SWAD-Europe: Mapping Semantic Web Data with RDBMSes
"We've tried to address:
1. I want to implement an RDF triple store. How can I do this with a relational database?
2. I have a legacy relational database. How can it be exposed as RDF?"
They say that BDB is an order of magntitude faster than RDBMS and attribute that to transaction support. They discuss that a two index approach is the best tradeoff (indexing subject+predicate and object). Also noted is the performance problems when changing the schema (they note Sesame's use of PostgreSQL). There's a mailing list and blog available too.