It does make sense when the goal of the language spec is interoperability, and it's something that everyone has to do, in practice, no matter the platform.

Call it a "hint", if you will, the meaning of which is implementation defined. But standardize the syntax for pete's sake.

Interoperability is achieved if the same query with the same inputs yields the same result. "In the same amount of time" (which is the part that indexes are aimed at) is manifestly not part of that picture, and that's by design.

And "hints" in the language are not exactly going to improve "interoperability" if their meaning is still allowed to be implementation-defined, are they ?

That sounds backwards to me. It's only lack of standardization that forces it to be an "implementation detail". To any application that needs to be able to query data in reasonable time, it's fundamental and necessary functionality.