Joins and linked servers query causing delay - sql-server

Question

Hey guys the code below is taking a really long time. I've been looking at it for quite a while. Is there anything that stands out as the obvious cause of delay?
[SQLSRV-3-JB] is a linked server BTW
Select count(cast (Unique_ID as bigint)) as [Count],
T.[Region_Code],
T.[Region_Name],
U.[Region_Code],
Y.[Examination_Year],
case when [Subject] = 'MUSIC THEORY' THEN 'Theory'
else 'Practical'
end
from [SQLSRV-3-JB].[X].[dbo].[Exam_and_Candidate_Details] Y
left join [SQLSRV-3-JB].[X].[dbo].[UK_Exam_Centre_Info] T
on Y.Centre_Code = T.Centre_Code
left join [SQLSRV-3-JB].[X].[dbo].[UK_Exam_Centres] U
on Y.Centre_Code = U.Centre_Code
where Y.[Examination_Year] between 2010 and 2016
group by Y.[Examination_Year],
T.[Region_Code],
T.[Region_Name],
U.[Region_Code],
case when [Subject] = 'MUSIC THEORY' THEN 'Theory'
else 'Practical'
end

Answer 1

Yes, there is one very obvious problem - the remote server. When you use a linked server like this, SQL Server has a disturbing habit of pulling all of the data in the remote tables to the local server before performing JOINs or filtration.
The correct way to handle this is to make this query a view on the remote server, and query the view with your WHERE clause on your end. So, your remote code would look like this:
-- Remote Server
USE X
GO
CREATE VIEW dbo.ExamCenterInfo
AS
Select count(cast (Unique_ID as bigint)) as [Count],
T.[Region_Code],
T.[Region_Name],
U.[Region_Code],
Y.[Examination_Year],
case when [Subject] = 'MUSIC THEORY' THEN 'Theory'
else 'Practical'
end
from dbo.[Exam_and_Candidate_Details] Y
left join dbo.[UK_Exam_Centre_Info] T
on Y.Centre_Code = T.Centre_Code
left join dbo.[UK_Exam_Centres] U
on Y.Centre_Code = U.Centre_Code
group by Y.[Examination_Year],
T.[Region_Code],
T.[Region_Name],
U.[Region_Code],
case when [Subject] = 'MUSIC THEORY' THEN 'Theory'
else 'Practical'
end
Then your local code:
-- Local Server
SELECT *
FROM [SQLSRV-3-JB].[X].[dbo].ExamCenterInfo ECI
where ECI.[Examination_Year] between 2010 and 2016
NOTE: There is some possibility that this will pull all of the view output before filtering by year. If this is still a problem, you will have to create a more complex mechanism to execute the view with filtration on the remote server.