WITH

post AS (SELECT pos.id AS pos_id,

                pos.user_id AS usr_id,

                 DATE_TRUNC('month', pos.creation_date)::date AS mon

         FROM stackoverflow.posts AS pos

         JOIN (SELECT id,

               creation_date

     FROM stackoverflow.users) AS us ON us.id=pos.user_id

          WHERE DATE_TRUNC('month', us.creation_date)::date BETWEEN '2008-09-01' AND '2008-09-30'

           AND (pos.id IS NOT NULL AND EXTRACT(month FROM pos.creation_date)='12')),

posts AS (SELECT DISTINCT user_id,

           DATE_TRUNC('month', creation_date)::date AS pos_month

           FROM stackoverflow.posts)    

SELECT COUNT(DISTINCT post.pos_id) AS cnt_post,

             EXTRACT(MONTH FROM posts.pos_month) AS month

FROM posts JOIN post ON post.usr_id=posts.user_id

GROUP BY month

ORDER by month DESC