Performance issues in T-SQL function - sql

Question

I have a table which provides me daily inventory status. I.e Inventory Item X, on a particular date had Y amount of quantity. I have created a function to obtain the purchase price on a particular day based on the last purchases.
When I run the query on the dailyinventorystatus table it completes within 3 minute for date > 2014-01-01. However, when I add in the function as a subquery it causes huge performance issues. It has been over 1.5 hours and the query is still running.
How do I improve this?
Here is the query:
SELECT
*,
RWReports.dbo.FindPurchasePrice(InventoryKey, Date , warehouse) as SalesPurchasePrice
FROM
DailyInventoryStatus
WHERE
Warehouse IN ('NYC,', 'CHICAGO', 'CHINA', 'ATLANTA')
AND Date >= '2014-01-01'
Here is the function:
CREATE FUNCTION [dbo].[FindPurchasePrice]
(#InventoryKey varchar(8), #InDate Date , #Warehouse varchar(30))
RETURNS REAL
AS
BEGIN
DECLARE #oPurchasePrice AS REAL ;
SELECT TOP (1)
#oPurchasePrice = UnitPurchasePrice
FROM
PurchaseTransactions
WHERE
InventoryKey = #InventoryKey
AND TransactionDate <= #InDate
AND Warehouse = #Warehouse
ORDER BY
TransactionDate DESC;
IF #oPurchasePrice IS NULL
SELECT
#oPurchasePrice = mw.cost
FROM
Rentalworks.dbo.masterwh mw
JOIN
Rentalworks.dbo.warehouse w ON w.warehouseid = mw.warehouseid
AND mw.masterid = #InventoryKey
AND w.warehouse = #Warehouse;
RETURN #oPurchasePrice;
END;
GO

Answer 1

This is how you could possibly convert this into an inline table valued function.
CREATE FUNCTION [dbo].[FindPurchasePrice]
(
#InventoryKey varchar(8)
, #InDate Date
, #Warehouse varchar(30)
)
RETURNS TABLE
AS
RETURN
SELECT ISNULL(pt.UnitPurchasePrice ,mw.cost) AS PurchasePrice
FROM Rentalworks.dbo.masterwh mw
JOIN Rentalworks.dbo.warehouse w on w.warehouseid = mw.warehouseid
AND mw.masterid = #InventoryKey
AND w.warehouse = #Warehouse
OUTER APPLY
(
SELECT TOP (1) UnitPurchasePrice
FROM PurchaseTransactions
WHERE InventoryKey = #InventoryKey
AND TransactionDate <= #InDate
AND Warehouse=#Warehouse
ORDER BY TransactionDate DESC
) pt
I of course can't test this but it syntax checks fine.
Now to include that in your original select statement you would do something like this.
SELECT dis.*
, fp.PurchasePrice
FROM DailyInventoryStatus dis
CROSS APPLY dbo.FindPurchasePrice(dis.InventoryKey, dis.Date, dis.warehouse) fp
WHERE Warehouse IN ('NYC,', 'CHICAGO', 'CHINA', 'ATLANTA')
AND Date >= '2014-01-01'

Answer 2

Here's one way to re-write without the function by incorporating all the logic into a single query:
with data as (
select
dis.*, pt.TransactionDate, pt.UnitPurchasePrice,
row_number() over (
partition by dis.InventoryKey, dis.Warehouse
order by TransactionDate desc
) as TransNumber
from
DailyInventoryStatus dis left outer join
PurchaseTransactions pt
on pt.InventoryKey = dis.InventoryKey
and pt.Warehouse = dis.Warehouse
and pt.TransactionDate < dis.Date
where dis.Date >= ?
)
select
*,
coalesce(
UnitPurchasePrice,
(
select mw.cost
from
Rentalworks.dbo.masterwh mw inner join
Rentalworks.dbo.warehouse w
on w.warehouseid = mw.warehouseid
where mw.masterid = data.InventoryKey
and w.warehouse = data.Warehouse
)
) as PurchasePrice
from data
where TransNumber = 1