SELECT 
  category_id, 
  parent_id 
FROM 
  tmall_categories 
WHERE 
  parent_id IN(204, 211, 215)

Query time 0.00761

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.013616989,
    "nested_loop": [
      {
        "table": {
          "table_name": "tmall_categories",
          "access_type": "range",
          "possible_keys": ["parent"],
          "key": "parent",
          "key_length": "3",
          "used_key_parts": ["parent_id"],
          "loops": 1,
          "rows": 9,
          "cost": 0.013616989,
          "filtered": 100,
          "index_condition": "tmall_categories.parent_id in (204,211,215)"
        }
      }
    ]
  }
}