SQL is what it is today because it is battle tested and has to handles a very hard problem of handling arbitrary concurrent reads/writes, so the likely scenario is that trying to replace general SQL wholesale will just end up making a worse, less tested version of SQL that developers are less familiar with. So, I think the best query language is probably whatever query feature that's already in your backend language, LINQ for C# for example. The only room for an SQL replacement in my opinion is if you are willing to trade flexibility for speed a la TigerBeetle.

The good thing about having built your own programming language via LLM nowadays is that you don't really have to speculate about a theoretical language when you can just have Codex/Claude implement it and try it out for yourself. I did it yesterday when I wanted to try out this theoretical high-performance database architecture that I had in mind and just added query functionalities to the language I already have.

If anyone is interested about the results, the default naive mode for this new database is ~0.2x the speed of concurrent durable mutation workloads, but if you specialize it to the particular application, you can get ridiculous 50-100x performance increases on filters and maps at the cost of flexibility and more upfront design. Experimental results are promising, definitely not production ready though.