BUG #7967: Wrong week number in extract function

noose@noose.pl wrote on 18.03.2013 09:23:

The following bug has been logged on the website:

Bug reference: 7967
Logged by: Pawel Kobylak
Email address: noose@noose.pl
PostgreSQL version: 9.1.3
Operating system: Debian
Description:

Hi,
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)

Result:
"2012-12-31";2012;1

Expected:
"2012-12-31";2012;53
OR
"2012-12-31";2013;1

This result is correct? Or that is little bug? :-)
Regards,
Pawel

Expected - or at least documented.

You are looking for "isoyear" instead of "year":

select extract(isoyear from date '2012-12-31')
Result: 2013

The same "option" is available for the to_char() function: IYYY vs. YYYY

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noose@noose.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

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