Re: BUG #3431: age() gets the days wrong

"Pelle Johansson" <pelle(at)morth(dot)org> writes:
> The age() function seem to work by first counting months until less than a
> month remains to to the second argument, then counting days left. This
> doesn't give the correct result, as shown by this example:

> # select column1, age(column1, '2006-11-02'), date '2006-11-02' +
> age(column1, '2006-11-02') from (values ('2007-01-31'::date),
> ('2007-02-01')) as alias;
> column1 | age | ?column?
> ------------+----------------+---------------------
> 2007-01-31 | 2 mons 29 days | 2007-01-31 00:00:00
> 2007-02-01 | 2 mons 29 days | 2007-01-31 00:00:00
> (2 rows)

I took another look at this example. I believe what is actually going
wrong here is that when timestamp_age converts a month into an
equivalent number of days, it uses the number of days in the first
month of the interval it's dealing with (ie, the month containing
the earlier of the two dates). This is just wrong, because interval
addition adds months first and then days. The appropriate conversion
to use is actually the length of the next-to-last month of the interval.

As an example, 8.2 and CVS HEAD produce

regression=# select age('2007-03-14', '2007-02-15');
age
---------
27 days
(1 row)

which is reasonable, but

regression=# select age('2007-04-14', '2007-02-15');
age
---------------
1 mon 27 days
(1 row)

is not so reasonable, nor is

regression=# select age('2007-03-14', '2007-01-15');
age
---------------
1 mon 30 days
(1 row)

If we change the code to use the next-to-last month of the interval
then these two cases produce '1 mon 30 days' and '1 mon 27 days'
respectively.

Another problem is that the code isn't doing the propagate-to-next-field
bit for negative fractional seconds. Hence it produces

regression=# select age('2007-02-14 01:00:00', '2007-01-15 01:00:00.4');
age
----------------------
30 days -00:00:00.40
(1 row)

which is maybe not incorrect, but certainly fairly inconsistent with

regression=# select age('2007-02-14 01:00:00', '2007-01-15 01:00:01');
age
------------------
29 days 23:59:59
(1 row)

Hence I propose the attached patch. This does not change any existing
regression test outputs, but it does change the example given in the
documentation: age(timestamp '2001-04-10', timestamp '1957-06-13')
will now produce '43 years 9 mons 28 days' not 27 days. Which actually
is correct if you try to add back the result to timestamp '1957-06-13'.
It also appears to fix Palle's example:

regression=# select column1, age(column1, '2006-11-02'), date '2006-11-02' +
age(column1, '2006-11-02') from (values ('2007-01-31'::date),
('2007-02-01')) as alias;
column1 | age | ?column?
------------+----------------+---------------------
2007-01-31 | 2 mons 29 days | 2007-01-31 00:00:00
2007-02-01 | 2 mons 30 days | 2007-02-01 00:00:00
(2 rows)

As I said earlier, I'm worried about changing the behavior of a function
that's been around for so long, so I'm disinclined to back-patch this.
But it seems like a reasonable change to make in 8.3. Comments?

regards, tom lane