You can concretely measure how much time it takes to scan X million rows of a table; how much slower it is for every join you add; how much slower an index is t

You can concretely measure how much time it takes to scan X million rows of a table; how much slower it is for every join you add; how much slower an index is to scan for every column that's added to it; how much slower it gets for every column you add to the select clause; etc. And determining optimal join order in my case isn't particularly hard (it's a lot like a star schema; the key thing is not to bring in the whole of the big table in the middle). I was a compiler engineer in my previous job; transforming and optimizing expression trees are my bread and butter. I'd give myself good odds of improving MySQL for my particular use case - I'd know where to look, what stats to collect and how to approach things differently. But likely my optimizations would be pretty specific to my situation, and not everybody would be happy with more predictable, sometimes slower performance.