SQL Server Query Plan Cache

In the SQL server, the query plan cache plays an important role in ensuring efficient and optimized query execution. In this article, we are going to dive deep into the basics of query plan cache and its workflow. We will also learn how to view the query plan cache in our SQL Server databases.

What is a Query Plan Cache?

The query plan cache, as its name suggests, is a dedicated memory structure that stores execution plans for Transact-SQL statements. When a T-SQL statement is executed for the first time, SQL Server analyzes the statement’s syntax and semantics, generating an execution plan that outlines the most efficient approach to access the requested data. This execution plan is then cached (temporarily stored) so that when the same query is run again SQL server doesn’t need to create another query plan rather it uses the cached query plan which improves database performance. Query plans that are used most frequently are cached for a longer duration of time.

How Does the Query Plan Cache Work?

Workflow of query plan cache in SQL Servers

Here is an explanation of how query plan cache works step-by-step:

1. Query Execution – A SQL query is sent to the database engine for execution.

2. Check for Cache – SQL Server checks the query plan cache to see if there is an already compiled plan.

3. Cache Hit – If there is a plan, it’s a cache hit. The cached plan is run without having to recompile the code again.

4. Cache Miss – If no existing plan exists, it results in a cache miss. The query optimizer compiles a new query plan.

5. Store Plan – After compilation, the new plan is also stored in memory cache to reuse in future before returning results to the calling client.

Why Query Plan Cache is Important for SQL Server?

The query plan cache plays a crucial role in maintaining optimal SQL Server performance, contributing to several key benefits:

• Reduces Query Execution Time: By caching execution plans, SQL Server bypasses the need for repeated compilation and optimization, saving valuable processing time and expediting query execution.

• Improved Query Throughput: The minimized execution time for cached queries results into enhanced query throughput, enabling SQL Server to handle a higher volume of queries efficiently. As a result the queries are executed at a much faster rate.

• Overall System Performance: The efficiency gains resulting from the query plan cache extend to the overall performance of the SQL Server instance, contributing to a more responsive and scalable system.

• Saves compilation resources like CPU and memory.

How can We View the Query Plan Cache in SQL Servers

To view the query plan cache’s we will have to use the Dynamic Management Views (DMV’S) which provided real time insights into various aspects of SQL server operations. Follow the below steps to view the query plan cache in SQL Servers:

Step1: Connect to SQL Server Management Studio(SSMS).

Step2: Open a new Query window.

Step3: Execute the following commands

SELECT cplan.usecounts, cplan.objtype, qtext.text, qplan.query_plan
FROM sys.dm_exec_cached_plans AS cp
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st
CROSS APPLY sys.dm_exec_query_plan(plan_handle) AS qp
ORDER BY cplan.usecounts DESC

Step4: The usecounts columns represents how many time a query plan was reused.

Step5: We can use the following command to clear the plan cache

DBCC FREEPROCCACHE

Step6: After clearing the plan cache let’s run a simple query multiple times to see how query plan works by reusing the query plan cache.

Select * from Employees  Where FirstName = "Steve"

After executing the query for a single time the usecount =1.

Now when we again execute this query the usecount = 2 which indicates that we were able to reuse the same query plan again.

Conclusion

In summary, the SQL Server query plan cache is essential for reusing execution plans and providing fast query performance to the users. Monitoring the cache at frequent intervals is an important DBA responsibility. Leveraging this cache capability allows SQL Server databases to run much more efficiently.

FAQ’s on Query Plan Cache

Q.1: How can I monitor the query plan cache?

Answer:

You can easily monitor the query plan cache via the Dynamic Management Views, also know as DMV.

Q.2: Can the Query Plan Cache be disabled?

Answer:

Yes, you can disable it however it is not at all recommended to disable query plan cache because it can decrease your server’s performance.

Q.3: Does the Query Plan Cache persist across server restarts?

Answer:

No , the cache is cleared when the SQL server’s instance is restarted.

Q.4: What factors influence the size of the query plan cache?

Answer:

The size of the query plan cache is basically determined by the amount of memory allocated to it. The frequency of query executions also play a crucial role in the cache size.