We develop an application that works with MS SQL Server - our customers run anything from SQL 2008 Express to 2017 Standard. Our queries are not parameterised and it is impractical to rewrite the whole application so that they are. We therefore have a lot of plans for the same query. I have seen that there is an option in SSMS against the database to set Parameterisation to Forced, so that there will be fewer query plans, but that this can then cause issues with Parameter Sniffing with Stored Procedures.
Before I try changing that option, can I just clarify that Stored Procedures are pieces of code that you explicitly create and store in the database; running queries directly from the application do NOT get turned into Stored Procedures (even temporarily), so it isn't a problem.
Parameter sniffing can happen to both stored procedure calls and parameterized queries. In your case, the best option is to fix your application, which will take considerable efforts. So before that can happen, set the Parameterisation to Forced will certainly help to reduce the number of plans and tighten the security.
and no, this will not change your queries to stored procedures.
Related
Setup:
Using SQL Server 2008 R2.
We've got a stored procedure that has been intermittently running very long. I'd like to test a theory that parameter sniffing is causing the query engine to choose a bad plan.
Question:
How can I copy the query's execution plans from one database to another (test) database?
Note:
I'm fully aware that this may not be parameter sniffing issues. However, I'd like to go through the motions of creating a test plan and using it, if at all possible. Therefore please do not ask me to post code and/or table schema, since this is irrelevant at this time.
Plans are not portable, they bind to object IDs. You can use planguides, but they are strictly tied to the database. What you have to do is test on a restored backup of the same database. On a restored backup you can use a planguide. But for relevance the physical characteristics of the machines should be similar (CPUs, RAM, Disks).
Normally though one does not need to resort to such shenanigans as copy the plans. Looking at actual execution plans all the answers are right there.
Have you tried using OPTIMIZE FOR clause? With it you can tune your procedure easier, and without the risk that plan that you copy from another database will be inappropriate due to differences in those databases (if copying the plan is even possible).
http://www.mssqltips.com/sqlservertip/1354/optimize-parameter-driven-queries-with-sql-server-optimize-for-hint/
I was wondering if you guys could help me get to the bottom of a weird problem I have recently had on SQL Server.
I have a stored procedure (lets call SPold) which is reasonably large with a lot of calculations (can't possibly do this in app as info for around 6000 users needs to come back in a one-er (I reduce this to 1000 based on Surname)). The stored procedure usually executes in a couple of seconds, and is called once every couple of minutes.
Now this morning, the stored procedure was suddenly taking 4-10 times as long to execute, causing a number of timeouts. I discovered that by making a copy of the procedure with a new name (SPnew) and executing, I would get the fast execution times again. This indicated to me that the execution plan was the problem with the original, SPold, so I decided to execute it with recompile. This would return the results a quicker (although not as fast as SPnew), but subsequent calls from users to SPold were once again slow. It was like the new plan wasn't being kept.
What I have done is to fix this is put Exec SPnew into SPold, and now calls to SPold are returning fast again.
Does anyone have any idea what is going on here? The only thing that updated overnight was the statistics, although I think that this should affect both SPold and SPnew.
Sounds like you are experiencing an incorrectly cached query plan due to parameter sniffing.
Can you post the stored procedure?
Batch Compilation, Recompilation, and Plan Caching Issues in SQL Server 2005
I Smell a Parameter!
In SQL Server 2005, you can use the OPTIMIZE FOR query hint for preferred values of parameters to remedy some of the problems associated with parameter sniffing:
OPTIMIZE FOR Instructs the query optimizer to use a particular value for a local
variable when the query is compiled and optimized. The value is used
only during query optimization, and not during query execution.
OPTIMIZE FOR can counteract the parameter detection behavior of the
optimizer or can be used when you create plan guides. For more
information, see Recompiling Stored Procedures and Optimizing Queries
in Deployed Applications by Using Plan Guides.
Although SQL Server 2005 does not support OPTIMIZE FOR UNKNOWN (introduced in SQL Server 2008) which
will eliminate parameter sniffing for a given parameter:
OPTION (OPTIMIZE FOR (#myParam UNKNOWN))
You can achieve the same effect in SQL Server 2005 by copying the parameter into a local variable, and then use the local variable in the query.
I've also encounterred two "strange" cases with Sql Server 2005, which might relate to your problem as well.
In the first case my procedure executed prety fast, when being run as dbo, and it was slow when being run from the application, under a different user account.
In the second case the query plan of the procedure got optimized for the parameter values with which the procedure was called for the first time, and this plan was then reused later for other parameter values as well, resulting in a slow execution.
