Why does a database query only go slow in the application?

Question

I have a webpage that takes 10 minutes to run one query against a database, but the same query returns in less than a second when run from SQL Server Management Studio.

Answer 1

Is there any difference between the command text of the query in the app and the query you are executing manually? Since you said that reindexing helps performance (which also updates statistics), it sounds like it may be getting stuck on a bad execution plan.

You might want to run a sql trace and capture the showplanxml event to see what the execution plan looks like, and also capture sql statement complete (though this can slow the server down if a lot of statements are coming through the system so be careful) to be sure the statement sent to SQL server is the same one you are running manually.

Answer 2

I would suspect parameter sniffing.

The cached execution plan used for your application's connection probably won't be usable by your SSMS connection due to different set options so it will generate a new different plan.

You can retrieve the cached plans for the stored procedure by using the query below. Then compare to see if they are different (e.g. is the slow one doing index seeks and bookmark lookups at a place where the other one does a scan?)

Use YourDatabase;

SELECT *
FROM sys.dm_exec_cached_plans 
CROSS APPLY sys.dm_exec_sql_text(plan_handle) 
CROSS APPLY sys.dm_exec_query_plan(plan_handle) 
cross APPLY sys.dm_exec_plan_attributes(plan_handle) AS epa
where sys.dm_exec_sql_text.OBJECTID=object_id('YourProcName') 
         and attribute='set_options'