If you only know how to use a hammer then everything else looks like a nail. In other words, re-implementing an inverted index in SQL is waste of time when you can use tools like Sphinx and Lucene - which are highly optimized to do inverted indexes and that can easily handle 2 billion words. The same can be said about FlockDB - it's possible to emulate a graph database, but is the effort really worth it when there are such tools like Redis and neo4j which seem to be optimized for graph like structures.