My experience as the performance “expert” at the company I work at isn’t necessarily that the a single query gets sped up that much absolutely, but that much re

My experience as the performance “expert” at the company I work at isn’t necessarily that the a single query gets sped up that much absolutely, but that much relatively is possible when dealing with a query that executes very frequently. In my experience, if one of our core queries regresses from taking 20ms to taking 600ms (which happened recently although only one certain customer databases with 100m+ rows in the table) that the query will suddenly be taking 5+ seconds. The problems compounds as the IO and compute resources of the database gets consumed and everything starts getting even slower. In my recent case the same query before and after was actually taking the same amount of time when the DB wasn’t “on fire”. The query had started using a lot more IO resources which became a bottleneck but you wouldn’t know by query time until suddenly we were IO constrained under load.