Comment by evanelias

> In Tao, there are two tables: objects and references. This has scaled well for Facebook.

That's a rather tremendous oversimplification, unless something major changed in recent years. When I worked on database infra at FB, MySQL-backed TAO objects and associations were mapped to distinct underlying tables for each major type of entity or relationship. In other words, each UDB shard had hundreds of tables. Also each MySQL instance had a bunch (few dozen?) of shards, and each physical host had multiple MySQL instances. So the end result of that is that each individual table was kept to a quite reasonable size.

Nor was it an EAV / KV pattern at all, since each row represented a full object or association, rather than just a single attribute. And the read workload for associations typically consisted of range scans across an index, which isn't really a thing with EAV.