inconsistency between month, day, second representation of interval data type - postgresql

Question

I understand why postgresql uses month,day and second fields to representate the sql interval datatype. A month is not always the same length and a day can have 23, 24 or 25 hours if a daylight savings time adjustment is involved. this is from postgresql documentation.
But I then do not understand why this is not consequently handled both for months and days. see the following query which calculates an exact interval where the number of seconds between two points in time is exactly calculatable:
select ('2017-01-01'::timestamp-'2016-01-01'::timestamp); -->366 days.
postgresql chooses to give a result in days. not in months and not in seconds.
But why is the result days and not seconds? it is NOT defined how long days are (they can be 23,24 or 25 hours long). so why does he not give output in seconds?
Then since the length of months is also not defined, why doesn't postgresql give an output of 12 month instead of 366 days?
He does not care that the length of days is not defined, but obviously he cares that the length of month is not defined.
Why this asymmetrie?
For further explanation, see this query:
select ('10 days'::interval-'24 hours'::interval); --> 10 days -24:00:00
you see that postgresql correctly refuses to answer with 9 days. He is pretty aware of the problem that days and hours cannot be interchanged. But then again why does the first query return days?

Answer 1

I can't answer your question, but I think I can point you in the right direction. I think the book SQL-99 Complete, Really is the most accessible source for understanding SQL intervals. It's available online: https://mariadb.com/kb/en/sql-99/08-temporal-values/.
SQL standards describe two kinds of intervals: year-month intervals and day-time intervals. It does this to prevent month parts and day parts from appearing in the same interval, because, as you already know, the number of days in a month is ambiguous. The number of days in the interval '3' month depends on which three months you're talking about.
I think this is the verbose, standard SQL way to write your first query.
select cast(timestamp '2017-01-01' - timestamp '2016-01-01' as interval day to hour) as new_column;
new_column
interval day to hour
--
366 days
I suspect that you'll find that SQL standards have rules for what a SQL dbms is supposed to do when things like interval day to hour are omitted. PostgreSQL might or might not follow those rules.
postgresql chooses to give a result in days. not in months and not in seconds.
Standard SQL prevents month parts and day parts from appearing in the same interval. Also, the range of valid seconds is from 0 to 59.
select interval '59' second;
interval
interval second
--
00:00:59
select interval '60' second;
interval
interval second
--
00:01:00