SELECT 
  category_id, 
  parent_id 
FROM 
  categories 
WHERE 
  parent_id IN(
    21415, 528, 1416, 15493, 747, 529, 531, 
    525, 1417, 532, 748, 527, 530, 551, 533, 
    1420, 749, 1421, 1412, 1411, 13821, 
    21453, 3637
  )

Query time 0.00068

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.65"
    },
    "table": {
      "table_name": "categories",
      "access_type": "range",
      "possible_keys": [
        "parent"
      ],
      "key": "parent",
      "used_key_parts": [
        "parent_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 45,
      "rows_produced_per_join": 45,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "5.16",
        "eval_cost": "4.50",
        "prefix_cost": "9.66",
        "data_read_per_join": "117K"
      },
      "used_columns": [
        "category_id",
        "parent_id"
      ],
      "attached_condition": "(`goimagine`.`categories`.`parent_id` in (21415,528,1416,15493,747,529,531,525,1417,532,748,527,530,551,533,1420,749,1421,1412,1411,13821,21453,3637))"
    }
  }
}