Postgres 15

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CREATE TABLE myTable
(
    rowid  int,
    userid      text,
    webid         text,
    ts timestamp
);

INSERT INTO myTable
    ("rowid", "userid", "webid", "ts")
VALUES ('1', 'A', '34', '2023-01-31 16:34:49.000'),
       ('2', 'A', '73', '2023-01-31 16:34:50.000'),
       ('3', 'A', '97', '2023-01-31 16:34:58.000'),
       ('4', 'A', '17', '2023-01-31 17:35:02.000'),
       ('5', 'A', '17', '2023-01-31 17:35:07.000'),
       ('6', 'A', '17', '2023-01-31 17:35:18.000'),
       ('7', 'A', '17', '2023-01-31 17:35:30.000'),
       ('8', 'A', '1', '2023-01-31 17:35:37.000'),
       ('9', 'A', '1', '2023-01-31 17:35:38.000'),
       ('10', 'A', '77', '2023-01-31 17:35:41.000'),
       ('11', 'A', '77', '2023-01-31 17:35:42.000'),
       ('12', 'A', '15', '2023-01-31 17:35:42.000'),
       ('13', 'A', '44', '2023-01-31 17:35:42.000'),
       ('14', 'A', '1', '2023-01-31 17:35:42.000'),
       ('15', 'A', '77', '2023-01-31 17:35:45.000'),
       ('16', 'A', '44', '2023-01-31 17:35:45.000'),
       ('17', 'A', '1', '2023-01-31 17:37:10.000'),
       ('18', 'A', '1', '2023-01-31 17:37:12.000'),
       ('19', 'A', '77', '2023-01-31 17:37:14.000'),
       ('20', 'A', '77', '2023-01-31 17:52:14.000'),
       ('21', 'A', '77', '2023-01-31 18:12:14.000'),
       ('22', 'A', '77', '2023-01-31 18:45:14.000'),
       ('23', 'A', '77', '2023-01-31 18:55:15.000'),
       ('1', 'B', '33', '2023-01-31 06:37:15.000'),
       ('2', 'B', '56', '2023-01-31 06:40:15.000')
  ;

select rowid, userid, webid, session_id, ts, min(ts) over(partition by userid, session_id order by ts) first_ts
from (
    select t.*,
        1 + count(*) 
            filter(where webid != lag_webid or ts > lag_ts + interval '30' minute) 
            over(partition by userid order by ts) session_id
    from (
        select t.*, 
            lag(webid, 1, webid) over(partition by userid order by ts) lag_webid,
            lag(ts, 1, ts)       over(partition by userid order by ts) lag_ts
        from mytable t
    ) t
) t

CREATE TABLE

INSERT 0 25

rowid	userid	webid	session_id	ts	first_ts
1	A	34	1	2023-01-31 16:34:49	2023-01-31 16:34:49
2	A	73	2	2023-01-31 16:34:50	2023-01-31 16:34:50
3	A	97	3	2023-01-31 16:34:58	2023-01-31 16:34:58
4	A	17	4	2023-01-31 17:35:02	2023-01-31 17:35:02
5	A	17	4	2023-01-31 17:35:07	2023-01-31 17:35:02
6	A	17	4	2023-01-31 17:35:18	2023-01-31 17:35:02
7	A	17	4	2023-01-31 17:35:30	2023-01-31 17:35:02
8	A	1	5	2023-01-31 17:35:37	2023-01-31 17:35:37
9	A	1	5	2023-01-31 17:35:38	2023-01-31 17:35:37
10	A	77	6	2023-01-31 17:35:41	2023-01-31 17:35:41
11	A	77	9	2023-01-31 17:35:42	2023-01-31 17:35:42
12	A	15	9	2023-01-31 17:35:42	2023-01-31 17:35:42
13	A	44	9	2023-01-31 17:35:42	2023-01-31 17:35:42
14	A	1	9	2023-01-31 17:35:42	2023-01-31 17:35:42
15	A	77	11	2023-01-31 17:35:45	2023-01-31 17:35:45
16	A	44	11	2023-01-31 17:35:45	2023-01-31 17:35:45
17	A	1	12	2023-01-31 17:37:10	2023-01-31 17:37:10
18	A	1	12	2023-01-31 17:37:12	2023-01-31 17:37:10
19	A	77	13	2023-01-31 17:37:14	2023-01-31 17:37:14
20	A	77	13	2023-01-31 17:52:14	2023-01-31 17:37:14
21	A	77	13	2023-01-31 18:12:14	2023-01-31 17:37:14
22	A	77	14	2023-01-31 18:45:14	2023-01-31 18:45:14
23	A	77	14	2023-01-31 18:55:15	2023-01-31 18:45:14
1	B	33	1	2023-01-31 06:37:15	2023-01-31 06:37:15
2	B	56	2	2023-01-31 06:40:15	2023-01-31 06:40:15

SELECT 25