SELECT 
  category_id, 
  parent_id 
FROM 
  categories 
WHERE 
  parent_id IN(
    20966, 20967, 20963, 20964, 16224, 20965, 
    16225, 23208, 20991, 23158, 20084, 
    20969, 17192, 16228, 20934, 16223, 
    16221
  )

Query time 0.00064

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "5.85"
    },
    "table": {
      "table_name": "categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 26,
      "rows_produced_per_join": 26,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "3.25",
        "eval_cost": "2.60",
        "prefix_cost": "5.85",
        "data_read_per_join": "68K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`goimagine`.`categories`.`parent_id` in (20966,20967,20963,20964,16224,20965,16225,23208,20991,23158,20084,20969,17192,16228,20934,16223,16221))"
    }
  }
}