2. A distributed database system according to claim 1, further comprising: first means for detecting whether or not a derived relation, resulting from said semijoining by said semijoin operating means, is a high-level relation; and second means for dividing said derived relation into low-level relations when said derived relation is detected to be a high-level relation.
2. A distributed database system according to claim 1, further comprising: first means for detecting whether or not a derived relation, resulting from said semijoining by said semijoin operating means, is a high-level relation; and second means for dividing said derived relation into low-level relations when said derived relation is detected to be a high-level relation. 3. A distributed data base system according to claim 2, wherein: said semijoin operating means semijoins two relations from among said low-level relations from said second means when it is detected that the quantity of transfer data resulting from said semijoining of said two relations is decreased, so that each derived relation resulting from said semijoining is added to a next relation set among the plurality of relation sets arranged by said relation set means.