Left outer join with left() function - sql

Question

I'm running the below query daily (overnight) and it's taking considerable time to run (1-1.5 hours). I'm certain the "Acc.DateKey >= LEFT (LocationKey, 8)" is the reason and if this part of the join is removed the query executes in around 5 minutes. I just cannot think of a more efficient way.
Acc.DateKey is a bigint typically 20180101 etc., with the location key being a bigint typically 201801011234 etc.
So far I've considered including a new column in the LO table "AccLocationKey" which will be inserted with the LEFT (LocationKey, 8) function when loaded.
I've decided to pose the question here first - could this be improved upon without changing the LO table?
SELECT
ISNULL(MAX(L.LocationKey),(SELECT MIN(LocationKey) FROM LO WHERE Location = Acc.Location)) AS LocationKey
FROM
Acc
LEFT OUTER JOIN
(
SELECT
LocationKey
,Location
FROM
LO
)AS L
ON Acc.Location = L.Location AND Acc.DateKey >= LEFT(LocationKey,8)

Answer 1

Let's rewrite the query without the subquery in the SELECT:
SELECT COALESCE(MAX(L.LocationKey), MIN(L.MIN_LocationKey)) AS LocationKey
FROM Acc LEFT OUTER JOIN
(SELECT MIN(l.LocationKey) OVER (PARTITION BY l.Location) as min_location,
l.*
FROM LO l
) L
ON Acc.Location = L.Location AND Acc.DateKey >= LEFT(l.LocationKey, 8);
Probably your best chance at performance is to add a computed column and appropriate index. So:
alter table lo add locationkey_datekey as (try_convert(bigint, LEFT(l.LocationKey, 8))) persisted;
Then, the appropriate index:
create index idx_lo_location_datekey on lo(location, locationkey_datekey);
Then use this in the query:
SELECT COALESCE(MAX(L.LocationKey), MIN(L.MIN_LocationKey)) AS LocationKey
FROM Acc LEFT OUTER JOIN
(SELECT MIN(l.LocationKey) OVER (PARTITION BY l.Location) as min_location,
l.*
FROM LO l
) L
ON Acc.Location = L.Location AND Acc.DateKey >= l.LocationKey_datekey;
Happily, this index will also work for the window function.