Row number with condition - sql

Question

I want to increase the row number of a partition based on a condition. This question refers to the same problem, but in my case, the column I want to condition on is another window function.
I want to identify the session number of each user (id) depending on how long ago was their last recorded action (ts).
My table looks as follows:
id ts
1 2022-08-01 09:00:00 -- user 1, first session
1 2022-08-01 09:10:00
1 2022-08-01 09:12:00
1 2022-08-03 12:00:00 -- user 1, second session
1 2022-08-03 12:03:00
2 2022-08-01 11:04:00 -- user 2, first session
2 2022-08-01 11:07:00
2 2022-08-25 10:30:00 -- user 2, second session
2 2022-08-25 10:35:00
2 2022-08-25 10:36:00
I want to assign each user a session identifier based on the following conditions:
If the user's last action was 30 or more minutes ago (or doesn't exist), then increase (or initialize) the row number.
If the user's last action was less than 30 minutes ago, don't increase the row number.
I want to get the following result:
id ts session_id
1 2022-08-01 09:00:00 1
1 2022-08-01 09:10:00 1
1 2022-08-01 09:12:00 1
1 2022-08-03 12:00:00 2
1 2022-08-03 12:03:00 2
2 2022-08-01 11:04:00 1
2 2022-08-01 11:07:00 1
2 2022-08-25 10:30:00 2
2 2022-08-25 10:35:00 2
2 2022-08-25 10:36:00 2
If I had a separate column with the seconds since their last session, I could simply add 1 to each user's partitioned sum. However, this column is a window function itself. Hence, the following query doesn't work:
select
id
,ts
,extract(
epoch from (
ts - lag(ts, 1) over(partition by id order by ts)
)
) as seconds_since -- Number of seconds since last action (works well)
,sum(
case
when coalesce(
extract(
epoch from (
ts - lag(ts, 1) over (partition by id order by ts)
)
), 1800
) >= 1800 then 1
else 0 end
) over (partition by id order by ts) as session_id -- Window inside window (crashes)
from
t
order by
id
,ts
ERROR: Aggregate window functions with an ORDER BY clause require a frame clause

Answer 1

Use LAG() window function to get the previous ts of each row and create flag column indicating if the difference between the 2 timestamps is greater than 30 minutes.
Then use SUM() window function over that flag:
SELECT
id
,ts
,SUM(flag) OVER (
PARTITION BY id
ORDER BY ts
rows unbounded preceding -- necessary in aws-redshift
) as session_id
FROM (
SELECT
*
,COALESCE((LAG(ts) OVER (PARTITION BY id ORDER BY ts) < ts - INTERVAL '30 minute')::int, 1) flag
FROM
tablename
) t
;
See the demo.