Is there a reliable way to extract years from a postgres interval? - postgresql

Question

In Postgres, if I do the following:
select (now() - created_at) from my_table
I get results like this:
854 days 12:04:50.29658
Whereas, if I do:
select age(now(), created_at) fro my_table
I get results like this:
2 years 4 mons 3 days 12:04:50.29658
According to pg_typeof(...) they are both of type interval
But if I try to extract the years:
select extract(years from age(now(), created_at)) from my_table
I get:
2
Whereas, with:
select extract(years from (now() - created_at)) from my_table
I get:
0
Is there a consistent way to extract the number of years from an interval value (no matter how it was generated)?
Note: I don't have write access to the db, so can't define stored procedures, etc. Needs to be a select statement.
------ UPDATE ------
justify_interval(...) was suggested below, but unfortunately it seems to be inaccurate in its calculations.
E.g:
select age('2018-01-03'::timestamp, '2016-01-05'::timestamp);
gives the correct answer:
1 year 11 mons 29 days
Whereas:
select justify_interval('2018-01-03'::timestamp - '2016-01-05'::timestamp);
gives:
2 years 9 days
I believe this is because it (incorrectly) assumes that all months have 30 days in them
(see justify_days
here: https://www.postgresql.org/docs/current/static/functions-datetime.html)

Answer 1

The function justify_interval does what you want. Use it in combination with EXTRACT to get the years:
SELECT EXTRACT(years FROM
justify_interval(INTERVAL '1 year 900 days 700 hours'));
date_part
-----------
3
(1 row)
If 30 days = 1 month isn't accurate enough for you, you'll have to use EXTRACT to get the number of days and divide by 365.25.
There is a theoretical limit how exact you can be, because the number of years in an interval somewhat depends on between which dates that interval is.
The two-element age function gives a precise result for the number of years between two dates.