Postgres get the next row with bigger value (coparing different columns) - postgresql

Question

Postgres version: 10
Table example:
CREATE TABLE log (
group_id INTEGER,
log_begin TIMESTAMP,
log_end TIMESTAMP
);
My goal: I want to know, for distinct groups, which log began right after the current log ends for each row or NULL if does not exists. Example: if the log of row 1 ends at 2022-07-15 15:30:00, the next log begins at 2022-07-15 16:00:00, so 2022-07-15 16:00:00 is the answer. If the log of row 4 ends at 2022-07-15 15:20:00, the next log begins at 2022-07-15 15:30:00, so it's the answer
Example data:
group_id
log_begin
log_end
1
2022-07-15 15:00:00
2022-07-15 15:30:00
1
2022-07-15 16:00:00
2022-07-15 16:30:00
1
2022-07-15 17:00:00
2022-07-15 17:30:00
2
2022-07-15 15:00:00
2022-07-15 15:20:00
2
2022-07-15 15:15:00
2022-07-15 15:40:00
2
2022-07-15 15:30:00
2022-07-15 16:30:00
My first solution was use a sub-query and search the next value for every row, but this table is very big, so the query result is correct, but it's very slow. Something like this:
SELECT *, ( SELECT _L.log_begin FROM log _L
WHERE _L.log_begin > L.log_end
AND _L.group_id = L.group_id
ORDER BY _L.log_begin ASC LIMIT 1 ) AS next_log_begin
FROM log L
My second solution was use a window function like LEAD as above
SELECT *, LEAD( log_begin, 1 ) OVER ( PARTITION BY group_id ORDER BY log_begin ) AS next_log_begin
FROM log
but the result isn't correct:
group_id
log_begin
log_end
next_log_begin
1
2022-07-15 15:00:00
2022-07-15 15:30:00
2022-07-15 16:00:00
1
2022-07-15 16:00:00
2022-07-15 16:30:00
2022-07-15 17:00:00
1
2022-07-15 17:00:00
2022-07-15 17:30:00
NULL
2
2022-07-15 15:00:00
2022-07-15 15:20:00
2022-07-15 15:15:00
2
2022-07-15 15:15:00
2022-07-15 15:40:00
2022-07-15 15:30:00
2
2022-07-15 15:30:00
2022-07-15 16:30:00
NULL
Because in row 4 it should get 2022-07-15 15:30:00 instead and row 5 should be NULL.
Correct output:
group_id
log_begin
log_end
next_log_begin
1
2022-07-15 15:00:00
2022-07-15 15:30:00
2022-07-15 16:00:00
1
2022-07-15 16:00:00
2022-07-15 16:30:00
2022-07-15 17:00:00
1
2022-07-15 17:00:00
2022-07-15 17:30:00
NULL
2
2022-07-15 15:00:00
2022-07-15 15:20:00
2022-07-15 15:30:00
2
2022-07-15 15:15:00
2022-07-15 15:40:00
NULL
2
2022-07-15 15:30:00
2022-07-15 16:30:00
NULL
Is there any way to do that using Postgres 10?
Window function are preferable but not a required resource

Answer 1

The data and the results you expect to see don't appear to line up with the logic you've outlined, but I think I get what you are saying.
If I understand you correctly, you want to look at the "next log begin" for every record, sorted by group then log start. If this is the case, you want to omit the "partition by" because it will yield a null any time the group id changes. It executes the lead within groups of whatever value(s) you specify in partition by, in this case group_id. So, for starters:
select
group_id, log_begin, log_end,
lead (log_begin) over (order by group_id, log_begin) as x
from log
Which looks for the next record, independent of changes to the group.
There is no way I'm aware of to evaluate the result of a window function within the expression that invokes it, so to do this you essentially would need to wrap it in a CTE and then evaluate it:
with cte as (
select
group_id, log_begin, log_end,
lead (log_begin) over (order by group_id, log_begin) as x
from log
)
select
group_id, log_begin, log_end,
x
from cte
And now you can compare x to any other field. I think the new field you want would look like this:
case
when log_end < x then x
end as next_log_begin
But again, it does not match your desired results. So either I misunderstood, your sample data might be off, or your assumptions might be off. All are equally possible.
Full query example:
with cte as (
select
group_id, log_begin, log_end,
lead (log_begin) over (order by group_id, log_begin) as x
from log
)
select
group_id, log_begin, log_end,
x,
case
when log_end < x then x
end as next_log_begin
from cte
-- EDIT 7/18/2022 --
I think I see now based on your revised question. I can't promise this will be efficient, but if you implement a scalar I think it will do what you think. Try this and let me know.
select
group_id, log_begin, log_end,
(select min (log_begin)
from log l2
where l1.group_id = l2.group_id
and l2.log_begin > l1.log_end) as next_log_begin
from log l1
order by group_id, log_begin