MySQL naar PostgreSQL:hoe wijzig ik deze SQL-query?

SELECT 
  extract(MONTH from created_at) AS month, 
  extract(YEAR from created_at) AS year 
FROM users 
GROUP BY extract(MONTH from created_at), extract(YEAR from created_at) 
ORDER BY extract(MONTH from created_at), extract(YEAR from created_at) 

Hier is de up-to-date handleiding
http://www.postgresql.org/docs/current/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT

Trouwens:dit is standaard (ANSI) SQL die ook op MySQL werkt.