SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    8215, 8200, 8318, 8312, 8349, 21158, 
    8360, 8585, 8387, 8406, 8398, 8320, 
    8392, 8330, 8181, 8457, 8465, 8479, 
    8481, 8485, 8484, 8420, 8513, 8552, 
    8278, 8248, 21142, 8619, 8381, 8472
  )

Query time 0.00044

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "6.74"
    },
    "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": 30,
      "rows_produced_per_join": 30,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "3.74",
        "eval_cost": "3.00",
        "prefix_cost": "6.74",
        "data_read_per_join": "77K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`maniamall_cscart`.`cscart_categories`.`parent_id` in (8215,8200,8318,8312,8349,21158,8360,8585,8387,8406,8398,8320,8392,8330,8181,8457,8465,8479,8481,8485,8484,8420,8513,8552,8278,8248,21142,8619,8381,8472))"
    }
  }
}