id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
36213	Add warning to documentation: QuerySet.update can execute two separate SQL queries when using MySQL	Babak		"When you have a QuerySet that has filter conditions based on related tables, the `QuerySet.update()` function will execute two separate SQL queries (a `SELECT`, followed by an `UPDATE`).

Examples:

`BlogPost.objects.filter(published=True).update(foo=""bar"")` (Causes a single `UPDATE`)

`BlogPost.objects.filter(published=True, author__name=""Foo Bar"").update(foo=""bar"")` (Causes `SELECT` THEN `UPDATE`)

As I was told in the [https://forum.djangoproject.com/t/queryset-update-silently-turns-into-select-update-mysql/39095/2 forum] (thanks charettes), this is an undocumented [https://github.com/django/django/blob/240421c7c4c81fe5df26274b807266bd4ca73d7f/django/db/backends/mysql/features.py#L165-L167 MySQL-only behaviour] because MySQL [https://dbfiddle.uk/Zfz_e9Kw doesn't allow self-select updates].

This will **silently cause nasty race conditions** since the update is no longer running as a single SQL statement.

Currently the docs for `QuerySet.update` say:

   Using update() also prevents a race condition wherein something might change in your database in the short period of time between loading the object and calling save().

But in the case that I described, it causes the exact same type of race condition that the docs suggest that it prevents.

If the users are aware of this behaviour they can take care to avoid such filter conditions or to use alternative transaction handling mechanisms to ensure atomic behaviour.

I'd like to suggest for a warning to be added to the documentation about this."	Uncategorized	new	Documentation		Normal		mysql self-select race-condition		Unreviewed	0	0	0	0	0	0