Subselect in the ON clause - sql

Question

Please dont bash me if there are already answers for this question, but I found none.
Basically I want to make a subselect in the ON clause of a Left join to get the newest entry in a timeframe.
(start and endtime are timestamps, hardcoded, in local variables or host variables in a Cobol program) to simplify I used integers in that question.
Select * from table1 as t1
left join table2 as t2 on
t1.primary = t2.secondary
and t2.timestamp = (
select max(t2a.timestamp) from table2 as t2a
where t2.primary = t2a.primary
and t2a.timestamp > starttime
and t2a.timestamp < endtime
)
Now this does not work, I get the following error:
AN ON CLAUSE IS INVALID. SQLCODE=-338
Because (see the docs)
The ON clause cannot contain a subquery.
Now what we can do to surround that is instead of joining table2 to join a already delimited subquery. But that surrounds the query optimizer what literally kills the performance:
Select * from table1 as t1
left join (
select t2a.secondary from table2 as t2a
where t2a.timestamp = (
select max(t2b.timestamp)
from table2 as t2b
where t2a.primary = t2b.primary
and t2b.timestamp > starttime
and t2b.timestamp < endtime
)
)as t2
on t1.primary = t2.secondary
Any idea how to slove this?
Example data table1:
t1.primary
1
2
3
Example data table2:
t2.primary t2.secondary t2.timestamp
1 1 4
2 1 5
3 1 10
4 2 4
5 2 5
Variables:
starttime = 3
endtime = 6
Expected result:
t1.primary t2.primary t2.secondary t2.timestamp
1 2 1 5 --Leftjoined the newest entry in range
2 5 2 5
3 NULL NULL NULL

Answer 1

This should work
select *
from table1 t1
left join (
select t2.primary, t2.secondary, t2.timestamp,
row_number() over (partition by t2.secondary order by t2.timestamp desc) rn
from table2 t2
where t2.timestamp between starttime and endtime
) t on t1.primary = t.secondary and t.rn = 1
If you have an index table2(timestamp, secondary, primary) or at least table2(timestamp, secondary) then it should run really fast. Without the indexes, it still works with quite good performance, since it leads to one sequential scan of the tables.

Answer 2

something like this. Just typed in before lunch so don't bash me if it doesn't work.
select * from table1 a left join
(select t2b.primary, max(t2b.timestamp) mxts
from table2 t2b
group by t2b.primary
) as b on a.primary = b.primary
left join table2 on b.primary = table2.secondary and
table2.timestamp = mxts and table2.timestamp between mystartts and myendts
NOte: Don't assume timestamps are unique and can be used to extract the last entry from a table because this will undobtly fraile.