SELECT 
  id_path 
FROM 
  cscart_categories 
WHERE 
  category_id IN (
    1, 21, 26, 27, 33, 35, 36, 39, 40, 46, 107, 
    240, 243, 249, 257, 261, 269, 306
  )

Query time 0.00040

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["category_id"],
      "rows": 18,
      "filtered": 100,
      "index_condition": "cscart_categories.category_id in (1,21,26,27,33,35,36,39,40,46,107,240,243,249,257,261,269,306)"
    }
  }
}