DBCC FREEPROCCACHE (Transact-SQL)

Άρθρο
01/13/2023

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

Removes all elements from the plan cache, removes a specific plan from the plan cache by specifying a plan handle or SQL handle, or removes all cache entries associated with a specified resource pool.

Note

DBCC FREEPROCCACHE doesn't clear the execution statistics for natively compiled stored procedures. The procedure cache doesn't contain information about natively compiled stored procedures. Any execution statistics collected from procedure executions will appear in the execution statistics DMVs: sys.dm_exec_procedure_stats (Transact-SQL) and sys.dm_exec_query_plan (Transact-SQL).

Transact-SQL syntax conventions

Syntax

Syntax for SQL Server and Azure SQL Database:

DBCC FREEPROCCACHE [ ( { plan_handle | sql_handle | pool_name } ) ] [ WITH NO_INFOMSGS ]

Syntax for Azure Synapse Analytics and Analytics Platform System (PDW):

DBCC FREEPROCCACHE [ ( COMPUTE | ALL ) ]
     [ WITH NO_INFOMSGS ]
[;]

Arguments

( { plan_handle | sql_handle | pool_name } )

plan_handle uniquely identifies a query plan for a batch that has executed and whose plan resides in the plan cache. plan_handle is varbinary(64) and can be obtained from the following dynamic management objects:

sql_handle is the SQL handle of the batch to be cleared. sql_handle is varbinary(64) and can be obtained from the following dynamic management objects:

pool_name is the name of a Resource Governor resource pool. pool_name is sysname and can be obtained by querying the sys.dm_resource_governor_resource_pools dynamic management view.

To associate a Resource Governor workload group with a resource pool, query the sys.dm_resource_governor_workload_groups dynamic management view. For information about the workload group for a session, query the sys.dm_exec_sessions dynamic management view.

WITH NO_INFOMSGS

Suppresses all informational messages.

COMPUTE

Purge the query plan cache from each Compute node. This is the default value.

ALL

Purge the query plan cache from each Compute node and from the Control node.

Note

Starting with SQL Server 2016 (13.x), ALTER DATABASE SCOPED CONFIGURATION CLEAR PROCEDURE_CACHE can be used to clear the procedure (plan) cache for the current database.

Remarks

Use DBCC FREEPROCCACHE to clear the plan cache carefully. Clearing the procedure (plan) cache causes all plans to be evicted, and incoming query executions will compile a new plan, instead of reusing any previously cached plan.

This can cause a sudden, temporary decrease in query performance as the number of new compilations increases. For each cleared cachestore in the plan cache, the SQL Server error log will contain the following informational message:

SQL Server has encountered %d occurrence(s) of cachestore flush for the '%s' cachestore (part of plan cache) due to 'DBCC FREEPROCCACHE' or 'DBCC FREESYSTEMCACHE' operations.

This message is logged every five minutes as long as the cache is flushed within that time interval.

The following reconfigure operations also clear the procedure cache:

access check cache bucket count
access check cache quota
clr enabled
cost threshold for parallelism
cross db ownership chaining
index create memory
max degree of parallelism
max server memory
max text repl size
max worker threads
min memory per query
min server memory
query governor cost limit
query wait
remote query timeout
user options

In Azure SQL Database, DBCC FREEPROCCACHE acts on the database engine instance hosting the current database or elastic pool. Executing DBCC FREEPROCCACHE in a user database clears the plan cache for that database. If the database is in an elastic pool, it also clears the plan cache in all other databases in that elastic pool. Executing the command in the master database has no effect on other databases on the same logical server. Executing this command in a database using Basic, S0, or S1 service objective may clear the plan cache in other databases using these service objectives on the same logical server.

Result sets

When the WITH NO_INFOMSGS clause isn't specified, DBCC FREEPROCCACHE returns:

DBCC execution completed. If DBCC printed error messages, contact your system administrator.

Permissions

Applies to: SQL Server, Analytics Platform System (PDW)

Requires ALTER SERVER STATE permission on the server.

Applies to: Azure SQL Database

Requires membership in server role ##MS_ServerStateManager##.

Applies to: Azure Synapse Analytics

Requires membership in the db_owner fixed server role.

Remarks for Azure Synapse Analytics and Analytics Platform System (PDW)

Multiple DBCC FREEPROCCACHE commands can be run concurrently.

In Azure Synapse Analytics or Analytics Platform System (PDW), clearing the plan cache can cause a temporary decrease in query performance as incoming queries compile a new plan, instead of reusing any previously cached plan.

DBCC FREEPROCCACHE (COMPUTE) only causes SQL Server to recompile queries when they are run on the Compute nodes. It doesn't cause Azure Synapse Analytics or Analytics Platform System (PDW) to recompile the parallel query plan that is generated on the Control node.

DBCC FREEPROCCACHE can be canceled during execution.

Limitations and restrictions for Azure Synapse Analytics and Analytics Platform System (PDW)

DBCC FREEPROCCACHE can't run within a transaction.

DBCC FREEPROCCACHE isn't supported in an EXPLAIN statement.

Metadata for Azure Synapse Analytics and Analytics Platform System (PDW)

A new row is added to the sys.pdw_exec_requests system view when DBCC FREEPROCCACHE is run.

Examples: SQL Server

A. Clear a query plan from the plan cache

The following example clears a query plan from the plan cache by specifying the query plan handle. To ensure the example query is in the plan cache, the query is first executed. The sys.dm_exec_cached_plans and sys.dm_exec_sql_text dynamic management views are queried to return the plan handle for the query.

The plan handle value from the result set is then inserted into the DBCC FREEPROCACHE statement to remove only that plan from the plan cache.

USE AdventureWorks2022;
GO
SELECT * FROM Person.Address;
GO
SELECT plan_handle, st.text
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st
WHERE text LIKE N'SELECT * FROM Person.Address%';
GO

Here's the result set.

plan_handle                                         text
--------------------------------------------------  -----------------------------
0x060006001ECA270EC0215D05000000000000000000000000  SELECT * FROM Person.Address;
  
(1 row(s) affected)

-- Remove the specific plan from the cache.
DBCC FREEPROCCACHE (0x060006001ECA270EC0215D05000000000000000000000000);
GO

B. Clear all plans from the plan cache

The following example clears all elements from the plan cache. The WITH NO_INFOMSGS clause is specified to prevent the information message from being displayed.

DBCC FREEPROCCACHE WITH NO_INFOMSGS;

C. Clear all cache entries associated with a resource pool

The following example clears all cache entries associated with a specified resource pool. The sys.dm_resource_governor_resource_pools view is first queried to obtain the value for pool_name.

SELECT * FROM sys.dm_resource_governor_resource_pools;
GO
DBCC FREEPROCCACHE ('default');
GO

Examples: Azure Synapse Analytics and Analytics Platform System (PDW)

D. DBCC FREEPROCCACHE basic syntax

The following example removes all existing query plan caches from the Compute nodes. Although the context is set to UserDbSales, the Compute node query plan caches for all databases will be removed. The WITH NO_INFOMSGS clause prevents informational messages from appearing in the results.

USE UserDbSales;
DBCC FREEPROCCACHE (COMPUTE) WITH NO_INFOMSGS;

The following example has the same results as the previous example, except that informational messages will show in the results.

USE UserDbSales;
DBCC FREEPROCCACHE (COMPUTE);

When informational messages are requested and the execution is successful, the query results will have one line per Compute node.

E. Grant permission to run DBCC FREEPROCCACHE

The following example gives the login David permission to run DBCC FREEPROCCACHE.

GRANT ALTER SERVER STATE TO David;
GO

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