How to resolve multiple execution plans cache issue?

In SQL Server, multiple execution plans being cached for the same query can lead to performance degradation. This happens due to inefficient memory usage and inconsistent query execution times. This is often caused by parameterization issues, query variations, or different SET options. Here’s how to resolve it:

1. Use Forced Parameterization

By default, SQL Server uses simple parameterization, where only certain parts of the query are parameterized. This can cause multiple execution plans to be cached for slight query variations. Enabling forced parameterization ensures that SQL Server treats all literals as parameters, resulting in fewer cached plans.

Steps to Enable Forced Parameterization:

1. Enable it at the database level: ALTER DATABASE [DatabaseName] SET PARAMETERIZATION FORCED;
2. Monitor query performance after enabling this feature.

• Caution: Forced parameterization might not be suitable for all workloads. Use it for queries with frequent literal variations.

2. Use Query Store

Query Store (available in SQL Server 2016 and later) helps identify and resolve issues with multiple cached execution plans.

Steps:

1. Enable Query Store: ALTER DATABASE [DatabaseName] SET QUERY_STORE = ON;
2. Use Query Store reports to:
   • Identify queries with multiple plans.
   • Force a specific plan using:
     • EXEC sp_query_store_force_plan @query_id, @plan_id;

3. Use Plan Guides

Plan guides allow you to enforce a single execution plan for a query without modifying the query itself.

Steps:

1. Identify the problematic query and its optimal execution plan.
2. Create a plan guide to force the use of the desired plan:
   • EXEC sp_create_plan_guide @name = N'GuideName', @stmt = N'SELECT * FROM Table WHERE Column = @Param', @type = N'SQL', @params = NULL, @hints = N'OPTION (OPTIMIZE FOR UNKNOWN)';

4. Parameterize Queries in Code

Ensure that applications use parameterized queries instead of embedding literals. For example:

Without Parameters:

SELECT * FROM Employees WHERE Department = 'HR';

With Parameters:

EXEC sp_executesql 
    N'SELECT * FROM Employees WHERE Department = @Department',
    N'@Department NVARCHAR(50)',
    @Department = 'HR';

5. Normalize Query Text

Queries with slight variations (e.g., differences in whitespace, capitalization, or comments) result in different execution plans. Use normalized query text to avoid this issue.

6. Address Different SET Options

Queries executed with different SET options (e.g., SET ANSI_NULLS, SET QUOTED_IDENTIFIER) are treated as separate queries, resulting in multiple plans.

• Ensure consistent SET options in your environment.
• Use:
  • SELECT * FROM sys.dm_exec_cached_plans CROSS APPLY sys.dm_exec_sql_text(plan_handle) WHERE text LIKE '%QueryText%'; to identify queries with differing SET options.

7. Use Stored Procedures

Using stored procedures has several benefits. SQL Server creates a single execution plan for each procedure. The server reuses the plan, reducing variations caused by ad hoc queries.

8. Clear Plan Cache (Temporary Measure)

If the cache is overwhelmed with multiple plans, you can clear it as a temporary fix:

Clear Entire Plan Cache:

DBCC FREEPROCCACHE;

Clear Plan Cache for Specific Query:

DECLARE @PlanHandle VARBINARY(64);

SELECT @PlanHandle = plan_handle
FROM sys.dm_exec_cached_plans 
CROSS APPLY sys.dm_exec_sql_text(plan_handle)
WHERE text LIKE '%QueryText%';

DBCC FREEPROCCACHE (@PlanHandle);

• Caution: Use with care in production environments.

9. Consolidate Indexing and Statistics

• Filtered Indexes: Reduce the need for query variations by indexing frequently queried subsets of data.
• Update Statistics: Ensure that SQL Server has accurate information for generating optimal plans.

10. Use OPTION (RECOMPILE) for Critical Queries

Force SQL Server to compile a new plan for each execution, avoiding plan caching entirely:

SELECT * FROM Employees WHERE Department = @Department OPTION (RECOMPILE);

• Note: This increases CPU usage due to repeated compilation.

11. Monitor and Adjust

Use Dynamic Management Views (DMVs) to monitor and troubleshoot plan caching issues:

• Identify queries with multiple cached plans:
  • SELECT text, plan_handle, usecounts, cacheobjtype FROM sys.dm_exec_cached_plans CROSS APPLY sys.dm_exec_sql_text(plan_handle) WHERE objtype = 'Adhoc';
• Remove specific plans if necessary:
  • DBCC FREEPROCCACHE (plan_handle);

Conclusion

To resolve the issue of multiple cached execution plans, focus on effective solutions. Consider using parameterization (forced or application-level). Utilize Query Store and implement stored procedures. Monitoring and adjusting your approach based on workload patterns ensures consistent performance and efficient memory usage.