What I don't understand: no system gets to maintain open transactions for free. MySQL keeps UNDO logs, so the effect there, much like Oracle, is possibly running out of UNDO space, as well as slowing down reads that have to apply UNDO to get the last-committed row version. So what gives? Did Uber Engineering fix the dangling transaction issues while migrating off, or are they relying on some other property of MySQL vs. Postgres?