SQL leaks for complex queries and schemas if performance needs to be optimized. I argue virtually all abstractions leak heavily when performance is considered,

SQL leaks for complex queries and schemas if performance needs to be optimized. I argue virtually all abstractions leak heavily when performance is considered, some more than others. SQL leaks relatively little in comparison to some other technologies IME. Also, SQL has well-established processes and formalisms to design schemas which generally result in solid performance by themselves. That's what RDBMS are around for, after all: enabling efficient and consistent record-oriented data manipulation. This is quite difficult to do correctly in reality; for example, if you write your own transaction mechanism for disk/solid-state storage, you are going to do it wrong. This is genuinely difficult stuff. There is a ton of internals that SQL abstracts so well that very few DB programmers know or (have to) care about them. Things like commit and rollback protocols, checkpointing, on-disk layouts, I/O scheduling, page allocation strategies, caching etc.