Optimizing SQL Server Workload with Plan Cache

Tuning workload in SQL Server effectively often involves understanding and optimizing the queries that your applications run. The Plan Cache in SQL Server is a key resource in this process, as it stores execution plans that SQL Server uses to execute queries. By analyzing the Plan Cache, you can identify inefficient queries, understand how SQL Server is executing them, and make informed decisions on indexing, query rewriting, or database design changes to improve performance. Here are some strategies for tuning your workload using the Plan Cache:

1. Viewing Cached Execution Plans

You can use the Dynamic Management Views (DMVs) sys.dm_exec_cached_plans and sys.dm_exec_query_plan to see the execution plans that are currently cached by SQL Server.

SELECT 
    cp.plan_handle,
    qp.query_plan
FROM 
    sys.dm_exec_cached_plans AS cp
    CROSS APPLY sys.dm_exec_query_plan(cp.plan_handle) AS qp
WHERE 
    cp.cacheobjtype = 'Compiled Plan';

2. Identifying High-Cost Queries

Combine sys.dm_exec_query_stats with sys.dm_exec_query_plan to identify queries that have high resource usage. This can help you pinpoint where tuning efforts should be focused.

SELECT 
    qs.total_logical_reads, qs.total_logical_writes,
    qs.total_worker_time,
    qs.total_elapsed_time, qs.execution_count,
    qp.query_plan
FROM 
    sys.dm_exec_query_stats AS qs
    CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) AS qp
ORDER BY 
    qs.total_worker_time DESC;

3. Analyzing Execution Plans for Optimization Opportunities

Look for scans, missing indexes, and other inefficiencies in the execution plans. SQL Server's graphical plan display in SQL Server Management Studio (SSMS) can help you visually identify potential issues.

4. Forcing Query Plans

In some cases, you might find that a particular execution plan is more efficient than others. SQL Server allows you to force a query to use a specific plan using Query Store or Plan Guides.

5. Removing Ad-Hoc Queries from the Plan Cache

Ad-hoc queries can bloat the Plan Cache, reducing the efficiency of SQL Server's caching mechanism. You can use the DBCC FREEPROCCACHE command to clear the cache, but use it with caution as it affects all queries. Alternatively, focus on parameterizing queries to reduce plan cache bloat.

6. Using Parameterized Queries

Parameterized queries help SQL Server reuse execution plans more effectively, reducing compilation overhead and improving performance for repeated query patterns.

7. Monitoring Plan Cache Size

Monitoring the size of the Plan Cache can help you understand if SQL Server has sufficient memory allocated to caching execution plans. Use the sys.dm_os_memory_clerks DMV to monitor memory usage by the Plan Cache.

SELECT 
    SUM(pages_kb)/1024.0 AS CacheSizeMB
FROM 
    sys.dm_os_memory_clerks 
WHERE 
    type = 'CACHESTORE_SQLCP';

Best Practices

Regularly review the Plan Cache to identify and address inefficient queries.
Use SQL Server's Index Tuning Wizard and Database Engine Tuning Advisor to analyze queries and get recommendations for indexing.
Consider the impacts of clearing the Plan Cache, as it can temporarily degrade performance until the cache is repopulated.
Stay proactive in query optimization to minimize the need for emergency performance tuning.

Tuning workload in SQL Server using the Plan Cache is a powerful method for improving database performance. By analyzing cached plans, you can gain insights into query performance and make targeted optimizations to enhance the efficiency of your SQL Server instances.

Tuning workload in SQL Server using Plan Cache