PURPOSE: To make total execution time for naturally connecting two relations minimum by dividing a connection operation into different jobs and scheduling the jobs in optimum among plural processors.

CONSTITUTION: The first relation is divided into plural partitions and the second relation is divided into plural partitions corresponding to them. The execution time of the respective jobs is shortened to be less than 1/P of whole execution time so that the present set of the jobs constituted by tasks connecting the respective partitions with the corresponding partitions fits to one processor. When estimated skew quantity fits to standard skew quantity, the present set of the jobs which are scheduled last is executed by using P processors and the two data base relations are connected with the minimum completion time skew on a common field. Thus, whole execution time for naturally connecting the two relations can be set to a minimum.