I said primary index, not primary key (primary key and primary index is synonymous in mysql Dropbox example). Primary index is database theory lingo for storing

I said primary index, not primary key (primary key and primary index is synonymous in mysql Dropbox example). Primary index is database theory lingo for storing all the primary row data inside a B-tree. It’s synonymous with what you say IOT although that’s a new term for me. You’re incorrect about IOT reordering the entire table at least wrt mysql. MySQL uses a B-tree to store rows, so at most it’s insertion sort on a B-tree node and rare b-tree rebalance. Most b-tree leaf nodes have empty space to allow for adding new data without shifting more than a few hundred other rows. Also, non-IOT tables also need to do a similar process to write to each of its indexes. Last, it’s sort of a tossup since if you’re appending to an IOT table frequently, the right edge of the B-tree is likely cached. (similarly for any small number of paths through the primary index B-tree). At worst Postgres heap will need to surface one new heap disk page for writing, although I’m sure they have some strategy for caching the pages they write new data to. Sorry to spam this info! Glad to see we both love databases and I’m always please to see engagement about this topic!