Optimizing sql server scalar-valued function - sql-server

Question

Here is my question,
I have a view calling another view. And that second view has a scalar function which obviously runs for each row of the table. For only 322 rows, it takes around 30 seconds. When I take out the calculated field, it takes 1 second.
I appreciate if you guys give me an idea if I can optimize the function or if there is any other way to increase the performance?
Here is the function:
ALTER FUNCTION [dbo].[fnCabinetLoad] (
#site nvarchar(15),
#cabrow nvarchar(50),
#cabinet nvarchar(50))
RETURNS float
AS BEGIN
-- Declare the return variable here
DECLARE #ResultVar float
-- Add the T-SQL statements to compute the return value here
SELECT #ResultVar = SUM(d.Value)
FROM
(
SELECT dt.*,
ROW_NUMBER()
OVER (PARTITION BY dt.tagname ORDER BY dt.timestamp DESC) 'RowNum'
FROM vDataLog dt
WHERE dt.Timestamp BETWEEN dateadd(minute,-15,getdate()) AND GetDate()
) d
INNER JOIN [SKY_EGX_CONFIG].[dbo].[vPanelSchedule] AS p
ON p.rpp = left(d.TagName,3) + substring(d.TagName,5,5)
+ substring(d.TagName,11,8)
AND right(p.pole,2) = substring(d.TagName,23,2)
AND p.site = #site
AND p.EqpRowNumber = #cabrow
AND p.EqpCabinetName= #cabinet
WHERE d.RowNum = 1
AND Right(d.TagName, 6) = 'kW Avg'
RETURN #ResultVar
END

Answer 1

Scalar-valued functions have atrocious performance. Your function looks like an excellent candidate for an inline table-valued function that you can CROSS APPLY:
CREATE FUNCTION [dbo].[fnCabinetLoad]
(
#site nvarchar(15),
#cabrow nvarchar(50),
#cabinet nvarchar(50)
)
RETURNS TABLE
AS RETURN
SELECT SUM(d.Value) AS [TotalLoad]
FROM
(
SELECT dt.*, ROW_NUMBER() OVER (PARTITION BY dt.tagname ORDER BY dt.timestamp DESC) 'RowNum'
FROM vDataLog dt
WHERE dt.Timestamp BETWEEN dateadd(minute,-15,getdate()) AND GetDate()) d INNER JOIN [SKY_EGX_CONFIG].[dbo].[vPanelSchedule] AS p
ON p.rpp = left(d.TagName,3) + substring(d.TagName,5,5) + substring(d.TagName,11,8)
AND right(p.pole,2) = substring(d.TagName,23,2)
AND p.site = #site
AND p.EqpRowNumber = #cabrow
AND p.EqpCabinetName= #cabinet
WHERE d.RowNum = 1
AND Right(d.TagName, 6) = 'kW Avg'
In your view:
SELECT ..., cabinetLoad.TotalLoad
FROM ... CROSS APPLY dbo.fnCabinetLoad(.., .., ..) AS cabinetLoad

Answer 2

My understanding is the returned result set is 322 rows, but if the vDataLog table is significantly larger, I would run that subquery first and dump that result set into a table variable. Then, you can use that table variable instead of a nested query.
Otherwise, as it stands now, I think the joins are being done on all rows of the nested query and then you're stripping them off with the where clause to get the rows you want.

Answer 3

You really don't need a function and get rid of nested view(very poor performant)! Encapsulate the entire logic in a stored proc to get the desired result, so that instead of computing everything row by row, it's computed as a set. Instead of view, use the source table to do the computation inside the stored proc.
Apart from that, you are using the functions RIGHT, LEFT AND SUBSTRING inside your code. Never have them in WHERE OR JOIN. Try to compute them before hand and dump them into a temp table so that they are computed once. Then index the temp tables on these columns.
Sorry for the theoretical answer, but right now code seems a mess. It needs to go through layers of changes to have decent performance.

Answer 4

Turn the function into a view.
Use it by restraining on the columns site, cabrow and cabinet and Timestamp. When doing that, try storing GetDate() and dateadd(minute,-15,getdate()) on a variable. I think not doing so can prevent you from taking advantage on any index on Timestamp.
SELECT SUM(d.Value) AS [TotalLoad],
dt.Timestamp,
p.site,
p.EqpRowNumber AS cabrow,
p.EqpCabinetName AS cabinet
FROM
( SELECT dt.*,
ROW_NUMBER() OVER (PARTITION BY dt.tagname ORDER BY dt.timestamp DESC)'RowNum'
FROM vDataLog dt) d
INNER JOIN [SKY_EGX_CONFIG].[dbo].[vPanelSchedule] AS p
ON p.rpp = left(d.TagName,3) + substring(d.TagName,5,5) + substring(d.TagName,11,8)
AND right(p.pole,2) = substring(d.TagName,23,2)
WHERE d.RowNum = 1
AND d.TagName LIKE '%kW Avg'