Question

I have a fairly simple query that I keep getting timeouts (it takes over three minutes to complete, I stopped it early so I could post this question) on when it is running in code, however when I run the same query from the same computer in Sql Server Management Studio the query will only take 2532 ms the first query when the data is not cached on the server and 524 ms for repeated queries.

Here is my c# code

using (var conn = new SqlConnection("Data Source=backend.example.com;Connect Timeout=5;Initial Catalog=Logs;Persist Security Info=True;User ID=backendAPI;Password=Redacted"))
                using (var ada = new SqlDataAdapter(String.Format(@"
SELECT [PK_JOB],[CLIENT_ID],[STATUS],[LOG_NAME],dt 
FROM [ES_HISTORY] 
inner join [es_history_dt] on [PK_JOB] = [es_historyid] 
Where client_id = @clientID and dt > @dt and (job_type > 4 {0}) {1}
Order by dt desc"
     , where.ToString(), (cbShowOnlyFailed.Checked ? "and Status = 1" : "")), conn))
{
    ada.SelectCommand.Parameters.AddWithValue("@clientID", ClientID);
    ada.SelectCommand.Parameters.AddWithValue("@dt", dtpFilter.Value);
    //ada.SelectCommand.CommandTimeout = 60;
    conn.Open();
    Logs.Clear();
    ada.Fill(Logs); //Time out exception for 30 sec limit.
}

here is my code I am running in SSMS, I pulled it right from ada.SelectCommand.CommandText

declare @clientID varchar(200)
set @clientID = '138'
declare @dt datetime
set @dt = '9/19/2011 12:00:00 AM'

SELECT [PK_JOB],[CLIENT_ID],[STATUS],[LOG_NAME],dt 
FROM [ES_HISTORY] 
inner join [es_history_dt] on [PK_JOB] = [es_historyid] 
Where client_id = @clientID and dt > @dt and (job_type > 4 or job_type = 0 or job_type = 1 or job_type = 4 ) 
Order by dt desc

What is causing the major discrepancy for the difference in time?

---

To keep the comment section clean, I will answer some FAQ’s here.

The same computer and logon is used for both the application and ssms.

Only 15 rows are returned in my example query. However, es_history contains 11351699 rows and es_history_dt contains 8588493 rows. Both tables are well indexed and the execution plan in SSMS says they are using index seeks for the look-ups so they are fast lookups. The program is behaving as if it is not using the indexes for the C# version of the query.

Answer 1

Your code in SSMS is not the same code you run in your application. This line in your application adds a NVARCHAR parameter:

 ada.SelectCommand.Parameters.AddWithValue("@clientID", ClientID);

while in the SSMS script you declare it as VARCHAR:

declare @clientID varchar(200)

Due to the rules of Data Type Precedence the Where client_id = @clientID expression in your query is not SARG-able where @clientID is of type NVARCHAR (I’m making a leap of faith and assume that client_id column is of type VARCHAR). The application thus forces a table scan where the SSMS query can do a quick key seek. This is a well know and understood issue with using Parameters.AddWithValue and has been discussed in many articles before, eg. see How Data Access Code Affects Database Performance. Once the problem is understood, the solutions are trivial:

• add parameters with the constructor that accepts a type: Parameters.Add("@clientID", SqlDbType.Varchar, 200) (and do pass in the explicit length to prevent cache pollution, see Query performance and plan cache issues when parameter length not specified correctly

• or cast the parameter in the SQL text: where client_id = cast(@clientID as varchar(200)).

The first solution is superior because it solves the cache pollution problem in addition to the SARG-ability problem.

I would also recommend you read Slow in the Application, Fast in SSMS? Understanding Performance Mysteries