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
35793	Add support for atomic upserts	Storm Heg	YashRaj1506	"Hello,

I took over maintenance of a package called wagtail-ab-testing which performs a raw SQL query to insert or update (UPSERT) a model atomically ([https://github.com/wagtail-nest/wagtail-ab-testing/blob/1ba57e4707061abf26c3c5829a8a79138fcaa2a5/wagtail_ab_testing/models.py#L450-L460 link to relevant source code])

This raw sql query confused me, but as it turns out it is there because this the Django ORM does not appear support atomic upserts without taking a lock on the row, which hurts performance

Specifically, the (performant) raw SQL for PostgreSQL looks like this:

{{{#!sql
INSERT INTO %s (ab_test_id, version, date, hour, participants, conversions)
VALUES (%ss, %ss, %%s, %%s, %%s, %%s)
ON CONFLICT (ab_test_id, version, date, hour)
  DO UPDATE SET participants = %s.participants + %%s, conversions = %s.conversions + %%s;
}}}


There are a few things to note here:
* There is an unique constraint on combination of `ab_test_id`, `version`, `date`, and `hour` columns that prevents duplicate objects from being created.
* The `participants` and `conversions` columns are updated atomically; That is: these columns are ''incremented'' by the given values, not set directly. This is important for atomicity. We don't want multiple concurrent database calls to overwrite each other.

The more common way to do this would be to use the [https://docs.djangoproject.com/en/5.1/ref/models/querysets/#update-or-create `update_or_create`] method, but this [https://github.com/django/django/blob/5865ff5adcf64da03d306dc32b36e87ae6927c85/django/db/models/query.py#L965-L969 internally takes a lock on the row in case of updates], which is not acceptable because this is a hot code path with a lot of concurrent requests. We can’t afford to take a lock on the row.

I think it would be great if Django ORM supported atomic upserts without taking a lock on the row. I’m not sure what the api for that would look like, suggestions are welcome.

As for database support, PostgreSQL and SQLite support this syntax since PostgreSQL 9.5 and SQLite 3.24.0 respectively. MySQL and MariaDB do not support this natively, but it can be emulated using `INSERT ... ON DUPLICATE KEY UPDATE` syntax and creating a unique index on the columns.

Oracle apparently supports something similar using the vastly different `MERGE` statement syntax. That might be a bit of a challenge.

Looking for feedback on this idea. Is this something that would be worth supporting in Django? I’ll admit it’s a bit of a niche feature, but it’s a very useful one in certain cases. I would love to get rid of the raw SQL query in `wagtail-ab-testing` and have something that works across all supported databases.

- Storm"	New feature	assigned	Database layer (models, ORM)		Normal			Storm Heg Lily Foote Simon Charette John Speno	Accepted	0	0	0	0	0	0