MySQL към PostgreSQL:Как да променя тази SQL заявка?

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) 

Ето актуалното ръководство
http://www.postgresql.org/docs/current/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT

Между другото:Това е стандартен (ANSI) SQL, който работи и на MySQL.