Models that use SET, SET_NULL, and SET_DEFAULT as on_delete handler
don't have to fetch objects for the sole purpose of passing them back to
a follow up UPDATE query filtered by the retrieved objects primary key.
This was achieved by flagging SET handlers as _lazy_ and having the
collector logic defer object collections until the last minute. This
should ensure that the rare cases where custom on_delete handlers are
defined remain uncalled when when dealing with an empty collection of
instances.
This reduces the number queries required to apply SET handlers from
2 to 1 where the remaining UPDATE use the same predicate as the non
removed SELECT query.
In a lot of ways this is similar to the fast-delete optimization that
was added in #18676 but for updates this time. The conditions only
happen to be simpler in this case because SET handlers are always
terminal. They never cascade to more deletes that can be combined.
Thanks Renan GEHAN for the report.
Simon Charette
django-bot
Mariusz Felisiak
Étienne Loks
Flavio Curella
Tim Graham
Jason Myers
Florian Apolloner