SELECT

actor_name as user,

COUNT(*) as cnt

FROM revision_userindex

JOIN change_tag

ON rev_id = ct_rev_id

JOIN actor_revision

ON actor_id = rev_actor

WHERE ct_tag_id = 

(SELECT ctd_id FROM change_tag_def

WHERE ctd_name = 'mw-rollback')

GROUP BY user

ORDER BY cnt DESC

LIMIT 500;