There may be a need to query the various kinds of documents as if they were the same. […] One possibility is to execute 3 separate queries (one for each entity type) and then aggregate the result sets for the end user to view. That may work, but it wouldn’t be very efficient nor elegant. Another possibility is to create a multi-map index that could be queried. Let’s take a look at that approach.
Nice feature. I assume something similar could be achieved in NoSQL databases supporting MapReduce by
mapping only the items of interest from the various collections in the database.
Original title and link: RavenDB Polymorphic Queries Using Multi-Map Indices ( ©myNoSQL)