Finding total session time of a user in postgres - sql

Question

I am trying to create a query that will give me a column of total time logged in for each month for each user.
username | auth_event_type | time | credential_id
Joe | 1 | 2021-11-01 09:00:00 | 44
Joe | 2 | 2021-11-01 10:00:00 | 44
Jeff | 1 | 2021-11-01 11:00:00 | 45
Jeff | 2 | 2021-11-01 12:00:00 | 45
Joe | 1 | 2021-11-01 12:00:00 | 46
Joe | 2 | 2021-11-01 12:30:00 | 46
Joe | 1 | 2021-12-06 14:30:00 | 47
Joe | 2 | 2021-12-06 15:30:00 | 47
The auth_event_type column specifies whether the event was a login (1) or logout (2) and the credential_id indicates the session.
I'm trying to create a query that would have an output like this:
username | year_month | total_time
Joe | 2021-11 | 1:30
Jeff | 2021-11 | 1:00
Joe | 2021-12 | 1:00
How would I go about doing this in postgres? I am thinking it would involve a window function? If someone could point me in the right direction that would be great. Thank you.

Answer 1

Solution 1 partially working
Not sure that window functions will help you in your case, but aggregate functions will :
WITH list AS
(
SELECT username
, date_trunc('month', time) AS year_month
, max(time ORDER BY time) - min(time ORDER BY time) AS session_duration
FROM your_table
GROUP BY username, date_trunc('month', time), credential_id
)
SELECT username
, to_char (year_month, 'YYYY-MM') AS year_month
, sum(session_duration) AS total_time
FROM list
GROUP BY username, year_month
The first part of the query aggregates the login/logout times for the same username, credential_id, the second part makes the sum per year_month of the difference between the login/logout times. This query works well until the login time and logout time are in the same month, but it fails when they aren't.
Solution 2 fully working
In order to calculate the total_time per username and per month whatever the login time and logout time are, we can use a time range approach which intersects the session ranges [login_time, logout_time) with the monthly ranges [monthly_start_time, monthly_end_time) :
WITH monthly_range AS
(
SELECT to_char(m.month_start_date, 'YYYY-MM') AS month
, tsrange(m.month_start_date, m.month_start_date+ interval '1 month' ) AS monthly_range
FROM
( SELECT generate_series(min(date_trunc('month', time)), max(date_trunc('month', time)), '1 month') AS month_start_date
FROM your_table
) AS m
), session_range AS
(
SELECT username
, tsrange(min(time ORDER BY auth_event_type), max(time ORDER BY auth_event_type)) AS session_range
FROM your_table
GROUP BY username, credential_id
)
SELECT s.username
, m.month
, sum(upper(p.period) - lower(p.period)) AS total_time
FROM monthly_range AS m
INNER JOIN session_range AS s
ON s.session_range && m.monthly_range
CROSS JOIN LATERAL (SELECT s.session_range * m.monthly_range AS period) AS p
GROUP BY s.username, m.month
see the result in dbfiddle

Answer 2

Use the window function lag() with a partition it by credential_id ordered by time, e.g.
WITH j AS (
SELECT username, time, age(time, LAG(time) OVER w)
FROM t
WINDOW w AS (PARTITION BY credential_id ORDER BY time
ROWS BETWEEN 1 PRECEDING AND CURRENT ROW)
)
SELECT username, to_char(time,'yyyy-mm'),sum(age) FROM j
GROUP BY 1,2;
Note: the frame ROWS BETWEEN 1 PRECEDING AND CURRENT ROW is pretty much optional in this case, but it is considered a good practice to keep window functions as explicit as possible, so that in the future you don't have to read the docs to figure out what your query is doing.
Demo: db<>fiddle