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 at least one hardware processor connected with said database and said query plan generator for executing said received query in accordance with said generated query plan, wherein said at least one hardware processor: repeats starting to execute one database operation of said plurality of database operations for executing said received query until the execution of said received query is completed; issues a data read request to a storage device storing said database to read data necessary for executing said database operation; in executing said database operation, upon completion of said data read request, applies said database operation to data read by said data read request and, if execution of another database operation of said plurality of database operations in accordance with a result of said application of said database operation to said data issue necessary for executing said received query, starts to recursively execute said another database operation of said plurality of database operations in accordance with said result of said application of said database operation to said data; and in starting to recursively execute said another database operation, generates a task and assigns execution of said another database operation to said task, whereby said at least one hardware processor performs execution of a database operation of said plurality of database operations assigned in said generated task, regardless of whether another execution of a database operation is completed.