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I have a SQL Server 2012 setup as a linked server on a SQL Server 2008 server.

The following queries executes in less than 1 second:

   SELECT kg.IdGarment
   FROM Products p 
   INNER JOIN ZAMStaffRentals r ON r.ProductID = p.UniqueID 
   INNER JOIN ZAMCustomer_Staff s ON r.StaffID = s.UniqueID 
   INNER JOIN ZAMRentalGarments g ON g.StaffRentalID = r.UniqueID 
   INNER JOIN [REMOTE_SERVER].[REMOTE_DB].dbo.t_ProcessIT_Garment kg on g.BarcodeRFID = kg.IdGarment 
   INNER JOIN ZAMKannChanges log on log.GarmentID = g.UniqueID 
   WHERE log.ActionType = 'I' 

t_ProcessIT_Garment contains 37,000 rows, the query returns two records, the IdGarment column is the Primary Key. No problem here.

However, if I run this query to do a remote update, it takes 24 seconds, and 2 rows is affected:

   UPDATE [REMOTE_SERVER].[REMOTE_DB].dbo.t_ProcessIT_Garment SET 
   IdGarment = IdGarment
   FROM Products p 
   INNER JOIN ZAMStaffRentals r ON r.ProductID = p.UniqueID 
   INNER JOIN ZAMCustomer_Staff s ON r.StaffID = s.UniqueID 
   INNER JOIN ZAMRentalGarments g ON g.StaffRentalID = r.UniqueID 
   INNER JOIN [REMOTE_SERVER].[REMOTE_DB].dbo.t_ProcessIT_Garment kg on g.BarcodeRFID = kg.IdGarment 
   INNER JOIN ZAMKannChanges log on log.GarmentID = g.UniqueID 
   WHERE log.ActionType = 'I' ;

I tested using IdGarment = IdGarment to keep things simple. The execution plan shows it uses Remote Query for the first query, but Remote Scan for the second query, which has 100% of the cost.

The table joins are identical in both queries, why is it using Remote Scan for the second query, and how do I fix this?

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3 Answers 3

A friend just had a similar problem and rewriting the query solved it.

I know this is an old question, and likely you have worked it out, but for anyone else you could try this and report back...

UPDATE kg
SET 
   IdGarment = IdGarment
   FROM Products p 
   INNER JOIN ZAMStaffRentals r ON r.ProductID = p.UniqueID 
   INNER JOIN ZAMCustomer_Staff s ON r.StaffID = s.UniqueID 
   INNER JOIN ZAMRentalGarments g ON g.StaffRentalID = r.UniqueID 
   INNER JOIN [REMOTE_SERVER].[REMOTE_DB].dbo.t_ProcessIT_Garment kg on g.BarcodeRFID = kg.IdGarment 
   INNER JOIN ZAMKannChanges log on log.GarmentID = g.UniqueID 
   WHERE log.ActionType = 'I' ;

I don't know why this helped, but using the UPDATE "alias" form of the statement meant that the whole UPDATE query was remoted, rather than pulling the data back to the local instance to perform the JOINs.

If anyone has any ideas why this behaviour was seen, I would love to know.

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1  
Welcome to Database Administrators. This is a useful answer, but feel free to edit it to add a description of how this rewrite helped your friend, and any other details you think would be helpful to future site visitors. –  Paul White Jul 26 '13 at 12:12
    
So the only change is the UPDATE clause at the beginning? I don't remember whether this got solved at the end, I'll report back if I find out. Thanks. –  Robo Jul 27 '13 at 13:48
    
Hi Robo, yes the only change was to use an alias for the UPDATE target and the whole query was then remoted. –  Mister Magoo Aug 7 '13 at 23:56

Do you have sufficient permissions on the table on the linked server? This behavior is documented in Books Online: Guidelines for Using Distributed Queries :

To create the best query plans when you are using a table on a linked server, the query processor must have data distribution statistics from the linked server. Users that have limited permissions on any columns of the table might not have sufficient permissions to obtain all the useful statistics, and might receive a less efficient query plan and experience poor performance. If the linked server is an instance of SQL Server, to obtain all available statistics, the user must own the table or be a member of the sysadmin fixed server role, the db_owner fixed database role, or the db_ddladmin fixed database role on the linked server.

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In Linked Server Properties > Security, I've set it to use a user that has db_owner database role, and it didn't make any difference. –  Robo Mar 18 '13 at 2:13

If your remote scan operator is returning all rows, you may be experiencing the similar problem that occur when updating a table on linked server as described in this KB 942982. Although that is SQL 2005 issue, not sure if it applies to SQL 2008/2012. Try testing in your test environment by adding UNIQUE constraint or PRIMARY KEY to BarcodeRFID column in ZAMRentalGarments table and see if it helps.

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