US Patent 7941423 Virtual pair algorithm for outer join resolution

A system and method for executing semijoin reductions on outer join operations are presented. Tuple lists are created based on a query. A sequence of semijoin reduction steps are performed, where each semijoin reduction step either switches one tuple list, or reduces all tuple lists at one relation. Virtual connector pairs are inserted into the tuple lists, and then all tuple lists are centralized at a common host. The join results are assembled at the common host, and negative row IDs are substituted. From the join result, the query result is retrieved by projecting to the requested attributes.