NUMROWS is used to provide an approximate number of rows that an external table can have.
NUMROWS用于提供一个外部表可能拥有的大致行数。
2
NUMROWS: the approximate number of rows that the external table can possibly have.
NUMROWS：外部表可能拥有的大致行数。
3
According to possessing the characteristics of approximate description of function expression, the control system structure employing mapping table to implement controlling is presented sequentially.