Quelle: https://piwik.org/faq/troubleshooting/#faq_42DELETE piwik_log_visit, piwik_log_link_visit_action
FROM piwik_log_visit INNER JOIN piwik_log_link_visit_action
WHERE piwik_log_visit.idvisit = piwik_log_link_visit_action.idvisit
AND visit_first_action_time <= DATE_SUB(CURDATE(), INTERVAL 30 DAY);
OPTIMIZE TABLE piwik_log_visit, piwik_log_link_visit_action;