The database system of the present invention decides a fragment length responding to a unit of a data process of a parallel arithmetic unit, and stores tuple data containing variable-length data into a fragment and metadata of the fragment into a fragment header, respectively, in a column store database. The database system refers to the metadata when executing a process for data stored in the column store database, decides the fragments to be assigned to each thread that is executed by the parallel arithmetic unit, assigns the fragments to each thread based upon the decided content, and causes each thread to execute a parallel arithmetic operation.