I am not a database person nor have I used many, however I have been aware of them for a long time. The following might include prior art:
software was released in 18761976:
http://en.wikipedia.org/wiki/Multics_Relational_Data_Store http://web.mit.edu/multics-history/source/Multics/doc/info_segments/mrds.info
Additional reverences can be found by google for multics mrds
