SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    596, 272, 273, 274, 386, 275, 276, 597, 
    277, 371, 366, 278, 281, 367, 282, 283, 
    284, 375
  )

Query time 0.00047

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "13.06"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 19,
      "rows_produced_per_join": 19,
      "filtered": "100.00",
      "index_condition": "(`zdrowy_db`.`cscart_categories`.`parent_id` in (596,272,273,274,386,275,276,597,277,371,366,278,281,367,282,283,284,375))",
      "cost_info": {
        "read_cost": "11.16",
        "eval_cost": "1.90",
        "prefix_cost": "13.06",
        "data_read_per_join": "121K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ]
    }
  }
}