frameworks for understanding databases

Document databases should be added to the nine types. I think the author would reply they’re a type of key-value database, where the values are a semi-structured data type, usually JSON; but they’re distinct from true KV stores (like LMDB and others of the “xxxDB” family) because they include indexing.

For example, CouchDB has map-reduce indexes, and Couchbase [my $DAYJOB] has the full range of secondary, FTS and vector indexes (even a SQL-derived query language.) MongoDB has indexes too but I know less about it.