The Cost of Ad Hoc Queries

Erin Stellato gives us a simple demonstration of why parameterization is important for performance:

From this screenshot you can see that we have about 3GB total dedicated to the plan cache, and of that 1.7GB is for the plans of over 158,000 adhoc queries. Of that 1.7GB, approximately 500MB is used for 125,000 plans that execute ONE time only. About 1GB of the plan cache is for prepared and procedure plans, and they only take up about 300MB worth of space. But note the average use count – well over 1 million for procedures. In looking at this output, I would categorize this workload as mixed – some parameterized queries, some adhoc.

Kimberly’s blog post discusses options for managing a plan cache filled with a lot of adhoc queries. Plan cache bloat is just one problem you have to contend with when you have an adhoc workload, and in this post I want to explore the effect it can have on CPU as a result of all the compilations that have to occur. When a query executes in SQL Server, it goes through compilation and optimization, and there is overhead associated with this process, which frequently manifests as CPU cost. Once a query plan is in cache, it can be re-used. Queries that are parameterized can end up re-using a plan that’s already in cache, because the query text is exactly the same. When an adhoc query executes it will only re-use the plan in cache if it has the exact same text and input value(s).

Read on to see an example of how long it takes a set of ad hoc queries to finish versus their parameterized equivalents. Erin’s test is at the behavioral extreme (100% parameterized versus 100% ad hoc) so real-world results won’t be quite this good.

Related Posts

What Compatibility Level 150 Gets You

Erik Darling explains the upsides and downsides of moving to SQL Server 2019 and compatibility level 150: In those versions, flipping compatibility level uses the new Cardinality Estimator (CE). That new Cardinality Estimator is real hit or miss. The worst part is that there’s practically no gain to be realized for using higher compatibility levels […]

Read More

Offloading Code Review Burdens with Automation

Ed Elliott argues that automation and testing can make code reviews easier: OK so if we break this down into what a DBA should be doing as part of a code review: – Ensure formatting is correct and any standards followed– Have they introduces a SQL injection vulnerability?– Consider any side effects of the actual […]

Read More

Categories

May 2019
MTWTFSS
« Apr Jun »
 12345
6789101112
13141516171819
20212223242526
2728293031