with t1 as (

SELECT  *,
lag(timestamp) over(partition by user_id order by timestamp) as timestamp_lag,
if(TIMESTAMP_DIFF(timestamp, lag(timestamp) over(partition by user_id order by timestamp),minute)<30,0,1) as session_start
FROM `table_name`

), t2 as (

select *,
sum(session_start) over(partition by user_id order by timestamp) as session_number
from t1

), t3 as (

select *,
concat(user_id,'_',cast(session_number as string)) as session_id,
min(timestamp) over(partition by user_id, session_number) as start_session_time,
max(timestamp) over(partition by user_id, session_number) as end_session_time
from t2

)

select user_id, session_id, start_session_time, end_session_time
from t3
group by 1,2,3,4