Parameter Sniffing with Database Scoped Configurations in SQL Server 2016
Eric Russo | | June 9, 2016
I have seen some of the basics that I learnt more than two decades don’t seem to change. One of the traditional performance tuning I used to get bumped into was around parameter sniffing working with SQL Server. Having said that, there are various settings in SQL Server which were at the server level till SQL Server 2014. Most of them can be set via sp_configure setting or trace flags at server level. Before we move forward and explain the feature, it is important to learn the basics – more of a primer to those who haven’t encountered this. Let us take a step back and understand parameter sniffing in SQL Server.
What is parameter sniffing?
Parameter sniffing is a phenomenon in SQL Server where SQL optimizer would “sniff” the value of parameter during compile time. SQL Server query optimizer has freedom to choose complete different plan for same query with different parameter values. Due to parameter sniffing, it is observed that cached plan is picked up which might cause slow performance.
Let’s look at example now to understand it. We would create database object.
USE [master] GO IFDB_ID('SniffingDB')ISNOTNULL BEGIN ALTERDATABASE [SniffingDB] SETSINGLE_USERWITHROLLBACKIMMEDIATE DROPDATABASE [SniffingDB] END GO CREATEDATABASE SniffingDB GO USE SniffingDB GO CREATETABLE SkewedDataTable( id INTidentity ,Gender CHAR(1) ,WideColumn CHAR(7000)DEFAULT'A' ) GO SETNOCOUNTON GO INSERTINTO SkewedDataTable DEFAULT VALUES('F') GO 995 INSERTINTO SkewedDataTable(Gender) VALUES ('M') GO 5 CREATEINDEX idx_SkewedDataTable_gender ON SkewedDataTable(Gender) GO
There is special thing with the data above. We have deliberately populated skewed data in gender column by inserting 995 values with “F” and 5 values with “M”. This would cause different query plan for both the values. We would create stored procedure which would use that column in predicate.
CREATEPROCEDURE CountGender @Gender CHAR(1) AS BEGIN SELECT* FROM SkewedDataTable WHERE Gender = @Gender END
If we run the parameter with “M” and “F”, we would get different plan, provided it’s not already generated. To demonstrate it, we can run below script in the database which uses DBCC
FREEPROCCACHE. USE SniffingDB GO SETSTATISTICSPROFILEON GO DBCC FREEPROCCACHE GO EXEC CountGender'M' GO EXEC CountGender'F' GO DBCC FREEPROCCACHE GO EXEC CountGender'F' GO EXEC CountGender'M' GO SETSTATISTICSPROFILEOFF
Once we run above, we would see 4 plans in the output. First two and last two would be same. This is because of plan reuse.
How do we solve it?
Parameter sniffing may not be a problem in all scenarios but if there is a skewed data distribution then we may want to disable the parameter sniffing. Microsoft has provided server level trace flag 4136 which can be used to disable it. This is documented in Microsoft knowledge base article 980653.
What is new in SQL Server 2016?
The problem with the trace flag is that it would affect all the databases on the instance. As we can see below in the database properties window, we do have option to turn ON and OFF for “Parameter Sniffing” option.
Here is the command to turn off the sniffing at database level. USE [SniffingDB] GO ALTERDATABASESCOPEDCONFIGURATIONSETPARAMETER_SNIFFING=OFF; GO
Once above command is run and if we execute the same procedure again, we can notice that irrespective of the value passed, the plan would be the same for all four executions.
You might wonder, what is the meaning of “Parameter Sniffing For Secondary” is? It is useful in the scenarios where availability group is deployed and we want different setting for optimizer on primary and secondary replica. There are three possible values:
- Primary – Same value on secondary which is set on primary
- ON – Turn it OFF.
- OFF – Turn it ON.
Other than parameter sniffing, there are more setting at database level which we would cover in some other blog.
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