SELECT 
  item_id 
FROM 
  cscart_selected_cart_items 
WHERE 
  user_id = 0

Query time 0.00029

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_selected_cart_items",
      "access_type": "ALL",
      "rows": 9,
      "filtered": 100,
      "attached_condition": "cscart_selected_cart_items.user_id = 0"
    }
  }
}