Re: BUG #7967: Wrong week number in extract function

noose(at)noose(dot)pl writes:
> I'm running that query and result is ... unexpected for me...

> Query:
> select '2012-12-31', EXTRACT(year from '2012-12-31'::date), EXTRACT(week
> from '2012-12-31'::date)

It's correct, because "week" follows the ISO definition of week
counting. According to that, 2012-12-31 falls in the first week of 2013.
(I have no idea how ISO arrived at their definition, but this is what it
says: weeks start on Mondays, and the first week of a year is the one
containing January 4.)

You should usually use isoyear when you are using week, so that the
results sync up.

This is all explained in
http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT
although I notice that the explanation of "week" fails to show
explicitly that late-December dates can be considered to fall into the
next year. I'll go fix that.

regards, tom lane