SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    8239, 8271, 8297, 8396, 8402, 8342, 8433, 
    8430, 8434, 8437, 8463, 8464, 8517, 
    8470, 8506, 8536, 8542, 8590, 8567, 
    8612, 8613, 21369, 8663
  )

Query time 0.00044

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "4.86"
    },
    "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": 23,
      "rows_produced_per_join": 23,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "2.56",
        "eval_cost": "2.30",
        "prefix_cost": "4.86",
        "data_read_per_join": "59K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`maniamall_cscart`.`cscart_categories`.`parent_id` in (8239,8271,8297,8396,8402,8342,8433,8430,8434,8437,8463,8464,8517,8470,8506,8536,8542,8590,8567,8612,8613,21369,8663))"
    }
  }
}