Tuesday, March 17, 2015

How to fill the plan cache efficiently - Optimize for ad hoc workloads option

It is good to keep all plans used for compiled queries in the memory, as it helps to optimize the query processing, allowing engine to reuse the cached plan without recompiling query again for subsequent executions. But what if users frequently issue one-time-execute adhoc queries, and very rare to issue the same again? In that case, we should not waste the memory allocated to plans with them and should hold them only if they are executed again. How do we do it?

We can instruct SQL Server not to cache the plan for adhoc queries at the first execution but at the second execution using a server level setting. It is not recommend to change the default behavior but if we are opening the database for a similar situation, it may offer some performance benefit for other queries specifically with stored procedures.

This setting is called as optimize for ad hoc workloads. By default, it is disable, and it can be enabled passing 1 for this setting using sp_configure.

Here is the code for enabling it.

sp_configure 'optimize for ad hoc workloads', 1;

Now it is enabled. Let's run a query and see how it is cached.

-- Cleaning buffer
-- Simple adhoc query with a filter
SELECT * FROM Sales.SalesOrderDetail
WHERE SalesOrderDetailID = 1;
-- Checking chached plans
-- We should see one record for above query
SELECT p.usecounts, p.cacheobjtype, p.objtype, t.text
FROM sys.dm_exec_cached_plans p
CROSS APPLY sys.dm_exec_sql_text(p.plan_handle) t
WHERE p.usecounts > 0 AND
TEXT LIKE '%SELECT * FROM Sales.SalesOrderDetail%'
ORDER BY p.usecounts DESC;

As you see, there is a record indicating that plan is cached but the type is different. It means that plan has not been fully cached and SQL Server will not be able to use it again for next execution. This small compiled plan stub helps to identify the query with next execution, and then plan is recreated and stored.

Execute the SELECT query again and see the plan cache.

It is cached. Execute again and see.

It is being reused now. This saves resources heavily but have to enable only if we are 100% sure that we always get dissimilar adhoc queries.

For more info on this setting, refer: https://msdn.microsoft.com/en-us/library/cc645587.aspx

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