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)
Voici le manuel à jour
http://www.postgresql.org/docs/current/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT
Au fait :il s'agit d'un SQL standard (ANSI) qui fonctionne également sur MySQL.