[da_chicken]

My point is that nothing stops you from modifying the value of an auto increment column, nor from inserting directly with a specific value. Yes, rollbacks don't roll back consumed values, but an auto increment column isn't immutable and the table isn't required to use the next value.

I've seen an application do things like an INSERT ROLLBACK SELECT LAST_INSERT_ID() to "reserve" IDs... or even perfectly acceptable things like reserving IDs out of a SEQUENCE. Those weren't all MySQL systems, but it did lead to confusion sometimes why gaps might appear or why timestamps might be "inconsistent".

The above one was a potential problem through 5.7 though, as it was possible to reuse some values since MySQL kept the auto increment value in memory only. INSERT followed by a ROLLBACK, then restart the server and you could get reused IDs. It's rare, but I've seen it. However, but it looks like they changed it with 8.0 to save the auto increment value to a system table now. That's a good thing.