Re: Date conversion using day of week

On 03/29/2011 04:24 PM, Adrian Klaver wrote:
> ...
> Well the strange part is only fails for SUN:...
> test(5432)aklaver=>select to_date('2011-13-SUN', 'IYYY-IW-DY');
> to_date
> ------------
> 2011-03-28
>
> ...
You specified Sunday as the day but the date returned is a Monday. I
would categorize that as a bug. (Hackers cc'd). Since Sunday is the last
day of an ISO week, it should have returned 2011-04-03.

My first inclination without consulting source or morning coffee is that
PostgreSQL is seeing Sunday as day zero. Note that while:
select to_date('2011-13-1', 'IYYY-IW-ID');
to_date
------------
2011-03-28

So does:
steve=# select to_date('2011-13-0', 'IYYY-IW-ID');
to_date
------------
2011-03-28

So something isn't right. All sorts of other stuff is allowed as well -
I don't know if that's by design or not:

steve=# select to_date('2011-13--23', 'IYYY-IW-ID');
to_date
------------
2011-03-04

steve=# select to_date('2011-13-56', 'IYYY-IW-ID');
to_date
------------
2011-05-22

> Agreed, maintaining ISO arguments across the board is the way to go:
>
> Monday
> select to_date('2011-13-1', 'IYYY-IW-ID');...
We have to distinguish Gregorian and ISO days when represented as an
integer since they define the start-of-week differently. Same with year.
I don't think I've ever seen and ISO-week-date written as 2011-13-SUN
but it *does* define a distinct date (which is not Monday). And even if
PostgreSQL were updated to throw an error on that mix of formats it
still leaves the problem of ISO day-of-week equal to zero.

Cheers,
Steve