Re: [PATH] Correct negative/zero year in to_date/to_timestamp

On 3/11/16, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Sun, Feb 28, 2016 at 9:38 PM, Vitaly Burovoy
> <vitaly(dot)burovoy(at)gmail(dot)com> wrote:
>>> However, I'm not sure we ought to tinker with the behavior in this
>>> area. If YYYY-MM-DD is going to accept things that are not of the
>>> format YYYY-MM-DD, and I'd argue that -1-06-01 is not in that format,
>>
>> It is not about format, it is about values.
>
> I disagree. In a format like "-1-06-01", you want the first minus to
> indicate negation and the other two to be a separator. That's not
> very far away from wanting the database to read your mind.

It is not my wish. The database does it just now:
postgres=# SELECT to_date('-1-06-01', 'YYYY');
to_date
---------------
0002-01-01 BC
(1 row)

>> Because it is inconvenient a little. If one value ("-2345") is passed,
>> another one ("2346 BC") is got. In the other case a programmer must
>> check for negative value, and if so change a sign and add "BC" to the
>> format. Moreover the programmer must keep in mind that it is not
>> enough to have usual date format "DD/MM/YYYY", because sometimes there
>> can be "BC" part.
>
> Yeah, well, that's life. You can write an alternative function to
> construct dates that works the way you like, and that may well be a
> good idea. But I think *this* change is not a good idea, and
> accordingly I vote we reject this patch.

My wish is to make the behavior be consistent.
Since there are two reverse functions ("extract" and "to_date"
["to_timestamp" in fact is the same]), I expect that is described as
"year" ("year"-"YYYY") means the same thing in both of them, the same
with pairs "isoyear"-"IYYY", "dow"-"DDD", "isodow"-"IDDD", etc.

Now "year" is _not_ the same as "YYYY" (but it cat be so according to
the documentation: there is no mentioning of any ISO standard),
whereas "isoyear" _is_ the same:
postgres=# SELECT y, to_date(y, 'YYYY')YYYY,to_date(y, 'IYYY')IYYY
postgres-# FROM(VALUES('-1-06-01'))t(y);
y | yyyy | iyyy
----------+---------------+---------------
-1-06-01 | 0002-01-01 BC | 0002-01-01 BC
(1 row)

and
postgres=# SELECT y, date_part('year', y)YYYY,date_part('isoyear', y)IYYY
postgres-# FROM(VALUES('0002-06-01 BC'::date))t(y);
y | yyyy | iyyy
---------------+------+------
0002-06-01 BC | -2 | -1
(1 row)

P.S.: proposed patch changes IYYY as well, but it is easy to fix it
and I'm ready to do it after finding a consensus.
--
Best regards,
Vitaly Burovoy