Abstract: When queries written in SQL are converted into query trees by the query optimizer (S1102). The query processor finds tasks to be processed directly from the query trees (S1104), and if the next tasks performing operation on processing results of the tasks that can be processed directly are project operation, the query processor retrieves both the tasks that can be processed directly and the tasks of the project operation, and combines these tasks to create tasks that combined relational operations (S1106). The query processor checks whether there is any group in the group queue collecting tasks that access the same relation (S1108). If a group is found, the query processor adds these tasks into the group (S1112). Otherwise, the query processor creates a new group for these tasks, adds these tasks into the group, and insert the group into the group queue. The query processor further collects the tasks having common sub-expression into sub-groups (S1116).