An optimisation scheme for coalesce/valid time selection operator sequences

Queries in temporal databases often employ the coalesce operator, either to coalesce results of projections, or data which are not coalesced upon stor age. Therefore, the performance and the optimisation schemes utilised for t his operator is of major importance for the performance of temporal DBMSs. Insofar, performance studies for various algorithms that implement this ope rator have been conducted, however, the joint optimisation of the coalesce operator with other algebraic operators that appear in the query execution plan has only received minimal attention. In this paper, we propose a schem e for combining the coalesce operator with selection operators which are ap plied to the valid time of the tuples produced from a coalescing operation. The proposed scheme aims at reducing the number of tuples that a coalescin g operator must process, while at the same time allows the optimiser to exp loit temporal indices on the valid time of the data.