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sqlsql-serversql-server-2008ado.netparameterized

SQL Parameter Slows Down Query

I have a query which I'm using with SQL Server 2008R2 via ADO.NET. When I use a LIKE clause inline, it works in less than a second, with 5 rows returned from 2 million. If I declare the paramater as I do in .NET at the start of the query in SSMS, it takes forever.

It's the same query, but parameterized.

The first (which works fine) is (which works fine):

;WITH Results_CTE AS (
    SELECT  ld.* , ROW_NUMBER() OVER (ORDER BY PK_ID) AS RowNum  
    FROM list..List_Data ld 
    WHERE Name IS NOT NULL  AND 
    Postcode LIKE 'SW14 1xx%' 
) SELECT * FROM Results_CTE

The second which takes forever is:

declare @postcode varchar(10) = 'SW14 1xx'
;WITH Results_CTE AS (
    SELECT  ld.* , ROW_NUMBER() OVER (ORDER BY PK_ID) AS RowNum  
    FROM list..List_Data ld
    WHERE Name IS NOT NULL  AND 
    Postcode LIKE @postcode +'%' 
) SELECT * FROM Results_CTE

I believe this has something to do with the inner workings of SQL Server but I really have no idea.

Solution

  • Use 

    SELECT * 
FROM Results_CTE 
OPTION (RECOMPILE)

    SQL Server does not sniff the value of the variable so it has no idea how selective it will be and will probably be assuming that the query will return significantly more rows than is actually the case and giving you a plan optimised for that.

    In your case I'm pretty sure that in the good plan you will find it is using a non covering non clustered index to evaluate the PostCode predicate and some lookups to retrieve the missing columns whereas in the bad plan (as it guesses the query will return a greater number of rows) it avoids this in favour of a full table scan.