PostgreSQL aggregate windows recursively - sql

Question

I have a series of Events which are produced by different users over time.
How can I aggregate this series by events that are close each other. Two events are close (in the same window) if:
b.user = a.user
and b.time >= a.time
and b.time - a.time <= interval '1 month'
This is a recursive condition. For example, the following dataset:
CREATE TABLE pg_temp.Data
("event" int, "user" int, "date" date, "value" int)
;
INSERT INTO pg_temp.Data
("event", "user", "date", "value")
VALUES
(1, 1, '2017-01-01', 5),
(2, 1, '2017-01-07', 3),
(3, 1, '2017-02-09', 2),
(4, 1, '2017-03-12', 4),
(5, 1, '2017-04-03', 7),
(6, 1, '2017-05-01', 6),
(7, 2, '2017-01-05', 9),
(8, 2, '2017-01-12', 1),
(9, 2, '2017-03-24', 6)
;
select * from pg_temp.Data
should be reduced to something like:
[
{
"init": "2017-01-01",
"latest": "2017-01-07",
"events": [
1,
2
],
"user": 1,
"value": 8
},
{
"init": "2017-02-09",
"latest": "2017-02-09",
"events": [
3
],
"user": 1,
"value": 2
},
{
"init": "2017-03-12",
"latest": "2017-05-01",
"events": [
4,
5,
6
],
"user": 1,
"value": 17
},
{
"init": "2017-01-05",
"latest": "2017-01-12",
"events": [
7,
8
],
"user": 2,
"value": 10
},
{
"init": "2017-03-24",
"latest": "2017-03-24",
"events": [
9
],
"user": 2,
"value": 6
}
]
Where init and latest are the window's time range and value is the sum of values in the window.
Note that events 6 and 4 are more than one month apart, but they've been aggregated into the same group because event 5 is in between them.

Answer 1

Use window functions:
SELECT min(date) AS init,
max(date) AS latest,
array_agg(event) AS events,
"user",
sum(value) AS value
FROM (SELECT event,
"user",
date,
value,
count(grp_start)
OVER (PARTITION BY "user" ORDER BY date) session_id
FROM (SELECT event,
"user",
date,
value,
CASE
WHEN date
> lag(date, 1, timestamp '-infinity')
OVER (PARTITION BY "user" ORDER BY date)
+ INTERVAL '1 month'
THEN 1
END grp_start
FROM data
) tagged
) numbered
GROUP BY "user", session_id
ORDER BY "user", init;
This will result in:
┌─────────────────────┬─────────────────────┬─────────┬──────┬───────┐
│ init │ latest │ events │ user │ value │
├─────────────────────┼─────────────────────┼─────────┼──────┼───────┤
│ 2017-01-01 00:00:00 │ 2017-01-07 00:00:00 │ {1,2} │ 1 │ 8 │
│ 2017-02-09 00:00:00 │ 2017-02-09 00:00:00 │ {3} │ 1 │ 2 │
│ 2017-03-12 00:00:00 │ 2017-05-01 00:00:00 │ {4,5,6} │ 1 │ 17 │
│ 2017-01-05 00:00:00 │ 2017-01-12 00:00:00 │ {7,8} │ 2 │ 10 │
│ 2017-03-24 00:00:00 │ 2017-03-24 00:00:00 │ {9} │ 2 │ 6 │
└─────────────────────┴─────────────────────┴─────────┴──────┴───────┘
(5 rows)
A word af advice: it is a good idea not to use column names like user that are reserved words. If you forget to use them inside double quotes, surprising things will happen (try it out).