SELECT 
  state_id 
FROM 
  cscart_states 
WHERE 
  code = '東京都' 
  AND country_code = 'JP'

Query time 0.00027

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_states",
      "access_type": "const",
      "possible_keys": [
        "cs",
        "code",
        "country_code"
      ],
      "key": "cs",
      "used_key_parts": [
        "country_code",
        "code"
      ],
      "key_length": "106",
      "ref": [
        "const",
        "const"
      ],
      "rows": 1,
      "filtered": 100
    }
  }
}