SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(
    284, 283, 280, 271, 264, 276, 281, 272, 
    265, 266, 273, 274, 267, 268, 275, 269, 
    270
  )

Query time 0.00025

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00957962,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_categories",
          "access_type": "index",
          "possible_keys": ["parent"],
          "key": "parent",
          "key_length": "3",
          "used_key_parts": ["parent_id"],
          "loops": 1,
          "rows": 20,
          "cost": 0.00957962,
          "filtered": 100,
          "attached_condition": "cscart_categories.parent_id in (284,283,280,271,264,276,281,272,265,266,273,274,267,268,275,269,270)",
          "using_index": true
        }
      }
    ]
  }
}