Plan change regression happens when SQL Database changes a plan for some T-SQL query, and the new plan has the worse performance than the previous one. SQL Server 2017 has Automatic Tuning feature that enables you to easily find plan change regressions and fix them. In this post you will see the demo script that you can use to cause plan change regression and manually fix it using new sys.dm_db_tuning_recommendations view.
If you are not familiar with plan regressions and new tuning recommendations in SQL Server 2017, I would recommend to read these two posts:
- What is plan regression in SQL Server?
- How to find query plan choice regressions with SQL Server 2017 CTP2
This would be enough to understand steps in this demo.
Our experience with plan regression recommendations has been uniformly positive so far. Those tests have been in dev and QA environments, but so far, there hasn’t been a terrible recommendation.