As it is, people already have to look into "why" for practical considerations. A database failing to perform will lead to a timeout somewhere else and a 500 err

As it is, people already have to look into "why" for practical considerations. A database failing to perform will lead to a timeout somewhere else and a 500 error served to an end user. With the lack of tooling support, information is extracted by modifying the query by trial and error (seeing what needs to be changed to flip it over to desired behavior), from dark corners of the internet, and by reading the source code of the storage engine. My latest expedition into these matters was a case where a table scan was technically faster (and favored by the query planner) than a low-cardinality index, but used a lot more CPU. So when the DB was hit with several cases of that particular query simultaneously, the server would run out of CPU and everything slowed to a crawl.