WITH top_user as

    (SELECT user_id

    FROM stackoverflow.posts

    GROUP BY user_id

    ORDER BY COUNT(distinct id) DESC

    LIMIT 1)

SELECT EXTRACT(WEEK FROM creation_date) as week, MAX(creation_date)

FROM top_user tp

JOIN stackoverflow.posts p ON tp.user_id=p.user_id

WHERE creation_date::date BETWEEN '2008-10-01' AND '2008-10-31'

GROUP BY EXTRACT(WEEK FROM creation_date)