How to use SQL_CALC_FOUND_ROWS function on active record with select method

I know we can use SQL_CALC_FOUND_ROWS  and FOUND_ROWS() like

PHP Code:

$this->db->select('SQL_CALC_FOUND_ROWS *', false);

// And get the result
$query = $this->db->query('SELECT FOUND_ROWS() AS `count`');
$totalRows = $query->row()->count; 


But if we want to select the column by our own like

PHP Code:

$this->db->select('SQL_CALC_FOUND_ROWS *', false);
$this->db->select('a.name, b.age'); 


This will work but come out the sql string like "SELECT SQL_CALC_FOUND_ROWS *, `a`.`name`, `b`.`age`", actually get everything from table a and table b, how to make it like "SELECT SQL_CALC_FOUND_ROWS `a`.`name`, `b`.`age`"