Most vendors use three indexes for triples and 4 or 6 for quads. All the indexes are covering, which is to say they triplicate all data—-in other words the database consists only of indexes.
I refer to RDF as the "absurd normal form". When my friends and I do DB design, it's almost inevitable we fall into what might inevitably become the "ThingThing" table thats a many-to-many joiner of everything to everything else. (That's when we giggle, leave the room, go to lunch, and then come back when we've returned to our senses.)
But, for RDF its exactly what I want, I'm not interested in schemas and such for this work, so it's perfect for my scenario.
But for my one hobby project, I'm using RDF and a triple store. Even with a "small" dataset, you can get an explosion of properties.
I want to be able to add arbitrary properties to arbitrary things and relate them all together. Build the graph organically.
So far, its working really well. But underneath, its (likely) just a couple of b+trees do all of the heavy lifting.