My query returns empty results with big tables. Any advice?

I have never had problems with CI so far, but yesterday I started a project in which I have to handle big amounts of data, the database has approximately 10m rows on several tables and I'm struggling with a simple search function


My controller looks something like this. Its need to retrieve the username of the user by entering the email, ID or viceversa:

PHP Code:

public function username_finder() {

  $type = $this->input->post('type');

  if ($type == 'email') {
    $this->form_validation->set_rules('input_string', 'Input String', 'trim|required|valid_email');
    if($this->form_validation->run() == FALSE) { } else {

      $input_string = $this->input->post('input_string');
      $result = $this->user_model->finder_engine($input_string, $type);
      return $result;
      }
   }
} 


The finder_engine on my model:

PHP Code:

public function finder_engine($input_string, $type) {


  if ($type == 'email') {
    $this->db->like('email', $input_string);
    $query = $this->db->get('members');
    return $query->result();
   }
} 


My members table looks like this:
id  | username | email | id_number | password

When I run this on the members table (10m rows) it returns and empty string.

I have tried with different versions but it always returns an empty array:

Code:

if ($type == 'email') {

   $query = "SELECT * FROM members WHERE email=?";
   $query = $this->db->query($query, array($input_string));
   return $query->result_array();

 }

I duplicated the table and cut it off to 25000 rows, still empty, but when I run the same queries on the same table with 5000 records it returns the values perfectly. I guess this is a problem with the memory or with the execution time, but I have no idea. How can I approach this properly? What are good measures for CI regarding handling this amount of data?

Thanks a lot in advance!