SQL Server provides a powerful command called DBCC FREEPROCCACHE that allows you to clear the procedural cache. The procedural cache stores execution plans for queries, which are reused to improve query performance. However, there may be situations where you need to remove specific execution plans or clear the entire cache. In this article, we will explore the usage of the DBCC FREEPROCCACHE command and its implications.

What is the Procedural Cache?

When a query is executed in SQL Server, the query optimizer generates an optimized execution plan and stores it in the procedural cache. This allows SQL Server to reuse the execution plan for subsequent executions of the same query, avoiding the need for full optimization. However, there are cases where a new execution plan may cause performance issues, such as high CPU and memory utilization. In such scenarios, you may want to remove specific execution plans or clear the entire cache.

Using DBCC FREEPROCCACHE to Clear a Specific Execution Plan

To remove a specific execution plan from the cache, you can use the DBCC FREEPROCCACHE command with the plan handle of the execution plan. The plan handle uniquely identifies a query execution plan in the cache. For example:

DBCC FREEPROCCACHE (0x05000900996DB224D002DAFF3802000001000000000000000000000000000000000000000000000000000000)

This command will remove the specified execution plan from the cache. You can validate this by re-running the query and observing the new execution plan generated.

Clearing All Execution Plans from the Cache

If you want to clear all execution plans cached for stored procedures, you can simply run the following command:

DBCC FREEPROCCACHE

This command will remove all execution plans from the cache. It is important to note that this command will also log an entry in the SQL Server error logs.

Clearing the Cache at the Resource Pool Level

In addition to clearing the cache for execution plans, you can also clear the cache at the resource pool level. Resource pools are used to manage the allocation of resources in SQL Server. To clear the cache for a specific resource pool, you can use the following command:

DBCC FREEPROCCACHE ('internal');

This command will clear the procedural cache for the specified resource pool. You can verify the cache memory and used memory for resource pools using the sys.dm_resource_governor_resource_pools dynamic management view.

Considerations and Best Practices

While the DBCC FREEPROCCACHE command can be useful in certain scenarios, it should be used with caution. Clearing the cache can increase the utilization of system processes such as CPU and memory. It is recommended to clear the cache only when it is crucial to do so, such as when you encounter performance issues caused by specific execution plans.

Alternatively, you can use other methods such as sp_recompile to recompile a stored procedure and generate a new execution plan. In SQL Server 2016 onwards, you can also use the ALTER SCOPED CONFIGURATION CLEAR PROCEDURE_CACHE command to clear the procedural cache in a specific database.

Conclusion

The DBCC FREEPROCCACHE command is a powerful tool in SQL Server that allows you to clear the procedural cache. By removing specific execution plans or clearing the entire cache, you can address performance issues caused by inefficient execution plans. However, it is important to use this command judiciously and consider alternative methods to resolve performance issues. Understanding the implications of the DBCC FREEPROCCACHE command will help you make informed decisions when managing the procedural cache in SQL Server.