Posts tagged query optimizer and parameter sniffing. This is because the filter is different, with the orderdate being may 30 th in the first query and may 31 st in the second query. Parameter sniffing is when the parameter values with the cached plan vary enough from the. Why parameter sniffing hurts your sql query performance. Incidentally, while you can assign the parameter to a local variable to circumvent sniffing, a more flexible option is to use the optimize for or recompile hints in queries that are particularly affected or with recompile on the whole stored procedure, but thats a little more drastic. This means that the execution in front of you was optimized for an input parameter value of 1, but actually you executed the stored procedure with a parameter value of 2. Nov 27, 2007 so, why is parameter sniffing so often a problem. Disable sql server parameter sniffing at the instance level. I agree, except the optimizer didnt do it best this time. Parameter sniffing only becomes a problem when the cached plan isnt anywhere close to being the optimal plan for given input parameters.
However, microsoft recently released a cumulative update which provides a trace flag to disable parameter sniffing at the instance level. Different approaches to correct sql server parameter sniffing. By first time, i really mean whenever sql server is forced to compile or recompile a stored procedures because it is not in the procedure cache. When a stored procedure is compiled the last time a query plan is created according to the incoming parameter, this query plan may not be suitable for the next parameter. Of course, sometimes it can also be a problem but there are some solutions available. In any case, parameter sniffing wont be an issue the table contents are never used to optimize the query plan. Exceptions second best idea instead of attempting to get sql server to deliver a good enough execution plan every time, its possible to provide branching logic for queries where parameter sniffing can produce bad results. The three most common methods for resolving bad parameter sniffing are, plan forcing using query store currently available in azure sql database and sql server 2016, i have written extensively on this topic, using a recompile hint to get a new plan every time, or, using the optimize for hint to get a plan based on a specific value or on the. The issue with optimize for unknown hints is that they can produce unpredictable performance, just like parameter sniffing option 5. Enhancing query performance with adaptive query processing in.
For years, microsoft has partnered with many healthcare organizations to envision and deliver on their digital future. Most of the time a proc will run just fine, but sometimes it will run very slow. Parameter sniffing is a phenomenon in sql server where sql optimizer would sniff the value of parameter during compile time. This process, as you can see, sometime causes your sp to execute a plan that was optimized based on a different set of parameters because of plan caching. Parameter sniffing is when the parameter values with the cached. The idea is that if the parameter values are known, then the appropriate column statistics can be used and the optimiser can estimate the number of rows that the. Parameter sniffing is enabled by default in sql server. Sql server tries always to generate the most optimized execution plan for.
Sql server parameter sniffing and optimize for unknown you can take advantage of the new query hint of optimize for unknown to simulate the local variable in the stored procedure. If a sql query has parameters, sql server creates an execution plan tailored to them to improve performance, via a process called parameter sniffing. For this first version of this adaptive query processing feature family, we have three newread more. These past few months brought on an unprecedented urgency, as healthcare organizations pivoted to manage the unexpected. In my last post, we have gone through the parameter sniffing and possible solutions for parameter sniffing. Sep 20, 2010 parameter sniffing can lead to inefficient execution plans sometimes. The optimize parameters grid operator is applied on it. Parameter sniffing is the result of using the query plan in the cache. Sql server may choose a key lookup plan for a small number of values, and a clustered index seek or scan for a large number of values. Indexes good idea often, inadequate covering indexes can be the root cause of parameter sniffing. If these values are typical and the data distribution is even in the underlying tables, all the calls to the stored procedure will benefit from this query plan since the plan is reused. Parameter sniffing, embedding, and the recompile options. We also know that the plan cache can store these execution plans so they can be reused the next time the same query needs to be executed again.
Parameter sniffing allows sql to compile a plan that is tailored to the type of parameter that is actually passed into the stored procedure. While the queries are very similar and will likely require the same execution plan, sql server will create a separate plan for each. For each subsequent execution the optimizer is going to assume the estimates are good even though the estimates may be way off. Optimize for hints when parameter sniffing is turned off dzone. Thats the parameter sniffing problem in sql server. Sql server parameter sniffing and optimize for unknown. You can see in the selected parameters window that the c and gamma parameters of the svm operator are selected. Postmigration validation and optimization guide sql server. Jun 26, 20 the data in your tables and your queries can lead to parameter sniffing not making sense, however. If youve ever searched for sql server parameter sniffing youve probably read all sorts of bad things about it.
Parameter sniffing occurs when a parameterized query uses cached cardinality estimates to make query plan decisions. The problem occurs when the first execution has atypical parameter values. When the database engine compiles a stored procedure, it may use the actual parameters supplied to prepare an execution plan. The compile time value is 1, and the runtime value is 2. In my previous post, i explained the basic of parameter sniffing, and then built on that to explain the lesser known mechanics of variable sniffing and cardinality sniffing. Like recompile query hint, this can be useful for parameter sniffing. Understanding how to identify if it is a problem and knowing various ways to fix it when it is a problem are valuable skills to have. Jun 24, 2012 parameter sniffing,slow in application fast in ssms, as we discusses the earlier, query optimization and plan generation are costly operations. Parameter sniffing on table valued parameters stack overflow. Parameter sniffing,slow in application fast in ssms, as we discusses the earlier, query optimization and plan generation are costly operations. At its heart, parameter sniffing is the ability of the sql server optimiser to know the values of parameters passed to a stored proc at the point that it compiles the procedure.
Optimize for hints when parameter sniffing is turned off. Dynamics 365 blog modernizing business process with cloud. When a stored procedure is compiled or recompiled, the parameter values passed for that invocation are sniffed and used for cardinality estimation. The query optimizer uses parameter sniffing to help determine what execution plan should be used to optimize execution of an sp. So how can we use optimize for unknown to avoid parameter sniffing. The cached execution plan may not be welloptimized for the new parameter. Dynamics 365 blog modernizing business process with. This is an issue that i tend to be seeing often as of late. Oct 20, 2014 and as you can see they are different.
Parameter sniffing is useful for reusing the same execution plan for the same. I have tried updating statistics on every table in the database and this has not solved the problem. It is easy to overcome the parameter sniffing for the stored procedure by declaring the local variable inside the stored procedure. Sql server parameter sniffing in depth gavin draper sql. If the parameters are atypical, the plan may be slow for typical parameters. Optimize for query hint is the query hint that instructs the query optimizer to use a specific value for a local variable when creating the query plan. How local variable or optimize for unknown resolve parameter. Misconceptions on parameter sniffing sql server fast. The cached execution plan may not be welloptimized for the new parameter, hence the second call of the query may be very slow. In the possible solutions except the local variable and optimize for unknown are very straight forward solution and we know how they helps us to resolve the issue.
In this article, i will explain what parameter sniffing is, and then explore different ways of dealing with the performance problems that are occasionally caused to parameter. This post was authored by joe sack, principal program manager, data systems group, microsoft sql server 2017 and azure sql database introduce a new generation of query processing improvements that will adapt optimization strategies to your application workloads runtime conditions. When parameter sniffing stinks, test methods to correct it and determine the best way to freshen the air in the. Some of these solutions are covered in my posts here and here however, microsoft recently released a cumulative update which provides a trace flag to.
Have a look at the edit parameter settings parameter of the optimize parameters grid operator. Nov 03, 2016 in my previous post, i explained the basic of parameter sniffing, and then built on that to explain the lesser known mechanics of variable sniffing and cardinality sniffing. Optimize for local variables if you have local variables used in the query then the sql server optimize wont be able to know about them on compilation time as theyll appear only at execution time. To avoid this cost, sql server will try to keep the generated execution plans in plan cache and reuse the same in subsequent execution. Thanks for a great explanation of the issues involved with parameter sniffing, etc. Sql server parameter sniffing posted by karthick p.
Sql server parameter sniffing simplest example sql authority. Incidentally, while you can assign the parameter to a local variable to circumvent sniffing, a more flexible option is to use the optimize for or recompile hints in queries that are particularly affected or with recompile on the. In this article, well look at the ways in which we can approach it. This is known as parameter sniffing because the optimizer sniffs the current parameter value during compilation.
May 31, 2017 if youve ever searched for sql server parameter sniffing youve probably read all sorts of bad things about it. Jul 28, 2015 enabling trace flag 46 disables parameter sniffing, which is equivalent to adding an optimize for unknown hint to each query which references a parameter. Jul 20, 2018 parameter sniffing is the result of using the query plan in the cache. If i used optimize for unknown instead of optimize for a value, the results would be the same as when i disable parameter sniffing the optimizer just uses an average instead of specific values.
Or simply from the options tab of the database properties window. In truth parameter sniffing is an optimization technique sql server uses to allow plans to be cache and reused with different parameter values and for the most part it works well and significantly improves overall performance vs not using parameter sniffing. Optimize for unknown will save you from the expense of compiling. Force the optimizer to generate one plan and use it for all parameter combinations leave the optimizer to generate a plan, but for every parameter combination use an optimal plan which one to use. Parameter sniffing is not always a bad idea and it is not always the reason of performance issues. Windows 2008 and windows 2008 r2 known issues related to working set memory. Sep 28, 2017 this post was authored by joe sack, principal program manager, data systems group, microsoft sql server 2017 and azure sql database introduce a new generation of query processing improvements that will adapt optimization strategies to your application workloads runtime conditions. For those who dont know, parameter sniffing is when sql server uses the precise values passed into a query as a parameter this means stored procedures or prepared statements to generate an execution plan from the statistics using the value from the parameter. In this post we will see how local variable and option for unknown are resolving the parameter sniffing issue. This behavior is called parameter sniffing and it is a good thing. When the optimizer creates an execution plan it sniffs the parameter values.
