Most database systems support multiple isolation levels that enable their users to trade off exposure to various types of application anomalies and bugs for (potentially small) increases in potential transaction concurrency. For decades, the highest level of “bug-free correctness” offered by commercial database systems was “SERIALIZABLE” isolation in which the database system runs transactions in parallel, but in a way that is equivalent to as if they were running one after the other. This isolation level was considered “perfect” because it enabled users that write code on top of a database system to avoid having to reason about bugs that could arise due to concurrency. As long as particular transaction code is correct in the sense that if nothing else is running at the same time, the transaction will take the current database state from one correct state to another correct state (where “correct” is defined as not violating any semantics of an application), then serializable isolation will guarantee that the presence of concurrently running transactions will not cause any kind of race conditions that could allow the database to get to an incorrect state.