Funnily enough I was having a conversation with my wife on exactly this as I
opened your email.
If the Wikipedia article is to be trusted, the following seems fitting:
SELECT EXTRACT(ORDINAL DECADE FROM '2020-01-01'::date);
date_part
---
201
And the default:
SELECT EXTRACT(CARDINAL DECADE FROM '2020-01-01'::date);
date_part
---
202
On Tuesday, 31 December 2019, 16:36:02 GMT, Bruce Momjian
wrote:
Does the next decade start on 2020-01-01 or 2021-01-01? Postgres says
it start on the former date:
SELECT EXTRACT(DECADE FROM '2019-01-01'::date);
date_part
---
201
SELECT EXTRACT(DECADE FROM '2020-01-01'::date);
date_part
---
202
but the _century_ starts on 2001-01-01, not 2000-01-01:
SELECT EXTRACT(CENTURY FROM '2000-01-01'::date);
date_part
---
20
SELECT EXTRACT(CENTURY FROM '2001-01-01'::date);
date_part
---
21
That seems inconsistent to me. /pgtop/src/backend/utils/adt/timestamp.c
has this C comment:
* what is a decade wrt dates? let us assume that decade 199
* is 1990 thru 1999... decade 0 starts on year 1 BC, and -1
* is 11 BC thru 2 BC...
FYI, these two URLs suggest the inconsistency is OK:
https://www.timeanddate.com/calendar/decade.html
https://en.wikipedia.org/wiki/Decade
--
Bruce Momjian http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
