It is worth noting that the only use case I can see where global indexes fill a functionality gap are with unique indexes which allow you to enforce uniqueness across an inheritance tree where the uniqueness is orthogonal to any partition key.
I could find large numbers of uses for that. That could also allow referential integrity to check against a root table rather than force partition explosion.Will
Otherwise the following mostly works:
Create table (like foo including all) inherits (foo);
So the gap this addresses is very real even if it is narrow.