1. A database management system for managing a database, comprising: a query interface for receiving a query to said database; a query plan generator connected with said query interface for generating a query plan from said received query, said generated query plan comprising a plurality of database operations necessary for execution of said received query; and a query executor connected with said database and said query plan generator for executing said received query in accordance with said generated query plan, wherein said query executor: executes the following steps for executing said received query: (A) a step of generating a task for executing a database operation associated to said task and starting to execute said generated task, (B) a step of issuing a data read request to a storage device storing said database to read data necessary for executing said database operation in executing said generated task, (C) when a plurality of tasks can be generated, in accordance with a result of execution of said database operation, for a next database operation which is necessary for executing said received query, a step of generating said plurality of tasks in accordance with said result of execution of said database operation, and (D) a step of further executing steps (B) and (C) for each of said plurality of generated tasks; whereby parallel processing of said generated tasks for said received query is performed; and after issuing said data read requests for said generated tasks in step (B), suspends execution of said generated tasks and thereafter, upon completion of data read of said data read requests, resumes execution of said generated tasks for starting to execute said database operations associated to said generated tasks with said read data for said generated tasks in the same order as the query executor starts to issue said data read requests.