Hallo!
It is written everywhere that the best way to change/update document in index is to delete an old one and then to add a new. Because re-indexing of the whole index is time consuming process.
Question is: What exactly will be reindexed in this process if not the whole index? segment?
Sorry, but for me this is not clear, because the changes could be really big, and something SHOULD be re-indexed in order to keep index updated..

Thank you in advance
Alex