Calculating datetime intervals taking into an account the current date - sql

Question

I have a sub request which returns this:
item_id, item_datetime, item_duration_in_days
1, '7-dec-2016-12:00', 3
2, '8-dec-2016-11:00', 4
3, '20-dec-2016-05:00', 10
4, '2-jan-2017-14:00', 50
5, '29-jan-2017-22:00', 89
I want to get "item_id" which falls into "now()". For that the algorithm is:
1) var duration_days = interval 'item_duration_in_days[i]'
2) for the very first item:
new_datetime[i] = item_datetime[i] + duration_days
3) for others:
- if a new_datetime from the previous step overlaps with the current item_datetime[i]:
new_datetime[i] = new_datetime[i - 1] + duration_days
- else:
new_datetime[i] = item_datetime[i] + duration_days
4) return an item for each iteration:
{id, item_datetime, new_datetime}
That is, there'll be something like:
item_id item_datetime new_datetime
1 7 dec 2016 10 dec 2016
2 11 dec 2016 15 dec 2016
3 20 dec 2016 30 dec 2016
4 2 jan 2017 22 feb 2017 <------- found because now() == Feb 5
5 22 feb 2017 21 may 2017
How can I do that? I think it should be something like "fold" function. Can it be done via an sql request? Or will have to be an PSQL procedure for intermediate variable storage?
Or please give pointers how to calculate that.

Answer 1

If I understand correctly your task, you need recursive call. Function take first row at first and process each next.
WITH RECURSIVE x AS (
SELECT *
FROM (
SELECT item_id,
item_datetime,
item_datetime + (item_duration_in_days::text || ' day')::interval AS cur_end
FROM ti
ORDER BY item_datetime
LIMIT 1
) AS first
UNION ALL
SELECT item_id,
cur_start,
cur_start + (item_duration_in_days::text || ' day')::interval
FROM (
SELECT item_id,
CASE WHEN item_datetime > prev_end THEN
item_datetime
ELSE
prev_end
END AS cur_start,
item_duration_in_days
FROM (
SELECT ti.item_id,
ti.item_datetime,
x.cur_end + '1 day'::interval AS prev_end,
item_duration_in_days
FROM x
JOIN ti ON (
ti.item_id != x.item_id
AND ti.item_datetime >= x.item_datetime
)
ORDER BY ti.item_datetime
LIMIT 1
) AS a
) AS a
) SELECT * FROM x;
Result:
item_id | item_datetime | cur_end
---------+---------------------+---------------------
1 | 2016-12-07 12:00:00 | 2016-12-10 12:00:00
2 | 2016-12-11 12:00:00 | 2016-12-15 12:00:00
3 | 2016-12-20 05:00:00 | 2016-12-30 05:00:00
4 | 2017-01-02 14:00:00 | 2017-02-21 14:00:00
5 | 2017-02-22 14:00:00 | 2017-05-22 14:00:00
(5 rows)
For seeing current job :
....
) SELECT * FROM x WHERE item_datetime <= now() AND cur_end >= now();
item_id | item_datetime | cur_end
---------+---------------------+---------------------
4 | 2017-01-02 14:00:00 | 2017-02-21 14:00:00
(1 row)