I've attached a patch for this. Turns out there was a comment in the source explaining that there is no interval_abs because it's not clear what to return; but I think it's clear that if i is an interval the larger of i and -i should be considered to be the absolute value, the same as would be done for any type; essentially, if the type is orderable and has a meaningful definition of unary minus, the definition of abs follows from those.
This does have some odd effects, as was observed in the previous discussion pointed at by John Naylor above (for which thanks!). But those odd effects are not due to abs(interval) itself but rather due to the odd behaviour of interval, where values which compare equal to '0'::interval can change a timestamp when added to it. This in turn comes from what the interval data type is trying to do combined with the inherently complicated nature of our timekeeping system.
I have included in the test case some testing of what happens with '1 month -30 days'::interval, which is "equal" to '0'::interval.
At least one thing concerns me about my code: Given an interval i, I palloc() space to calculate -i; then either return that or the original input depending on the result of a comparison. Will I leak space as a result? Should I free the value if I don't return it?
In addition to adding abs(interval) and related @ operator, I would like to update interval_smaller and interval_larger to change < and > to <= and >= respectively. This is to make the min(interval) and max(interval) aggregates return the first of multiple distinct "equal" intervals, contrary to the current behaviour:
odyssey=> select max (i) from (values ('1 month -30 days'::interval), ('-1 month 30 days'))t(i);
max
------------------
-1 mons +30 days
(1 row)
odyssey=> select min (i) from (values ('1 month -30 days'::interval), ('-1 month 30 days'))t(i);
min
------------------
-1 mons +30 days
(1 row)
odyssey=>
GREATEST and LEAST already take the first value:
odyssey=> select greatest ('1 month -30 days'::interval, '-1 month 30 days');
greatest
----------------
1 mon -30 days
(1 row)
odyssey=> select least ('1 month -30 days'::interval, '-1 month 30 days');
least
----------------
1 mon -30 days
(1 row)
odyssey=>