For this second case the solution was to copy the parameter values into local variables in the procedure, and then using the variables in the queries instead of the parameters.
in my recent subject ,I have to do some queries through dynamic SQL,But I'm curious about
the efficiency in different ways:
1)combine the sql sentences in my server and then send them to the database ,do the query
2)send my variables to database and combine them in a certain procedure and finally do the query
Hope someone can help
BTW(I use .Net and Sqlserver)
Firstly, one of the main things you should do is to parameterise your SQL - whether that be by wrapping it up as a stored procedure in the DB, or by creating the SQL statement in your application code and then firing the whole thing in to the DB. This will mean:
prevention against SQL injection attacks by not directly concatenating user-entered values into a SQL statement
execution plan reuse (subsequent executions of that query, regardless of parameter values, will be able to reuse the original execution plan) (NB. this could be done if not parameterised yourself, via Forced Parameterisation)
Stored procedures do offer some extra advantages:
security ,only need to grant EXECUTE permissions to the stored procedures, you don't need to grant the user direct access to underlying db tables
maintainability, a change to a query does not involve an application code change, you can just change the sproc in the DB
network traffic, not necessarily a major point but you're sending less over the wire especially if the query is pretty large/complex
Personally, I use stored procedures most of the time. Though the times I need to build up SQL dynamically in application code, it is always parameterised.
Best is to use stored procedure and pass parameters from your application, as Stored procedures are precompiled queries and have execution plan ready which saves lot of time.
You can refer this url which has details http://mukund.wordpress.com/2005/10/14/advantages-and-disadvantages-of-stored-procedure/
Happy coding!!
Related
LINQ-to-SQL vs stored procedures?
I have heard a lot of talk back and forth about the advantages of stored procedures being pre compiled. But what are the actual performance difference between LINQ and Stored procedures on Selects, Inserts, Updates, Deletes? Has anyone run any tests at all to see if there is any major difference. I'm also curious if a greater number of transactions makes a difference.
My guess is that LINQ statements get cached after the first transaction and performance is probably going to be nearly identical. Thoughts?
LINQ should be close in performance but I disagree with the statement above that says LINQ is faster, it can't be faster, it could possibly be just as as fast though, all other things being equal.
I think the difference is that a good SQL developer, who knows how to optimize, and uses stored procedures is always going to have a slight edge in performance. If you are not strong on SQL, let Linq figure it out for you, and your performance is most likely going to be acceptable. If you are a strong SQL developer, use stored procedures to squeeze out a bit of extra performance if you app requires it.
It certainly is possible if you write terrible SQL to code up some stored procedures that execute slower than Linq would, but if you know what you are doing, stored procedures and a Datareader can't be beat.
Stored procedures are faster as compared to LINQ query they can take the full advantage of SQL features.
when a stored procedure is being executed next time, the database used the cached execution plan to execute that stored procedure.
while LINQ query is compiled each and every time.
Hence, LINQ query takes more time in execution as compared to stored procedures.
Stored procedure is a best way for writing complex queries as compared to LINQ.
LINQ queries can (and should be) precompiled as well. I don't have any benchmarks to share with you, but I think everyone should read this article for reference on how to do it. I'd be especially interested to see some comparison of precompiled LINQ queries to SPROCS.
There is not much difference except that LINQ can degrade when you have lot of data and you need some database tuning.
LINQ2SQL queries will not perform any differently from any other ad-hoc parameterized SQL query, other than the possibility that the generator may not optimize the query in the best fashion.
The common perception is that ad-hoc sql queries perform better than Stored Procedures. However, this is false:
SQL Server 2000 and SQL Server version
7.0 incorporate a number of changes to statement processing that extend many
of the performance benefits of stored
procedures to all SQL statements. SQL
Server 2000 and SQL Server 7.0 do not
save a partially compiled plan for
stored procedures when they are
created. A stored procedure is
compiled at execution time, like any
other Transact-SQL statement. SQL
Server 2000 and SQL Server 7.0 retain
execution plans for all SQL statements
in the procedure cache, not just
stored procedure execution plans.
-- SqlServer's Books Online
Given the above and the fact that LINQ generates ad-hoc queries, my conclusion is that there is no performance difference between Stored Procedures & LINQ. And I am also apt to believe that SQL Server wouldn't move backwards in terms of query performance.
Linq should be used at the business logic layer on top of views created in sql or oracle. Linq helps you insert another layer for business logic, maintenance of which is in the hands of coders or non sql guy. It will definitely not perform as well as sql coz its not precompiled and you can perform lots of different things in sps.
