Da MySQL a PostgreSQL:come modificare questa query SQL?

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) 

Ecco il manuale aggiornato
http://www.postgresql.org/docs/current/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT

A proposito:questo è un SQL standard (ANSI) che funziona anche su MySQL.