Optimize update_or_create when defaults is empty / False-y

[Timothy Schilling]

The defaults parameter to update_or_create is used to change the values on a model instance after it's been fetched from the database. However, it's not required to pass anything in through defaults. In these cases where defaults is False-y, Django is performing an update on the model instance that shouldn't be changing any values. Instead the call to [object.save()](​https://github.com/django/django/blob/stable/4.2.x/django/db/models/query.py#L971-L972) should be skipped.

The concern would be needing to check for fields that use something like auto_now.

[Mariusz Felisiak]

Thanks for this ticket. An empty set of defaults is always a subset of concrete_field_names so ​the "else" branch is not reachable in this case. We could skip ​save() when update_fields is empty but this would be backward incompatible e.g. for users with database triggers. I'm not convinced it's worth changing, users who don't want to update any fields should use get_or_create() instead. Hope that makes sense.