Kresten Krab Thorup continues his experiments becoming more and more of an advocate of Riak:
The module riak_link_index provides a mechanism that can be applicable for some situations, providing means to create synthetic (secondary link index) objects triggered by a Riak commit hook. The mechanism work by defining a function that names these synthetic objects as a function of the primary object body.
As far as I can tell Basho is already working on adding secondary indices to Riak.
Original title and link: Riak Secondary Indices using Links (NoSQL databases © myNoSQL)