Relational dbs are mostly keyval stores inside. An index is a keyval store projection of another keyval store (but say, keying it for another subset of the valu

Relational dbs are mostly keyval stores inside. An index is a keyval store projection of another keyval store (but say, keying it for another subset of the value). B-tree indices and hashmaps are ways to represent keyval stores for quick look-up (b-trees are convenient as they allow range look-ups, & are automatically sorted, while hashmaps aren't, but have lower overhead for lookup and storage). In essence everything is keyval. A sparse array is an ordered keyval store with integer keys (also technically everything is ordered, too, but some orders are stable, and useful, while others aren't). A dense array is an adjacency-optimized version of a sparse array where the key is implicit based on computable offset within a larger dense array, your address space. RAM address space is also variations on that theme. Raw disk storage. And file systems. Everything is. Maybe I spend too much time messing with storage, but I can't see it any other way at this point.