SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    33868, 33903, 33865, 33856, 36185, 2474, 
    2554, 6631, 7225, 2557, 2538, 6632, 
    6839, 2463, 918, 2960, 6638, 912, 6578, 
    5393, 5152, 5521, 16633, 2890, 23795
  )

Query time 0.00032

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "6.26"
    },
    "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": "3.27",
        "eval_cost": "3.00",
        "prefix_cost": "6.27",
        "data_read_per_join": "77K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`maniamall_cscart`.`cscart_categories`.`parent_id` in (33868,33903,33865,33856,36185,2474,2554,6631,7225,2557,2538,6632,6839,2463,918,2960,6638,912,6578,5393,5152,5521,16633,2890,23795))"
    }
  }
}

Result