Manage statistics with drupal
Your data should not be nodes, as they do not keep any readable content on their own, they only make sense in connection to a product and in statistics, summaries et cetera, and shouldn't really be displayed on their own. So, you have two ways of dealing with them:


Bad way - make them just a multi-value field in product content type. Not optimal for storage, not really good for filtering.
Make them entities. You only need creation date and product reference on them. Making them entities will expose them to Views for easy filtering and aggregation, to search api if you will ever need to sort by "most sold", and, well, to each and every module that knows about entities. There are many answers and tutorials out there. Hint - your entity should not be a fieldable one. Non-fieldable entities always have only fields defined at creation, so storage is much simpler for them. And simpler == faster.