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
29215	Document potential change to behaviour of QuerySet methods when upgrading to Python 3.6	Matt Fisher	nobody	"Python 3.6 introduces [https://docs.python.org/3/whatsnew/3.6.html#whatsnew36-pep468 a change] that preserves the ordering of keyword arguments - [https://www.python.org/dev/peps/pep-0468/ PEP 468: Preserving Keyword Argument Order]

Since this is not a difference between Python 2 and 3 per se, it is not mentioned in the [https://docs.djangoproject.com/en/1.11/topics/python3/ “Porting to Python 3”] Django 1.11 topic page.

Preserving keyword argument ordering can change the SQL produced by QuerySet methods that accept `**kwargs`, specifically .get, .filter, and .exclude, and potentially also .values, .annotate, .aggregate, .create, .get_or_create, .update, and .update_or_create.

As an example, 
`.filter(b__in=some_qs, a=1)` 
could produce 
`... WHERE (a=1 AND b IN (SELECT …))` in Python < 3.6 and 
`... WHERE (b IN (SELECT …) AND a=1)` in Python >= 3.6. 
The query is semantically equivalent but the postgres query planner can produce dramatically different query plans and resultant performance if the query has a significant number of joins.

We had one particular frequently-run query that ran in 2-3 seconds in Python 2, and 80+ seconds in Python 3.6. The problem was only apparent in production, and presented as severely degraded responsiveness from the postgres RDS database. The additional load on the database made most other queries take longer as well, which made it difficult to track down the cause as it was not obvious that any queries could have changed. To compound the problem, the different query execution caused many more temp block writes on the RDS instance, which burned through our IOPS burst balance and further degraded site performance. Because the query didn’t change semantically, all our automated tests passed and we didn’t see the issue until it was in production under full load. Googling the problem produced no good results, which suggests it is not common, but in need of a useful resource.

People may be affected by this change when upgrading from Python 2.7 to Python 3.6+ using Django 1.x, in which case it would be helpful to have a warning in the [https://docs.djangoproject.com/en/1.11/topics/python3/ 1.11 porting topic], but it will also occur when people are upgrading from Python 3.[0-5] to 3.6+ while potentially using Django 2, in which case the porting topic is no longer in the current docs. Not sure what would be an appropriate place for a warning in this case.
"	Cleanup/optimization	closed	Documentation	2.0	Normal	invalid	documentation	Simon Charette	Unreviewed	0	0	0	0	0	0