Brent Ozar looks at a case when adding a suggested index monkeys with stats:
The query runs faster, make no mistake – but check out the estimates:
- Estimated number of rows = 1
- Actual number of rows = 165,367
Those estimates are built by SQL Server’s cardinality estimator (CE), and there have been major changes to it over the last couple of versions. You can control which CE you’re using by changing the database’s compatibility level. This particular StackOverflow database is running in 2016 compat mode – so what happens if we switch it back to 2012 compat mode?
Based on this result, there might be further optimizations available. Read on for more of Brent’s thoughts.