Randolph West has another nice post on temporal tables, in which he lets the cat out of the bag:
Look at the log records. They are identical to before. In fact, because a trigger is called in the same implicit transaction as the
UPDATEstatement, we didn’t even need to wrap the
INSERTstatement in a transaction.
What have we learned?
Temporal tables are doing exactly what an update or delete trigger (or stored procedure) would do, based on what we saw in the log records.
This is an in-depth look at what, exactly, is happening when temporal tables get updated.