abs function for interval

Hi,

Sometimes you want to answer if a difference between two timestamps is
lesser than x minutes but you are not sure which timestamp is greater
than the other one (to obtain a positive result -- it is not always
possible). However, if you cannot obtain the absolute value of
subtraction, you have to add two conditions.

The attached patch implements abs function and @ operator for
intervals. The following example illustrates the use case:

postgres=# create table xpto (a timestamp, b timestamp);
CREATE TABLE
postgres=# insert into xpto (a, b) values(now(), now() - interval '1
day'),(now() - interval '5 hour', now()),(now() + '3 hour', now());
INSERT 0 3
postgres=# select *, a - b as t from xpto;
a | b | t
----------------------------+----------------------------+-----------
2019-10-31 22:43:30.601861 | 2019-10-30 22:43:30.601861 | 1 day
2019-10-31 17:43:30.601861 | 2019-10-31 22:43:30.601861 | -05:00:00
2019-11-01 01:43:30.601861 | 2019-10-31 22:43:30.601861 | 03:00:00
(3 rows)

postgres=# select *, a - b as i from xpto where abs(a - b) < interval '12 hour';
a | b | i
----------------------------+----------------------------+-----------
2019-10-31 17:43:30.601861 | 2019-10-31 22:43:30.601861 | -05:00:00
2019-11-01 01:43:30.601861 | 2019-10-31 22:43:30.601861 | 03:00:00
(2 rows)

postgres=# select @ interval '1 years -2 months 3 days 4 hours -5
minutes 6.789 seconds' as t;
t
-----------------------------
10 mons 3 days 03:55:06.789
(1 row)

--
Euler Taveira Timbira -
http://www.timbira.com.br/
PostgreSQL: Consultoria, Desenvolvimento, Suporte 24x7 e Treinamento

Hi,

On 2019-10-31 23:20:07 -0300, Euler Taveira wrote:
> diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
> index 1dc4c820de..a6b8b8c221 100644
> --- a/src/backend/utils/adt/timestamp.c
> +++ b/src/backend/utils/adt/timestamp.c
> @@ -2435,6 +2435,23 @@ interval_cmp(PG_FUNCTION_ARGS)
> PG_RETURN_INT32(interval_cmp_internal(interval1, interval2));
> }
>
> +Datum
> +interval_abs(PG_FUNCTION_ARGS)
> +{
> + Interval *interval = PG_GETARG_INTERVAL_P(0);
> + Interval *result;
> +
> + result = palloc(sizeof(Interval));
> + *result = *interval;
> +
> + /* convert all struct Interval members to absolute values */
> + result->month = (interval->month < 0) ? (-1 * interval->month) : interval->month;
> + result->day = (interval->day < 0) ? (-1 * interval->day) : interval->day;
> + result->time = (interval->time < 0) ? (-1 * interval->time) : interval->time;
> +
> + PG_RETURN_INTERVAL_P(result);
> +}
> +

Several points:

1) I don't think you can do the < 0 check on an elementwise basis. Your
code would e.g. make a hash out of abs('1 day -1 second'), by
inverting the second, but not the day (whereas nothing should be
done).

It'd probably be easiest to implement this by comparing with a 0
interval using inteval_lt() or interval_cmp_internal().

2) This will not correctly handle overflows, I believe. What happens if you
do SELECT abs('-2147483648 days'::interval)? You probably should
reuse interval_um() for this.

> --- a/src/test/regress/expected/interval.out
> +++ b/src/test/regress/expected/interval.out
> @@ -927,3 +927,11 @@ select make_interval(secs := 7e12);
> @ 1944444444 hours 26 mins 40 secs
> (1 row)
>
> +-- test absolute operator
> +set IntervalStyle to postgres;
> +select @ interval '1 years -2 months 3 days 4 hours -5 minutes 6.789 seconds' as t;
> + t
> +-----------------------------
> + 10 mons 3 days 03:55:06.789
> +(1 row)
> +
> diff --git a/src/test/regress/sql/interval.sql b/src/test/regress/sql/interval.sql
> index bc5537d1b9..8f9a2bda29 100644
> --- a/src/test/regress/sql/interval.sql
> +++ b/src/test/regress/sql/interval.sql

> @@ -308,3 +308,7 @@ select make_interval(months := 'NaN'::float::int);
> select make_interval(secs := 'inf');
> select make_interval(secs := 'NaN');
> select make_interval(secs := 7e12);
> +
> +-- test absolute operator
> +set IntervalStyle to postgres;
> +select @ interval '1 years -2 months 3 days 4 hours -5 minutes 6.789 seconds' as t;
> --
> 2.11.0

This is not even remotely close to enough tests. In your only test abs()
does not change the value, as there's no negative component (the 1 year
-2 month result in a positive 10 months, and the hours, minutes and
seconds get folded together too).

At the very least a few boundary conditions need to be tested (see b)
above), a few more complicated cases with different components being
of different signs, and you need to show the values with and without
applying abs().

Greetings,

Andres Freund

Em qui, 31 de out de 2019 às 23:45, Andres Freund <andres(at)anarazel(dot)de> escreveu:
>
> 1) I don't think you can do the < 0 check on an elementwise basis. Your
> code would e.g. make a hash out of abs('1 day -1 second'), by
> inverting the second, but not the day (whereas nothing should be
> done).
>
> It'd probably be easiest to implement this by comparing with a 0
> interval using inteval_lt() or interval_cmp_internal().
>
Hmm. Good idea. Let me try it.

--
Euler Taveira Timbira -
http://www.timbira.com.br/
PostgreSQL: Consultoria, Desenvolvimento, Suporte 24x7 e Treinamento

On Fri, Nov 01, 2019 at 12:48:50AM -0300, Euler Taveira wrote:
> Hmm. Good idea. Let me try it.

Marked as RwF, as this has not been updated in four weeks. Please
feel free to resubmit later once you have an updated version.
--
Michael