I think the solution is to upgrade SQL the language. There's a very low complexity threshold after which you have to trade readability/composability for decent performance (in other words, there are a lot of cases where a human can notice a permissible optimization, but a database can't because it might break correctness for some odd edge case). We're solving that problem with programming languages (notably Rust); I'm not convinced that there's something inherently different about relational databases.