Missing documentation for uses of extract(field from interval)

(Note: I'm not subscribed to pgsql-docs, so please CC me on any
replies.  Thanks!)

I pulled up the documentation today to see if I could get just the
number of days in an interval using EXTRACT.  The answer provided was
vague: While you can use EXTRACT(field from interval) (shown in table
9-27), section 9.9.1 only lists:

day

    The day (of the month) field (1 - 31)

    SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
    Result: 16



It turns out that EXTRACT(day FROM interval) does work as expected --
so perhaps the documentation should be updated to something like:

day

    The day (of the month) field (1 - 31), or the number of days in the interval

    SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
    Result: 16

    SELECT EXTRACT(DAY FROM INTERVAL '42 days 21:00:00');
    Result: 42

Likewise for month, year, century and maybe some others.  (hour,
minute, and *second{s?} probably don't need updated text).

There also should probably be some warning in here about EXTRACT on
intervals however.  Namely cases like this:

SELECT EXTRACT(YEAR FROM INTERVAL '400 days')  --> Result: 0
(because the interval is '400 days' not '1 year 35 days')
SELECT EXTRACT(DAY FROM INTERVAL '1 year')  --> Result: 0    (because
the interval is '1 year' not '365 days')
SELECT EXTRACT(DAY FROM INTERVAL '1 month')  --> Result: 0

Though, this works:
SELECT EXTRACT(CENTURY FROM INTERVAL '99 years')  --> Result: 0
SELECT EXTRACT(CENTURY FROM INTERVAL '100 years')  --> Result: 1
--
Daniel Grace
AGE, LLC
System Administrator and Software Developer