In the most basic situation, the tables discussions and posts are used.
If you insert anything directly in the database, any additional behavior added by extensions will be skipped, including sending emails, sending notifications, linking mentions, parsing custom post content, ... In all cases you will most likely need a running version of the TextFormatter to prepare the post content before inserting in the database.
Ideally you should use the Discussion::startDiscussion() method via an extension to make sure all logic is properly applied.
Maybe you can describe in more details what you are trying to achieve ? Where does your bulk data comes from ?