SQL View Outer Apply Speed - sql-server

Question

I am seeing some strange query speed results when using a view with an outer apply, I am doing a distinct count on 2 different columns in the view, 1 is done in less than 0.1 seconds, the other takes 4-6 seconds, is the second count query returned slower because it is part of the outer apply? If so - how could I speed this query up?
The fast distinct count is -
SELECT DISTINCT ISNULL([ItemType], 'N/A') AS Items FROM vwCustomerItemDetailsFull
The slow distinct count is -
SELECT DISTINCT ISNULL([CustomerName], 'N/A') AS Items FROM vwCustomerItemDetailsFull
The view is -
SELECT I.ItemID,
IT.Name AS ItemType,
CASE
WHEN CustomerItemEndDate IS NULL
OR CustomerItemEndDate > GETDATE() THEN CustomerItems.CustomerName
ELSE NULL
END AS CustomerName,
CASE
WHEN CustomerItemEndDate IS NULL
OR CustomerItemEndDate > GETDATE() THEN CustomerItems.CustomerNumber
ELSE NULL
END AS CustomerNumber,
CASE
WHEN CustomerItemEndDate IS NULL
OR CustomerItemEndDate > GETDATE() THEN CustomerItems.CustomerItemStartDate
ELSE NULL
END AS CustomerItemStartDate,
FROM tblItems I
INNER JOIN tblItemTypes IT
ON I.ItemTypeID = IT.ItemTypeID
OUTER APPLY (SELECT TOP 1 CustomerName,
CustomerNumber,
StartDate AS CustomerItemStartDate,
EndDate AS CustomerItemEndDate
FROM tblCustomerItems CI
INNER JOIN tblCustomers C
ON C.CustomerID = CI.CustomerID
WHERE CI.ItemID = I.ItemID
ORDER BY EndDate DESC) AS CustomerItems

Answer 1

Check the execution plan, this speed difference is not strange at all, since it is an outer apply and not a cross apply, and within it you are limiting the results to top 1, it means that your outer apply has no influence on the number of results of the query, or the column ItemType.
Therefore when you select from the view and don't use any columns from the outer apply, the optimiser is smart enough to know it doesn't need to execute it. So in essesnce your first query is:
SELECT DISTINCT ISNULL([ItemType], 'N/A') AS Items
FROM ( SELECT tblItems
FROM Items
INNER JOIN tblItemTypes IT
ON I.ItemTypeID = IT.ItemTypeID
) vw
Whereas your second query has to execute the outer apply.
I have previously posted a longer answer which could also be helpful.
EDIT
If you wanted to change your query to a JOIN it could be rewritten as so:
SELECT I.ItemID,
IT.Name AS ItemType,
CustomerName,
CustomerNumber,
CustomerItemStartDate,
FROM tblItems I
INNER JOIN tblItemTypes IT
ON I.ItemTypeID = IT.ItemTypeID
LEFT JOIN
( SELECT ci.ItemID,
CustomerName,
CustomerNumber,
StartDate AS CustomerItemStartDate,
EndDate AS CustomerItemEndDate,
RN = ROW_NUMBER() OVER (PARTITION BY ci.ItemID ORDER BY EndDate DESC)
FROM tblCustomerItems CI
INNER JOIN tblCustomers C
ON C.CustomerID = CI.CustomerID
) AS CustomerItems
ON CustomerItems.ItemID = I.ItemID
AND CustomerItems.rn = 1
AND CustomerItems.CustomerItemEndDate < GETDATE();
However I don't think this will improve performance much since you said the most costly part is the sort on EndDate, and for your first query it will negatively impact performance because the optimiser will no longer optimise out the outer apply.
I expect the best way to improve the performance will be adding indexes, without knowing your data size or distribution I can't accurately guess the exact index you need, if you run the query on it's own showing the actual execution plan SSMS will suggest an index for you which would be better than my best guess.