SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    21278, 21280, 21279, 21281, 21282, 21283, 
    11792, 11793, 11796, 11797, 11798, 
    11788, 11821, 11735, 11803, 11804, 
    11805, 21284, 21285, 21286, 20910, 
    20911, 21287, 21288, 21289, 21290, 
    21292, 21291, 21293, 21294, 21302, 
    21303, 21295, 21296, 21297, 21298, 
    19441, 21299, 21300, 21301, 21304, 
    21305, 21306, 21307, 21308, 21309, 
    21315, 21310, 21311, 21313, 21312, 
    21314
  )

Query time 0.00089

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": 61,
      "rows_produced_per_join": 61,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "6.96",
        "eval_cost": "6.10",
        "prefix_cost": "13.06",
        "data_read_per_join": "158K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`maniamall_cscart`.`cscart_categories`.`parent_id` in (21278,21280,21279,21281,21282,21283,11792,11793,11796,11797,11798,11788,11821,11735,11803,11804,11805,21284,21285,21286,20910,20911,21287,21288,21289,21290,21292,21291,21293,21294,21302,21303,21295,21296,21297,21298,19441,21299,21300,21301,21304,21305,21306,21307,21308,21309,21315,21310,21311,21313,21312,21314))"
    }
  }
}

Result