1. Parameter Sniffing in SQL Server Stored Procedures
Miloš Radivojević
Parameter Sniffing in SQL Server Stored Procedures

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3. Miloš Radivojević Data Platform MVP
Principal Database Consultant, bwin GVC Vienna, Austria
Co-Founder: SQL Pass Austria
Conference Speaker, Book Author
Contact:
Twitter: @MilosSQL

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5. Agenda Query Execution Functions in the WHERE Clause
Data Type Conversion
Local Variables
APPLY vs. JOIN
Database Constraints and Performance
Other Tips
A typical example is a web search page with optional search criteria
The result set can be very large, but also empty
It is challenge to optimize the implementation for all search requests
Too many search criteria sometimes – application design problem

6. Query Execution Transact SQL is a declarative language
SQL Server generates an execution plan (cost-based optimization)
What can affect the query execution?
Which tables are involved in the query
Are there indexes available on the tables
Cardinality estimation by reading table statistics
How the query is written?
A typical example is a web search page with optional search criteria
The result set can be very large, but also empty
It is challenge to optimize the implementation for all search requests
Too many search criteria sometimes – application design problem

7. Functions in the WHERE Clause
Tip: Avoid functions in the WHERE clause with table columns as arguments!
An index will not be used at all or it will be used inefficiently
A typical example is a web search page with optional search criteria
The result set can be very large, but also empty
It is challenge to optimize the implementation for all search requests
Too many search criteria sometimes – application design problem

8. Data Type Conversion Tip: Choose the right data type! Otherwise:
Conversion overhead
String Conversion costs can be significant (a “non-Unicode” value will be converted to a “Unicode” value)
Estimation problem (for instance with the LIKE Operator)
A typical example is a web search page with optional search criteria
The result set can be very large, but also empty
It is challenge to optimize the implementation for all search requests
Too many search criteria sometimes – application design problem

9. Local Variables
Tip: Understand how local variables affect query execution
When local variables are used the SQL Server optimizer cannot generate the optimal execution plan because the variable value is not known at the compile time
Tip: Use OPTION (RECOMPILE) to get a good execution plan
A typical example is a web search page with optional search criteria
The result set can be very large, but also empty
It is challenge to optimize the implementation for all search requests
Too many search criteria sometimes – application design problem

10. Local Variables
Tip: Do not underestimate discrepancy in the cardinality estimation!
Otherwise you could see something like this:
A typical example is a web search page with optional search criteria
The result set can be very large, but also empty
It is challenge to optimize the implementation for all search requests
Too many search criteria sometimes – application design problem

11. Database Constraints
Tip: Database Constrains help SQL Server to make a better execution plan.
The main purpose of constraints is data integrity, but Unique Constraints, Check Constraints and Foreign Keys help SQL Server optimizer to make the right decision about query execution
A typical example is a web search page with optional search criteria
The result set can be very large, but also empty
It is challenge to optimize the implementation for all search requests
Too many search criteria sometimes – application design problem

12. Other Tips
Tip: Use UNION ALL when you know that sets are not overlapped and when duplicates are allowed!
Tip: IN vs. EXISTS! perform the same. Use what you are more comfortable with!
The same tip for EXISTS vs. COUNT(*)
A typical example is a web search page with optional search criteria
The result set can be very large, but also empty
It is challenge to optimize the implementation for all search requests
Too many search criteria sometimes – application design problem

13. Thanks to our sponsors!
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