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 at least three database operations and a sequence thereof necessary for execution of said received query; a query executor connected with said database and said query plan generator for executing said received query by issuing a plurality of data read requests in accordance with said generated query plan; and an I/O optimizer connected with said query executor and said database for controlling said plurality of issued data read requests by using a plurality of queues having different priority degrees, wherein said query executor: generates a first task for executing a first database operation of said sequence of said database operations, starts to execute said first task, and stores a data read request into one of said queues of said I/O optimizer corresponding to a priority degree representative of an emergency of said first database operation to read data necessary for said first database operation to be executed for said first task from said database; after executing said first database operation for said first task, generates a plurality of second tasks, each second task for executing a second database operation of said sequence of said database operations for a separate part of a result of execution of said first database operation executed for said first task, starts to execute each of said plurality of second tasks, and stores a data read request into one of said queues of said I/O optimizer corresponding to a priority degree representative of an emergency of said second database operation to read data necessary for said second database operation to be executed for each of said plurality of second tasks for said separate part of said result of execution of said first database operation executed for said first task from said database; after executing said second database operation for one of said plurality of second tasks, generates a plurality of third tasks, each third task for executing a third database operation of said sequence of said database operations for a separate part of a result of execution of said second database operation executed for said one of said plurality of second tasks, starts to execute each of said plurality of third tasks, and stores a data read request into one of said queues of said I/O optimizer corresponding to a priority degree representative of an emergency of said third database operation to read data necessary for said third database operation to be executed for each of said plurality of third tasks for said separate part of said result of execution of said second database operation executed for said one of said plurality of second tasks from said database; upon completion of data read of said data read request issued by one of said plurality of third tasks, executes said third database operation with said read data for said one of said plurality of third tasks and thereafter eliminates said one of said plurality of third tasks; upon completion of data read of said data read request issued by one of said plurality of second tasks, executes said second database operation with said read data for said one of said plurality of second tasks and thereafter eliminates said one of said plurality of second tasks; upon completion of data read of said data read request issued by said first task, executes said first database operation with said read data for said first task and thereafter eliminates said first task; and executes in parallel at least two tasks of said plurality of generated first, second and third tasks; wherein said I/O optimizer: selects a higher priority degree queue of said plurality of queues of said I/O optimizer, taking priority over other queues and issues a plurality of data read requests stored in said selected queue to a storage device storing said database in a sequence based on block addresses corresponding to said plurality of data read requests stored in said selected queue.