Codeigniter选择SUM给mysqld cpu高

Question

I'm still facing issue with calculating total sum amount of transaction, while it return the total sum it keep cpu high.

Cpu usage

Model class code

public function get_topup_sender_balance()
{

$this->db->select('SUM(total) as total');
$this->db->from('transactions');
$array = array('sender' => $this->session->id, 'status' => 'Processed', 'payment_type' => 'Topup', 'payment_method' => 'BalanceTopup');
$this->db->where($array);
$this->db->group_by('id');
$res = $this->db->get();
if($res->num_rows() > 0)
{
    return round($res->result(),2);
}else{
     return '0';
}

}

Helper custom library

public function transaction_balance() {

    $sender_topup = $this->CI->myac_model->get_topup_sender_balance();
    return number_format($sender_topup, 2, '.', '');
}

On Controller class and views page i call $this->helper_api->transaction_balance() to get customer balance and is return well balance but mysqld cpu will be high

And on mysql logs i get

# Query_time: 0.353698  Lock_time: 0.000111 Rows_sent: 0  Rows_examined: 484565
SET timestamp=1543791344;
SELECT SUM(total) as total
FROM `transactions`
WHERE `sender` = '4'
AND `status` = 'Processed'
AND `payment_type` = 'Topup'
AND `payment_method` = 'BalanceTopup'
GROUP BY `id`;

Any solution/idea to solve this pleases.