CODEWITHSUNDEEP
sqlsql-serverstored-proceduressql-server-2005parameter-sniffing
Malik Daud Ahmad Khokhar
Malik Daud Ahmad Khokhar

Reputation: 13730

SP taking 15 minutes, but the same query when executed returns results in 1-2 minutes

So basically I have this relatively long stored procedure. The basic execution flow is that it SELECTS INTO some data into temp tables declared with the # sign and then runs a cursor through these tables to generate a 'running total' into a third temp table which is created using CREATE. Then this resulting temp table is joined with other tables in the DB to generated the result after some grouping etc. The problem is, this SP had been running fine until now returning results in 1-2 minutes. And now, suddenly, its taking 12-15 minutes. If I extract the query from the SP and executed it in management studio by manually setting the same parameters, it returns results in 1-2 minutes but the SP takes very long. Any idea what could be happening? I tried to generate the Actual Execution plans of both the query and the SP but it couldn't generate it because of the cursor. Any idea why the SP takes so long while the query doesn't?

Upvotes: 38

Views: 84704

Answers (12)

LCJ
LCJ

Reputation: 22661

As mentioned in other answers here, most likely it is because of parameter sniffing.

For solving parameter sniffing, many recommend to not to use parameter directly in queries. Instead store the parameter in a variable first and then use the variable for all queries. In my case, this didn't help some times. The problem came back after some executions. Looks like when a variable used from parameter, it uses something like OPTION (OPTIMIZE FOR UNKNOWN) Why does wrong constant defeat parameter sniffing?

What really worked for me is removing the plan from cache - DBCC FREEPROCCACHE(@PlanHandle). My stored proc is highly important compared to other stored procedures in the system. So, I selectively do free proc cache of this procedure only. The following procedure is called just before my actual procedure is called.

Refer Targeted Plan Cache Removal

CREATE PROCEDURE [dbo].[up_DBCC_Free_Proc_Cache_By_ObjectNames]
(
    @StoredProcName VARCHAR(250)
)
AS
BEGIN
    DECLARE @PlanHandle VARBINARY(64);
 
    SELECT @PlanHandle = deps.plan_handle
    FROM sys.dm_exec_procedure_stats AS deps
    WHERE OBJECT_NAME(deps.object_id) LIKE @StoredProcName+'%'
 
    IF @PlanHandle IS NOT NULL
    BEGIN
        DBCC FREEPROCCACHE(@PlanHandle);
    END
END
GO

enter image description here

Upvotes: 0

Adhikar Patil
Adhikar Patil

Reputation: 43

This is because of parameter snipping. But how can you confirm it?

Whenever we supposed to optimize SP we look for execution plan. But in your case, you will see an optimized plan from SSMS because it's taking more time only when it called through Code.

For every SP and Function, the SQL server generates two estimated plans because of ARITHABORT option. One for SSMS and second is for the external entities(ADO Net).

ARITHABORT is by default OFF in SSMS. So if you want to check what exact query plan your SP is using when it calls from Code.

Just enable the option in SSMS and execute your SP you will see that SP will also take 12-13 minutes from SSMS. SET ARITHABORT ON EXEC YourSpName SET ARITHABORT OFF

To solve this problem you just need to update the estimate query plan.

There are a couple of ways to update the estimate query plan. 1. Update table statistics. 2. recompile SP 3. SET ARITHABORT OFF in SP so it will always use query plan created for SSMS (this option is not recommended) For more options please refer to this awesome article - http://www.sommarskog.se/query-plan-mysteries.html

Upvotes: -1

Ammar
Ammar

Reputation: 693

its due to parameter sniffing. first of all declare temporary variable and set the incoming variable value to temp variable and use temp variable in whole application here is an example below.

