1. A database management system comprising: a query interface for receiving a query; a query plan generator for generating a query plan from said received query; and a query executor for executing said received query in accordance with said generated query plan, wherein said query executor: executes a database processing for a database in accordance with said query plan; in said database processing, generates a new task each time each data is accessed, in accordance with a plurality set of data contained in said database, a task from which said new task originates processing data following said data accessed; issues a data read request for reading data from said database in the generated new task, acquires said data from said database, and executes said database processing for said acquired data; executes said database processing for each of a plurality of generated new tasks, and thereafter eliminates each of said new tasks; and executes in parallel at least two tasks of said plurality of generated new tasks.