SELECT 
  destination_id, 
  rate_value, 
  rate_type 
FROM 
  cscart_tax_rates 
WHERE 
  tax_id = 8 
  AND destination_id = 12

Query time 0.00021

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "message": "Impossible WHERE noticed after reading const tables"
    }
  }
}