Parameter sniffing vs variables vs recompile vs optimize for. We are going to run the above 2 queries here also but with the optimize query hint in the second query. However, it is not possible to use the same plan, if the connection has different set options. If you dont find parameter sniffing useful, you can disable it. Queries that meet any of the following conditions still undergo parameter sniffing even if the trace flag is set. You can take advantage of the new query hint of optimize for unknown to simulate the local variable in the stored procedure.
In other words, if you know that using a specific value for that parameter will generate the best plan. Parameter sniffing nelsonaloor, 20120629 first published. Plan reuse is an important part of how execution plans are managed. With this a plan is calculated every time, and adapted to each parameter value sent to the sp. I can only imagine that my problem is parameter sniffing. Parameter sniffing is the process whereby sql server creates an optimal plan for a stored procedure by using the calling parameters that are passed the first time a stored procedure is executed. It may be enabled in a future sql server release though which is a good reason to use one of the other options outlined in this chapter if you have a choice. In other words, if you know that using a specific value for that parameter will generate the best plan, you can specify that value in the query hint as a. We also know that the plan cache can store these execution plans so they can be reused. As i mentioned in a previous post, parameter sniffing is a good thing. In my article what is parameter sniffing you can find an example with this type of query hint.
Parameter sniffing vs variables vs recompile vs optimize. Sql server 2008 r2 cu2, sql server 2008 sp1 cu7 and sql. Sql server parameter sniffing in depth gavin draper. Depends on business requirements goals of parameter sniffing optimization. As a result, hardcoded literals in queries will result in different execution plans for each different value that is used in the query. Now i will demonstrate a parameter sniffing issue with an example. Generally speaking, this feature allows more efficient stored procedure execution plans, but a key requirement for everything to work as expected is that the parameter values used for compilation be typical.
But if you want, starting the instance with trace flag 46 set will disable parameter sniffing. Taking the resulting sql and running it in ssms runs in 3 or less seconds. Sql server tries to optimize the execution of your stored procedures by creating compiled execution plans. The query optimizer will use the same execution plan for the next call of the stored procedure.
For consistent stored procedure sproc performance parameter sniffing should be eliminated. So using local variable is like disabling the parameter sniffing which is not good so far. Parameter sniffing is most commonly a problem when data distribution is skewed, when theres some parameter values that return a small number of rows and some that return a large number of rows. The parameter sniffing problem can be tricky to handle. Microsoft recommend full testing before you do this though. The net effect is that the plan is optimized as if those specific parameter values were. A common cause for this type of situation is parameter sniffing. How local variable or optimize for unknown resolve. But we still need to decide which value we want to use that suits most of the. Enabling trace flag 46 disables parameter sniffing, which is equivalent to adding an optimize for unknown hint to each query which references a parameter. Just occasionally, it isnt, and you can then hit performance problems, as greg larsen explains. However because it was common to use local variables to defeat parameter sniffing in sql server 7.
Nov 24, 2015 for years, microsoft has partnered with many healthcare organizations to envision and deliver on their digital future. Dec 12, 2016 the three most common methods for resolving bad parameter sniffing are, plan forcing using query store currently available in azure sql database and sql server 2016, i have written extensively on this topic, using a recompile hint to get a new plan every time, or, using the optimize for hint to get a plan based on a specific value or on the. Same effect as using the local variable technique by totally disabling parameter sniffing, unless optionrecompile, with recompile or optimize for is used. Sql server 2016 parameter sniffing as a database scoped. Sql server query optimizer has freedom to choose complete different plan for same query with different parameter values. Enhancing query performance with adaptive query processing.
This plan is stored and reused since it is usually the best execution plan. We can see that by default parameter sniffing is set to on, so queries will use this by feature by default. Understand that parameter sniffing is not a bad thing it is an integral part of sql server. Nov 09, 2014 thanks for a great explanation of the issues involved with parameter sniffing, etc. First of all, dont be scared about parameter sniffing. Unlike traditional software design patterns, which more revolve around code and. Rewrite the stored procedure to use the option optimize for unknown. Aug 28, 20 however, for most workloads, disabling parameter sniffing entirely is the wrong solution, and may even be a disaster. In most cases the database workload is homogeneous so parameter sniffing wont be a problem. Sql parameter sniffing due to data skews microsoft tech.
739 735 1584 695 1222 453 1342 603 1444 945 455 387 1217 1357 954 1328 1468 344 1608 225 635 909 124 415 595 725 1488 1118 197 969 193 1434