WITH events AS
  (SELECT DATE_TRUNC('month', event_time)::date AS event_month,
          COUNT(DISTINCT event_id) AS events_cnt
   FROM tools_shop.events e
   JOIN tools_shop.orders o ON e.user_id = o.user_id
   WHERE event_name = 'view_item'
   GROUP BY event_month)
SELECT event_month,
       events_cnt,
       SUM(events_cnt) OVER (ORDER BY event_month) AS cum_sum
FROM events;