2019-06-03 23:18 UTC

Fun MySQL fact of the day: MVCC-caused replication delay

Looking way back to March 11, we had discussed InnoDB's MVCC implementation. We also had discussed how InnoDB (by default) needs to keep around all versions of a record in the InnoDB undo log until all queries referencing the record either commit or roll-back. And while it may not ... read more

2019-03-13 19:06 UTC

Fun MySQL fact of the day: implicit rollbacks

Last Thursday, I suggested that you may not always want your transactions automatically rolled-back on an ACID violation, and, on Monday, I hinted that transaction rollbacks are nearly the most expensive thing you can do in MySQL/InnoDB. And, well, it's because of the very same "undo logs" about ... read more

2019-03-12 20:00 UTC

Fun MySQL fact of the day: too much history

Yesterday, we discussed that InnoDB stores "old" record versions in an undo log, but we didn't discuss where undo logs are stored. While it's amusing to think it's turtles all the way down, it's actually a lot simpler: undo logs are stored inside InnoDB (and in redo logs, ... read more

2019-03-11 23:38 UTC

Fun MySQL fact of the day: nothing isn't free

Nothing isn't free in InnoDB. When a transaction rolls-back in MySQL/InnoDB, the "or-nothing" part of "all-or-nothing" is close to being the most expensive thing you can do in MySQL. While we can dig deep into the internals to find out why, we've already gone over all the building blocks, and ... read more

2019-03-04 23:15 UTC

Fun MySQL fact of the day: REPEATABLE READ isn't REPEATABLE WRITE

Since we're on the topic of transaction isolation, you might have previously had a mental model that transaction isolation levels are a layered model, each one stronger than the next. Unfortunately, this isn't quite true; and if that's not annoying enough, you might even be surprised to learn that ... read more

2019-03-01 21:18 UTC

Fun MySQL fact of the day: InnoDB read-only optimisation

The I in ACID stands for Isolation, which is a property that determines whether or not a transaction can see the progress of other transactions. Transaction isolation is a complex system generally defined by the allowance of certain "read phenomena". ... read more