MSSQL slow query in .NET application but quick in management studio [duplicate]

I have been having this problem for a couple of weeks now. The problem is that the query takes 4-5 minutes to run on the website and at most 2 or 3 seconds to run in ssms. Also I found that after I make a change to this query like adding the customerId variable it will start running quickly on the web page but by the next day it is slow again. The query in question is this one:

DECLARE @customerID INT
SET @customerID = @CustID
DECLARE @MyTable table(
Iden int NOT NULL IDENTITY(1,1),
ProductID int)

INSERT INTO @MyTable(ProductID)
SELECT P.ProductID FROM Product P WITH (NOLOCK)
left join Compunix_ProductMMY cpmmy with (nolock) on p.ProductID = cpmmy.ProductID
left join Compunix_CustomerMMY ccmmy with (nolock) on ccmmy.mmyid = cpmmy.mmyid
WHERE P.Deleted=0 AND P.Published=1 and (ccmmy.customerid = @customerID OR cpmmy.productid IS NULL)

SELECT c.Name, c.SeName, c.CategoryID
FROM Category c WITH (NOLOCK) 
JOIN ProductCategory PC With (NOLOCK) ON C.CategoryID = PC.CategoryID
JOIN @MyTable MT ON PC.ProductID=MT.ProductID
WHERE C.Published = 1
GROUP BY c.Name, c.SeName, c.CategoryID
ORDER BY c.Name

I have the same query running on 2 other site which work just fine. The only difference between the sites is that they are run on different databases and the slow site has a bit more than double the products (54000 products) on it compared to the other 2. All three sites and their databases are hosted on the same machine.

Chances are that you are hitting on a problem with parameter sniffing.

I suggest reading Slow in the Application, Fast in SSMS? by Erland Sommarskog to get a full understanding of the issue (long article but very good).

Take a look at sys.dm_exec_sessions for your ASP.Net application and for your SSMS session. I will hazard a guess that at least one of your SET settings is different. This can contribute to different plans (ultimately this gets attributed to parameter sniffing) and the app side usually ends up worse off.

See these other questions for a lot more details:

Stored procedure slow when called from web, fast from Management Studio

Procedure times out from ADO.NET but not in SSMS

Query times out when executed from web, but super-fast when executed from SSMS

ADO .NET vs. SQL Server Management Studio - ADO performs worse

I had same issue, in my case it was related to MARS, so I removed MultipleActiveResultSets=True; from connection string and now running time btween SSMS and asp.net are almost same (0.2s comparing to 4.5s before)

Note: MARS = Multiple Active Result Sets. If you set this property on a connection string, you can run multiple queries on the same connection in an interleaved fashion. It's mainly intended to permit you to submit UPDATE statements as you are iterating through a result set.

Doing following steps at the same time worked for me.

1. Try stored procedure recompiling
2. Clear the plan cache
3. Updating stats on the DB