I've run across code where someone wants to create a dynamic table, run a select statement to populate that table, then run a series of stored procedures using that table's data. All of this is done using inline SQL from .NET code.
What they've basically done is (pseudo code):
declare table;
insert IDs into table
exec storedprocedure table
exec storedprocedure table
exec storedprocedure table
exec storedprocedure table
In my opinion, and I don't have anything to stand on, is that this probably isn't as efficient as creating a stored procedure that does all of this and the .NET code just invokes that encapsulating stored procedure.
Are there performance hits or gains from either approach?
Are there performance hits or gains from either approach?
Not normally.
The only real difference (which is very unlikely to matter) is the size of the request data sent to the SQL Server, and the cost of hashing the TSQL batch to determine if there's a cached plan for it.
Once a cached plan is found for the TSQL batch, execution is the same as if an RPC request had invoked a stored procedure.
Another potential issue if this batch is run at a very high frequency is that the temp table metadata will not be cached. So if you're running this 1000s of times/sec and have a single tempdb file you can get metadata page latch contention in TempDb.
This is the kind of thing that sort of mattered a little a very long time ago, but which now is only noticable in edge cases.
Related
We have a complex stored procedure that is sometimes subject to parameter sniffing. It is a large, "all-in-one" procedure that is called by many different parts of the system and so it stands to reason that one query plan would not fit all use cases.
This works fine except periodically ONE particular report goes from seconds to minutes. In the past, a quick exec sp_recompile would speed it back up immediately. Now that never works. The report just eventually "fixes itself" in a day or two, meaning it goes back to taking seconds.
Refactoring the stored procedure is currently not an option and I don't want to do the other recommended approaches (saving parameters to local variables, WITH RECOMPILE, OPTIMIZE FOR UNKNOWN) as those are said to have other side effects.
So I have these questions:
Why wouldn't exec sp_recompile speed it up like before?
How can I tell if exec sp_recompile actually cleared the query plan cache? What should be run before, and after, the exec? I've tried some queries from the web but can't clearly tell if something changed, so a specific recipe would be great to have.
Would it be reasonable to clone the procedure with a different name, and call that clone just for this one report? The goal would be to get SQL Server to cache a separate plan just for the report. But I'm not sure if SQL Server is caching plans by procedure name, or if it caches the various queries inside the stored procedures. (If it's the latter, then there's no use to this approach, as the any clones of the procedure would have the same queries.)
Using several CTEs, especially complex queries (just like when joining with views) can potentially cause the query optimiser problems with producing an optimal execution plan.
If you have a lot of CTE definitions used, SQL Server will be attempting to construct a single monolithic execution plan and you could have a plan compilation timeout resulting in a sub-optimal plan being used.
You could instead replace the CTEs with temp tables - using intermediate results often has better performance as each query executes in isolation with a dedicated optimal (or at least better) plan. This can help the optimizer make a better choice for joins and index usage.
If you can benefit from two key different types of parameters that ideally require their own optimal plan then an option would be, as you suggest, to duplicate the procedure specific to each use-case.
You can confirm that this results in a separate execution plan by querying for your procedure name using dm_exec_sql_text
select s.plan_handle, t.text
from sys.dm_exec_query_stats s
cross apply Sys.dm_exec_sql_text(s.plan_handle)t
where t.text like '%proc name%'
You will note you have a different plan_handle for each procedure.
I have run into an enigma of sorts while researching a performance issue with a specific stored procedure. I did not create that stored procedure, and the logic is fairly ugly with nested selects in join statements, etc...
However, when I copy the logic of the stored procedure directly into a new query window, add the parameters at the top and execute it, this runs in under 400 milliseconds. Yet, when I call the stored procedure and execute it with the exact same parameter values, it takes 23 seconds to run!
This makes absolutely no sense at all to me!
Are there some server-level settings that I should check which could potentially be impacting this?
Thanks
Recompile your stored procedure.
