Deductive Databases
General idea: some relations are stored (extensional), others are defined by datalog queries (intensional).
Many research projects (MCC, Stanford, Wisconsin) [Great Ph.D theses!]
SQL3 realized that recursion is useful, and added linear recursion.
Hard problem: optimizing datalog performance.
Ideas from ddb’s made it into practice (recursion, magic sets, pred. movearound).