SELECT 
  state_id 
FROM 
  cscart_states 
WHERE 
  code = 'W' 
  AND country_code = 'AT'

Query time 0.00013

JSON explain

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