But you can definitely add a programming detail and segregate the business logic from core sql tables and database objects using Linq.
See LINQ-to-SQL vs stored procedures for help - I think that post has most of the info. you need.
Unless you are trying to get every millisecond out of your application, whether to use a stored procedure or LINQ may need to be determined by what you expect developers to know and maintainability.
Stored procedures will be fast, but when you are actively developing an application you may find that the easy of using LINQ may be a positive, as you can change your query and your anonymous type that is created from LINQ very quickly.
Once you are done writing the application and you know what you need, and start to look at optimizing it, then you can look at other technologies and if you have good unit testing then you should be able to compare different techniques and determine which solution is best.
You may find this comparison of various ways for .NET 3.5 to interact with the database useful.
http://toomanylayers.blogspot.com/2009/01/entity-framework-and-linq-to-sql.html
I don't think I would like having my database layer in compiled code. It should be a separate layer not combined. As I develop and make use of Agile I am constantly changing the database design, and the process goes very fast. Adding columns, removing columns or creating a new tables in SQL Server is as easy as typing into Excel. Normalizing a table or de-normalizing is also pretty fast at the database level. Now with Linq I would also have to change the object representation every time I make a change to the database or live with it not truly reflecting how the data is stored. That is a lot of extra work.
I have heard that Linq entities can shelter your application from database change but that doesn't make sense. The database design and application design need to go hand in hand. If I normalize several tables or do some other redesign of the database I wouldn't want a Linq object model to no longer reflect the actual database design.
And what about advantage of tweaking a View or Stored Procedure. You can do that directly at the database level without having to re-compile code and release it to production. If I have a View which shows data from several tables and I decide to change the database design all I have to do is change that View. All my code remains the same.
Consider a database table with a million entries, joined to another table with a million entries... do you honestly think that doing this on the webserver (be it in LINQ or ad-hoc SQL) is going to be faster or more efficient than letting SQL Server do it on the database?
For simple queries, then LINQ is obviously better as it will be pre-compiled, giving you the advantage of having type checking , etc. However, for any intensive database operations, report building, bulk data analysis that need doing, stored procedures will win hands down.
<script>alert("hello") </script> I think that doing this on the webserver (be it in LINQ or ad-hoc SQL) is going to be faster or more efficient than letting SQL Server do it on the database?
For simple queries, then LINQ is obviously better as it will be pre-compiled, giving you the advantage of having type checking , etc. However, for any intensive database operations, report building, bulk data analysis that need doing, stored procedures will win hands dow
Why is it that if I run my query as a parameterized procedure it runs 10 times faster then if I run it directly as a parameterized query?
I'm using the exact same query in both cases, and it doesn't matter if I'm calling from Management Studio or an SqlCommand from code.
EDIT: The execution plan looks different. So, why? I'm calling it with EXACTLY the same set of parameters.
EDIT: After more testing it seems the 10x slowdown only occurs when running the parameterized query from SQL Management Studio.
One thing I've seen recently is that if you set up the query parameters wrong it can cause major problems.
For example, say you have a parameter for an indexed varchar column and set it up from .Net using the SqlCommand's AddWithValue() method. You're in for a world of hurt with this scenario. .Net uses unicode strings and will set up your parameter as an nvarchar rather than varchar. Now sql server won't be able to use your index, and you'll see a significant performance penalty.
Find out if they are using the same execution plan is to display it when running. Use "include actual execution plan" in management studio and see what is the difference.
The connection-level settings can be critical in some cases, especially ANSI NULLS, CONCAT NULL YIELDS NULL, etc. In particular, if you have calculated persisted indexed columns (including promoted "xml" columns), then it won't trust the pre-calculated, index value if the settings aren't compatible, and will recalculate for each row (i.e. table scan instead of index seek).
Parameterized queries have a lot of advantages, including often a hefty performance increases.
Caching of queries
String Concatenation problems minimized
addressing SQL injection
Data does not have to be converted to a string before processing
Parameter sniffing may be affecting the stored procedure performance.
http://omnibuzz-sql.blogspot.com/2006/11/parameter-sniffing-stored-procedures.html
Stored procedures can run faster because the execution plan is cached by sql server.
But 10 times performance is suspicious.
Does it run the same the First time after you've cleared the stored execution plans? You can use these commands to clear the cache. But they clear the whole servers cache so do it only on development servers.
DBCC FREEPROCCACHE
DBCC FLUSHPROCINDB (<dbid>)
Are you running these directly on the SQL server to eliminate any network I/O from the performance testing?
My guess is that it is running slowly the first time, then once it is cached it is running faster.