SQL Server query improve performance on linked server

Question

How can I improve the next SQL Server query?

SELECT SUM(Qty) 
FROM Products
WHERE Type = 'SODA'
AND (Code LIKE 'A5%'
OR Code IN('DHA2','JHU8','KML2','LQA1','ZSX2')) 

Takes a lot of time in execution.

Thanks in advance.

I'm getting data from a DB2 database through Linked Server. I can't build indexes just read.

Id   int
Name nvarchar(100)
Type nvarchar(100)
Code nvarchar(100)
Qty  int

Answer 1

First, try creating an index on products(type, code, qty) . That might greatly improve performance.

Then, if that doesn't work, try rewriting the query as:

select sum(qty)
from ((select qty
       from products
       where type = 'SODA' and
             code like 'A5%'
      ) union all
      (select qty
       from products
       where type = 'SODA' and
             code IN ('DHA2','JHU8','KML2','LQA1','ZSX2'))
      )
     ) t;

This may look more complicated but sometimes or interferes with query optimization.

Answer 2

This is a linked server query. You don't state how you're executing it so I'm assuming it is using the linked server name in the FROM. If you look at the query plan, all you will see is Remote Query --> Computer Scalar --> Select. That basically means all of the data you are wanting to compute is copied into tempDB then calculated.

If you do the query as an EXEC at or OPENQUERY the query is executed remotely then the only thing to return is the result.

Try this:

select *
from 
openquery(LinkedServerName, '   
SELECT SUM(Qty) Total
FROM Products
WHERE Type = ''SODA''
AND (Code LIKE ''A5%''
OR Code IN(''DHA2'',''JHU8'',''KML2'',''LQA1'',''ZSX2'')) ')

Answer 3

Since you using a linked server and also you are string comparisons, I am pretty sure your slow performance is because SQL Server has to bring the whole Product table from the DB2 database to a local copy, then compare the strings according to your current collation settings. You see, comparing strings may have different results on different collation settings, and on different database systems. When linking between Microsoft SQL Servers databases you can enable Collation compatibility so when the query is executed it will trust the linked server decision about string comparison and sorting. I don't know if collation compatibility is possible between Microsoft SQL Server and DB2. So the solution to the problem will be to use OPENROWSET instead of a linked server.

Something like this:

 SELECT * FROM OPENROWSET 
 ('DB2OLEDB',Netlib=SNA;NetAddr=;NetPort=;RemoteLU=OLYMPIA;LocalLU=LOCAL;ModeName=QPCSUPP;User ID=WNW3XX;Password=WNW3XX;InitCat=OLYMPIA;Default Schema=WNW3XX;PkgCol=WNW3XX;TPName=;Commit=YES;IsoLvl=NC;AccMode=;CCSID=37;PCCodePage=1252;BinAsChar=NO;Data Source=Sample',
 'SELECT SUM(Qty) 
FROM Products
WHERE Type = ''SODA''
AND (Code LIKE ''A5%''
OR Code IN(''DHA2'',''JHU8'',''KML2'',''LQA1'',''ZSX2'')) ' )

See more info in linked server to DB2 using Microsoft OLE DB provider for DB2

And in OPENROWSET (Transact-SQL)

Answer 4

If it takes that long, I suggest you create a table with the same structure, then build some helpful indexes, then execute your query against this local table:

DELETE FROM Products

INSERT INTO Products
SELECT * FROM RemoteServer.database_name..Products

SELECT SUM(Qty) 
FROM Products
WHERE Type = 'SODA'
AND (Code LIKE 'A5%'
OR Code IN('DHA2','JHU8','KML2','LQA1','ZSX2'))