We’re running SQL Server 2019 with database compatibility level 150, and after recent tuning (RAID-10 tempdb with even files and RCSI enabled) performance is stable; however our application vendor is asking us to drop the database’s compatibility level all the way down to 110 (SQL 2012). My concern is that this would disable many 2019 optimizer features and potentially cause new regressions. Is there any good reason to change the whole database to 110, or would it be better to keep 150 and handle any vendor-specific query with targeted options like QUERY_OPTIMIZER_COMPATIBILITY_LEVEL_110 or LEGACY_CARDINALITY_ESTIMATION (or plan forcing) instead? If you’ve been in this situation, what did you choose and what were the results—and would you recommend testing on a copy before making any change in production?