SQL Server 2008 is reaching end of support this year, so upgrading your SQL Server might be on your mind. One of the big changes when you upgrade your SQL Servers is upgrading the compatibility level, which by default will upgrade the cardinality estimator (CE).
This can change query performance, for better or for worse. This post won’t focus on whether it’s good or bad, but instead I want to show you how you can check to see what CE was used by your queries.
It’s not a 100% guarantee, but generally I’ve found the new estimator to be superior.