ALTER PROCEDURE [dbo].[Sp_GetAllCustomerRecords]
@customerId INT 
AS
declare @customerIdTemp INT
set @customerIdTemp = @customerId
BEGIN
SELECT * 
FROM   Customers e Where
CustomerId = @customerIdTemp 
End

try this approach

Upvotes: 14

HotTester
HotTester

Reputation: 5778

I too faced a problem where we had to create some temp tables and then manipulating them had to calculate some values based on rules and finally insert the calculated values in a third table. This all if put in single SP was taking around 20-25 min. So to optimize it further we broke the sp into 3 different sp's and the total time now taken was around 6-8 mins. Just identify the steps that are involved in the whole process and how to break them up in different sp's. Surely by using this approach the overall time taken by the entire process will reduce.

Upvotes: -1

Jebli
Jebli

Reputation: 2825

1) When you run the query for the first time it may take more time. One more point is if you are using any corellated sub query and if you are hardcoding the values it will be executed for only one time. When you are not hardcoding it and run it through the procedure and if you are trying to derive the value from the input value then it might take more time.

2) In rare cases it can be due to network traffic, also where we will not have consistency in the query execution time for the same input data.

Upvotes: -1

Bill Martin
Bill Martin

Reputation: 4943

I usually start troubleshooting issues like that by using "print getdate() + ' - step '". This helps me narrow down what's taking the most time. You can compare from where you run it from query analyzer and narrow down where the problem is at.

Upvotes: 0

RBarryYoung
RBarryYoung

Reputation: 56755

This is the footprint of parameter-sniffing. See here for another discussion about it; SQL poor stored procedure execution plan performance - parameter sniffing

There are several possible fixes, including adding WITH RECOMPILE to your stored procedure which works about half the time.

The recommended fix for most situations (though it depends on the structure of your query and sproc) is to NOT use your parameters directly in your queries, but rather store them into local variables and then use those variables in your queries.

Upvotes: 81

HLGEM
HLGEM

Reputation: 96630

I'd also look into parameter sniffing. Could be the proc needs to handle the parameters slighlty differently.

Upvotes: 1

AdaTheDev
AdaTheDev

Reputation: 147334

For a start it doesn't sound like the SQL is going to perform too well anyway based on the use of a number of temp tables (could be held in memory, or persisted to tempdb - whatever SQL Server decides is best), and the use of cursors.

My suggestion would be to see if you can rewrite the sproc as a set-based query instead of a cursor-approach which will give better performance and be a lot easier to tune and optimise. Obviously I don't know exactly what your sproc does, to give an indication as to how easy/viable this is for you.

As to why the SP is taking longer than the query - difficult to say. Is there the same load on the system when you try each approach? If you run the query itself when there's a light load, it will be better than when you run the SP during a heavy load.

Also, to ensure the query truly is quicker than the SP, you need to rule out data/execution plan caching which makes a query faster for subsequent runs. You can clear the cache out using:

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS

But only do this on a dev/test db server, not on production. Then run the query, record the stats (e.g. from profiler). Clear the cache again. Run the SP and compare stats.

Upvotes: -1

pjp
pjp

Reputation: 17639

Try recompiling the sproc to ditch any stored query plan

exec sp_recompile 'YourSproc'

Then run your sproc taking care to use sensible paramters.

Also compare the actual execution plans between the two methods of executing the query.

It might also be worth recomputing any statistics.

Upvotes: 3

Gavin
Gavin

Reputation: 17392

I would guess it could possible be down to caching. If you run the stored procedure twice is it faster the second time?

To investigate further you could run them both from management studio the stored procedure and the query version with the show query plan option turned on in management studio, then compare what area is taking longer in the stored procedure then when run as a query.

Alternativly you could post the stored procedure here for people to suggest optimizations.

Upvotes: -1

Russell
Russell

Reputation: 17739

I would suggest the issue is related to the type of temp table (the # prefix). This temp table holds the data for that database session. When you run it through your app the temp table is deleted and recreated.
You might find when running in SSMS it keeps the session data and updates the table instead of creating it. Hope that helps :)

Upvotes: -1

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