Re: BUG #18348: Inconsistency with EXTRACT([field] from INTERVAL);

jian he  writes:
> you already mentioned "Not all fields are valid for every input data type".
> interval data type don't even have a unit "quarter",
> so the following should generate an error?
> select extract(quarter from interval '2011 year 12 month 48 hour
> 1005min 2 sec 11 ms');

I'm not especially persuaded by that reasoning.  Intervals don't have
century or millisecond fields either, but we allow extracting those.

If your argument is that we shouldn't allow it because we don't take
the input INTERVAL '1 quarter', I'd be much more inclined to add that
as valid input than to take away existing extract functionality.
But I'm dubious about the proposition that extract's list of valid
fields should exactly match the set of allowed input units.  The
semantics aren't really the same (as per the '80 minutes' example)
so such a restriction doesn't seem to have much basis in reality.

			regards, tom lane