I understand that document-oriented NoSQL DBs are "extensions" of the KV model in that they allow you to query more than just a single lookup key. But once something is a "document", I feel like it already has a relational model baked into it:

```
"myJson": {
    "fizz": 4,
    "buzz": "true",
    "widget" : {
        ...etc.
    }
}

```

To me, I don't see the difference between this JSON, and a `json_objects` table with a `fizz` and `buzz` field, and a foreign key relationship to a second `widgets` table.

And "columnar" DB's like Cassandra just sound like straight-up relational/table DBs.

So I ask: what is so different about document- and column-oriented DBs, and so distinguishing (from RDBMSes) about them? What problems are they best suited to solve that render them superior to relational DBs under certain circumstances? Thanks in advance!