Assign session IDs with a running sum

WITH flagged AS (
  SELECT user_id,
         event_time,
         CASE
           WHEN event_time - LAG(event_time) OVER (
             PARTITION BY user_id
             ORDER BY event_time
           ) > INTERVAL '30 minutes'
           OR LAG(event_time) OVER (
             PARTITION BY user_id
             ORDER BY event_time
           ) IS NULL
           THEN 1 ELSE 0
         END AS is_new_session
  FROM user_events
)
SELECT user_id,
       event_time,
       SUM(is_new_session) OVER (
         PARTITION BY user_id
         ORDER BY event_time
         ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
       ) AS session_id
FROM flagged;