Update в postgresql это по факту вставка плюс удаление за счёт его механизма работы.
We know https://www.postgresql.org/docs/current/routine-vacuuming.html#VACUUM-FOR-SPACE-RECOVERY that Postgresql does not update a table row in place. Rather, it writes a new version of the row (the Postgresql term for a row version is “tuple”) and leaves the old row version in place to serve concurrent read requests. VACUUM later removes these “dead tuples”.
https://www.cybertec-postgresql.com/en/is-update-the-same-as-delete-insert-in-postgresql