SELECT 
  category_id, 
  parent_id 
FROM 
  cscart_categories 
WHERE 
  parent_id IN(318, 286)

Query time 0.00042

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_categories",
          "access_type": "ALL",
          "possible_keys": ["parent"],
          "rows": 70,
          "filtered": 25.7142849,
          "attached_condition": "cscart_categories.parent_id in (318,286)"
        }
      }
    ]
  }
}