAL type? (It is only defined for numeric types.)
> (http://www.postgresql.org/docs/9.5/static/functions-math.html)

The only thing that comes to mind is you can get some strange
circumstances with intervals, like '-1 mons +4 days'. I don't think that
precludes sign() though.

> What I have come up with is this rather inelegant and error-prone case
> statement:

How is it error prone?

> case when x is null then null x>interval '0' then +1 when x<interval
> '0' then -1 when x=interval '0' then 0 end

You don't need to handle null explicitly. You could do

SELECT CASE WHEN x > interval '0' THEN 1 WHEN x < interval '0' THEN -1
WHEN x = interval '0' THEN 0 END

Or, you could do...

CREATE FUNCTION sign(interval) RETURNS int LANGUAGE sql STRICT IMMUTABLE
AS $$
SELECT CASE WHEN $1 > interval '0' THEN 1 WHEN x < interval '0' THEN -1
ELSE 0 END
$$;

That works because a STRICT function won't even be called if any of it's
inputs are NULL.


> Is there a more obvious way to do sign(interval)? Would it be
> technically difficult to make it "just work"?

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.
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com

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.

            regards, tom lane

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)

Thanks,
Dan