Optimizing sum() over(order by...) clause throwing 'resources exceeded' error - sql

Question

I'm computing a sessions table from event data from out website in BigQuery. The events table has around 12 million events (pretty small). After I add in the logic to create sessions, I want to sum all sessions and assign a global_session_id. I'm doing that using a sum()over(order by...) clause which is throwing a resources exceeded error. I know that the order by clause is causing all the data to be processed on a single node and that is causing the compute resources to be exceeded, but I'm not sure what changes I can make to my code to achieve the same result. Any work arounds, advice, or explanations are greatly appreciated.
with sessions_1 as ( /* Tie a visitor's last event and last campaign to current event. */
select visitor_id as session_user_id,
sent_at,
context_campaign_name,
event,
id,
LAG(sent_at,1) OVER (PARTITION BY visitor_id ORDER BY sent_at) as last_event,
LAG(context_campaign_name,1) OVER (PARTITION BY visitor_id ORDER BY sent_at) as last_event_campaign_name
from tracks_2
),
sessions_2 as ( /* Flag events that begin a new session. */
select *,
case
when context_campaign_name != last_event_campaign_name
or context_campaign_name is null and last_event_campaign_name is not null
or context_campaign_name is not null and last_event_campaign_name is null
then 1
when unix_seconds(sent_at)
- unix_seconds(last_event) >= (60 * 30)
or last_event is null
then 1
else 0
end as is_new_session
from sessions_1
),
sessions_3 as ( /* Assign events sessions numbers for total sessions and total user sessions. */
select id as event_id,
sum(is_new_session) over (order by session_user_id, sent_at) as global_session_id
#sum(is_new_session) over (partition by session_user_id order by sent_at) as user_session_id
from materialized_result_of_sessions_2_query
)
select * from sessions_3

Answer 1

If might help if you defined a CTE with just the sessions, rather than at the event level. If this works:
select session_user_id, sent_at,
row_number() over (order by session_user_id, sent_at) as global_session_id
from materialized_result_of_sessions_2_query
where is_new_session
group by session_user_id, sent_at;
If that doesn't work, you can construct the global id:
You can join this back to the original event-level data and then use a max() window function to assign it to all events. Something like:
select e.*,
max(s.global_session_id) over (partition by e.session_user_id order by e.event_at) as global_session_id
from events e left join
(<above query>) s
on s.session_user_id = e.session_user_id and s.sent_at = e.event_at;
If not, you can do:
select us.*, us.user_session_id + s.offset as global_session_id
from (select session_user_id, sent_at,
row_number() over (partition by session_user_id order by sent_at) as user_session_id
from materialized_result_of_sessions_2_query
where is_new_session
) us join
(select session_user_id, count(*) as cnt,
sum(count(*)) over (order by session_user_id) - count(*) as offset
from materialized_result_of_sessions_2_query
where is_new_session
group by session_user_id
) s
on us.session_user_id = s.session_user_id;
This might still fail if almost all users are unique and the sessions are short.