Mainstream Relational Database technology is split into two camps.
On the one hand are the systems that are directly or indirectly based upon techniques and algorithms derived from IBM's System R project. Most published research also falls into this group.
In the second camp are systems that have been inspired by Oracle. Oracle has a very interesting history. Although Oracle started with the specifications of System R, it actually did not use any of the System R ideas. A good summary of how Oracle evolved can be found here.
SimpleDBM is based upon the System R heritage. The main reason for this is that I do not work in a company that makes DBMSes. Hence I do not have access to information other than published research. Since Oracle does not publish details of its algorithms, it would be much harder for me to implement a system based upon Oracle. I have, however, given much thought to some of the techniques used in Oracle, and hope to discuss some of these in future blogs and papers.