When to Use Optimize for Ad Hoc Workloads in SQL Server for Perf

Ad hoc workloads in SQL Server refer to queries that are executed infrequently or just once.

Unlike repetitive queries that benefit from SQL Server’s ability to reuse execution plans, ad hoc queries can lead to inefficiencies when it comes to memory utilization and plan cache management.

The optimize for ad hoc workloads option is specifically designed to address these inefficiencies.

How It Works

When enabled, the optimize for ad hoc workloads option modifies SQL Server’s behavior in caching execution plans for single-use queries. Instead of storing the full execution plan upon first execution, SQL Server stores a stub, or a compact representation of the plan. This approach significantly reduces memory consumption in environments where ad hoc queries are prevalent.

Decision Criteria for Enabling

Memory Pressure

If your server experiences memory pressure, enabling this option can alleviate some of the stress by reducing the memory footprint of the plan cache.

Nature of Workloads

Use performance monitoring tools and DMVs (Dynamic Management Views) like

sys.dm_exec_cached_plans

and

sys.dm_exec_query_stats

Systems with a high volume of unique, one-time queries stand to benefit the most. For systems primarily executing a small set of repeated queries, the benefits may be less pronounced.

Performance Analysis

to analyze the nature of your workloads and the impact of ad hoc queries on your system.

Enabling the Option:

-- Connect to your SQL Server instance

DECLARE @Servername nvarchar(128) = 'YOUR_SERVER_NAME';

DECLARE @UserName nvarchar(50) = 'YOUR_USERNAME';

DECLARE @Password nvarchar(50) = 'YOUR_PASSWORD';

CONNECT @Servername, @UserName, @Password;

Practical Implications

 

-- Check plan cache size before
DECLARE @BeforeSize int; EXEC sys.dm_os_memory_clerks WITH OBJECT = 'plan cache' OUTPUT, size = @BeforeSize; PRINT 'Plan cache size before: ' + CAST(@BeforeSize AS nvarchar(10)) + ' KB';

-- Check plan cache size after 

DECLARE @AfterSize int; EXEC sys.dm_os_memory_clerks WITH OBJECT = 'plan cache' OUTPUT, size = @AfterSize; PRINT 'Plan cache size after: ' + CAST(@AfterSize AS nvarchar(10)) + ' KB'; 

-- Calculate and print size difference 

DECLARE @Difference int = @AfterSize - @BeforeSize; PRINT 'Size difference: ' + CAST(@Difference AS nvarchar(10)) + ' KB';

Troubleshooting and Optimization

 

 Testing Ad-Hoc Query Performance

This code executes a simple ad-hoc query before and after enabling the option to compare performance:

-- Check performance before 

DECLARE @StartTime datetime = GETDATE(); SELECT TOP 100 * FROM YourTable; DECLARE @EndTime datetime = GETDATE(); PRINT 'Execution time before: ' + CAST(DATEDIFF(ms, @StartTime, @EndTime) AS nvarchar(10)) + ' ms'; 

-- ... (code to enable option, if needed) ... 

-- Check performance after 

DECLARE @StartTime datetime = GETDATE(); SELECT TOP 100 * FROM YourTable; DECLARE @EndTime datetime = GETDATE(); PRINT 'Execution time after: ' + CAST(DATEDIFF(ms, @StartTime, @EndTime) AS nvarchar(10)) + ' ms';

While this option can improve performance, it also complicates the analysis of execution plans for ad hoc queries. Since only a plan stub is stored after the first execution, detailed analysis requires additional steps to capture the full execution plan.

• Implementation Strategy

Consider your server’s workload characteristics before enabling this option. It’s not a universal solution but can be highly effective in the right context.

• Monitoring and Adjusting

After enabling, monitor your server’s performance and the size of your plan cache. Adjustments should be based on empirical data and specific performance goals.

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