The Microsoft documentation says
When a procedure is compiled for the first time or recompiled, the procedure's query plan is optimized for the current state of the database and its objects. If a database undergoes significant changes to its data or structure, recompiling a procedure updates and optimizes the procedure's query plan for those changes. This can improve the procedure's processing performance.
and
Another reason to force a procedure to recompile is to counteract the "parameter sniffing" behavior of procedure compilation. When SQL Server executes procedures, any parameter values that are used by the procedure when it compiles are included as part of generating the query plan. If these values represent the typical ones with which the procedure is subsequently called, then the procedure benefits from the query plan every time that it compiles and executes. If parameter values on the procedure are frequently atypical, forcing a recompile of the procedure and a new plan based on different parameter values can improve performance.
If you run the SP's queries yourself (in SSMS maybe) they get compiled and run.
How to recompile your SP? (See the doc page linked above.)
You can rerun its definition to recompile it once. That may be enough if the procedure was first defined long ago in the life of your database and your tables have grown a lot.
You can put CREATE PROCEDURE ... WITH RECOMPILE AS into its definition so it will recompile every time you run it.
You can EXEC procedure WITH RECOMPILE; when you run it.
You can restart your SQL Server. (This can be a nuisance; booting the server magically makes bad things stop happening and nobody's quite sure why.)
Recompiling takes some time. But it takes far less time than a complex query with a bad execution plan would.
So... I ended up turning the nested selects on the joins to table variables, and now the sproc is executing in 60-milliseconds, while the in-line sql is taking 250=ms.
However, I still do not understand why the sproc was performing so much slower than the in-line sql version with the original nested sql logic?
I mean, both were using the exact same sql logic, so why was the sproc taking 23-seconds while the in-line was 400-ms?
I have a stored procedure which gets data from 5 tables. Tables are updated approximately 1000 records and 1000 updates in one hour. After inserting and updating, the stored procedure runs into a timeout.
When I rebuild one of the index of a table which is referenced in the stored procedure, it starts working normal again.. but it breaks down again after each new 1000 records updated.
What should I do?
Ok I think you are mistaken here when you say rebuilding the Index is solving the problem.
I think it is actually that rebuilding indexes invalids the the cached execution plan and on next execution after rebuilding the index will force sql server to recompile the execution plan.
Normally SQL Serve would use a cached execution plan for a stored procedure, but there are some factors that can cause sql server to recompile an execution plan for a stored procedure even if there a cached execution plan in proc cache memory. Rebuilding or any changes to an index that is being used in the execution of a stored procedure will result in recompilation of execution plan.
Since you are inserting 1000 rows every hour and you would also want to keep your statistics updated. I would say run an nightly job to update statistics.
But for your Store Procedure use WITH RECOMPILE option in your procedure's definition or use this option when executing this Stored Procedure and I think it will solve the issue.
to add this option in sp's definition
ALTER PROCEDURE myProc
WITH RECOMPILE
AS.......
Or to add this option when executing your stored procedure you can do as follows
EXECUTE myProc WITH RECOMPILE
Or you can also use a system stored procedure sp_recompile to force sql server to compile an execution plan even if there is one in cache memory.
EXECUTE sp_recompile N'dbo.MyProc';
GO
EXECUTE dbo.MyPrco;
GO
I am breaking my head on this issue since long. I have stored procedure in MS SQl and when I try to execute that procedure by providing all the parameters in SQL Query, it takes long time to execute but when I try to directly run the query which is there in SP it executes in no time. This is affecting my application performance also as we are using stored procedures to fetch the data from DB Server.
Please help.
Regards,
Vikram
Looks like parameter sniffing.
Here is a nice explanation: I Smell a Parameter!
Basically, sql server has cached query execution plan for the parameters it was first run with so the plan is not optimal for the new values you are passing. When you run the query directly the plan is generated at that moment so that's why it's fast.
You can mark the procedure for recompilation manually using sp_recompile or use With Recompile option in its definition so it is compiled on every run.
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?
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.
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
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.
I'd also look into parameter sniffing. Could be the proc needs to handle the parameters slighlty differently.
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.
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.
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.
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.
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.
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
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 :)