SQL Server Option (Recompile): When and Why to Use It
Understanding SQL Server's Query Optimization
SQL Server, like any database management system, optimizes queries to execute them efficiently. It analyzes the query and chooses the best execution plan based on existing data statistics. However, sometimes these statistics become outdated, leading to suboptimal query plans. This is where the OPTION (RECOMPILE) hint comes in.
The Problem with Outdated Statistics
Consider this scenario:
SELECT *
FROM Customers
WHERE City = 'New York';
Initially, the table Customers has 10% of its data residing in 'New York'. SQL Server builds an execution plan based on this information. Now, let's imagine that over time, the percentage of 'New York' customers grows to 50%. The old execution plan, optimized for 10%, might not be efficient anymore.
The OPTION (RECOMPILE) Solution
The OPTION (RECOMPILE) hint instructs SQL Server to recompile the query every time it's executed. This ensures that a fresh execution plan is created based on the current data statistics. This can improve performance in cases where the query's data distribution has significantly changed since the last compile.
When to Use OPTION (RECOMPILE):
- Dynamic Queries: Queries with variables that affect the query plan, such as
WHEREclauses with parameters, can benefit from recompilation. - Changing Data Distributions: When data significantly shifts over time, recompiling can ensure optimal plan generation.
- Temporary Tables: Since statistics on temporary tables are not updated frequently,
OPTION (RECOMPILE)can improve execution plans.
Important Considerations:
- Performance Overhead: Recompiling a query incurs a small performance overhead. Use it judiciously.
- Alternatives: Updating statistics can be a more sustainable solution for outdated data distributions.
- Optimization Level: The query optimizer's behavior is affected by the
OPTIMIZEserver setting.
Practical Examples:
-
Dynamic WHERE Clause:
SELECT * FROM Customers WHERE City = @City OPTION (RECOMPILE);This query will be recompiled every time it's executed with a different value for
@City. -
Improving Query for Temporary Table:
SELECT * FROM #TempTable OPTION (RECOMPILE);This will ensure that the execution plan for the query on the temporary table is based on the current data.
In Conclusion:
OPTION (RECOMPILE) is a useful tool for situations where the query optimizer might not have the latest data information. However, it's crucial to use it judiciously and weigh its benefits against potential performance overheads. Understanding the underlying mechanism of query optimization and data statistics helps in making informed decisions regarding this hint.
Additional Resources:
