SELECT 
  COUNT(*) 
FROM 
  cscart_states 
WHERE 
  country_code = 'VN'

Query time 0.00016

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_states",
          "access_type": "ref",
          "possible_keys": ["cs", "country_code"],
          "key": "country_code",
          "key_length": "8",
          "used_key_parts": ["country_code"],
          "ref": ["const"],
          "rows": 32,
          "filtered": 100,
          "attached_condition": "cscart_states.country_code = 'VN'",
          "using_index": true
        }
      }
    ]
  }
}