The first three database features we looked at are commonly found in distributed data stores. However, databases such as Riak and Voldemort are purely key value stores; these databases have no knowledge of the internal structure of a record that's stored in a particular key. This means useful functions such as updating only part of a record, reading only certain fields from a record, or retrieving records that contain a particular value in a given field are not possible.
Relational databases such as PostgreSQL, document stores such as MongoDB, and, to a limited extent, newer key-value stores such as Redis, do have a concept of the internal structure of their records, and most application developers are accustomed to taking ...
