I think even before we get there, one of the big issues with SQL is how we write optimized queries. It's basically involves memorizing a bunch of hacks and hoping that those hacks compile into something that is performant.
Before we use these 5th generation languages to write natural language queries we need an API that is sufficiently high level and possible to logically optimize without resorting to hacks.
Exactly. I always say SQL servers are kernel of beautiful relational theory wrapped in about 4 feet of duck-tape and bubble gum.
The SQL server presents itself as a black box, saying "just say what you want and I'll figure out how to get it" and then completely fails to deliver on this and you have to open up the black box and find that its insides are dark and scary and full of jagged sharp ends.
Before we use these 5th generation languages to write natural language queries we need an API that is sufficiently high level and possible to logically optimize without resorting to hacks.