Query optimization and processing in Federated Database Systems

this paper, we have selected a minimal set of core operations that includes the set of relational operations, i.e. foe,ß,1,\Gamma,[g, as well as three other operators which are useful in specifying the federated views. These are 2-way Outerjoin Operator

Information integration for graph databases

Philip S. Yu, Jiawei Han, Christos Faloutsos</p