A key thing to come to grips with is that the queries you need Solr to support completely drive your Solr schema design. This is very important to understand. Conversely, relational databases typically use standard third normal form decomposition of the data, largely because they have strong SQL relational-join support. Since queries drive the Solr schema design, all the data needed to match a document, that is, the criteria, should be in the document matched, not in a related one. To satisfy that requirement, data that would otherwise exist in one place is copied into related documents that need it to support a search. For example, an artist's name in MusicBrainz will not just exist on an artist document but also in a track document to support searching for tracks by artist. Solr 4's new Join support allows this design principle to be relaxed; however, it's not as capable as a SQL join and is often slow, so only consider this as a last resort.