clear select

Hi folks,
Using CI3
is there a way to clear / flush the select array ?

heres a basic example to try and explain better

PHP Code:

$this->db->select('user.*');


if ($rs_type == 'small_select'){

// clear the select options and start again
 $this->db->select('user.user_id'); // need to only select single item

 $query = $this->db->get('users');
 $result = $query->result();
 
 } else {

 // here we select everything as normal

 $query = $this->db->get('users');
 $result = $query->result();

 } 


thanks in advance