MariaDB does not support functional group by dependencies

[Matthew Somerville]

Requested to be opened from comment 11 on #34978. If you're using MariaDB with ONLY_FULL_GROUP_BY turned on, all queries that end up using a GROUP BY fail. Given a model such as:

class Author(models.Model):
    pass

class Play(models.Model):
    title = models.CharField(max_length=100)
    authors = models.ManyToManyField(Author)

Then list(Play.objects.annotate(Count('authors'))) generates the SQL:

SELECT app_play.id, app_play.title, COUNT(app_play_authors.author_id) AS authors__count
FROM app_play
LEFT OUTER JOIN app_play_authors ON (app_play.id = app_play_authors.play_id)
GROUP BY app_play.id
ORDER BY NULL

Which errors with django.db.utils.OperationalError: (1055, "'mariadbtest.app_play.title' isn't in GROUP BY")

Unlike MySQL since 5.7.5 (release notes: "MySQL now recognizes when a nonaggregated selected column is functionally dependent on (uniquely determined by) GROUP BY columns."), MariaDB does not support this feature. But Django 4.2 assumes both MySQL and MariaDB support it.

Simon's comment on #34978 said: "The solution will likely be to turn off the allows_group_by_selected_pks feature on MariaDB when ONLY_FULL_GROUP_BY mode is turned on. Note that the allows_group_by_selected_pks feature is different from the allows_group_by_pk feature removed in 041551d716b69ee7c81199eee86a2d10a72e15ab."

[David Sanders]

Thanks for creating this Matthew 🏆

[Nathaniel Conroy]

I think I understand the problem here, and I was able to test an simple solution locally.

However, this is my first ticket, so it may be a few days before I have a patch ready. I need to get more acquainted with the tests and the general workflow.

[Mariusz Felisiak]

​PR

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0257426f:

Fixed #34992 -- Fixed DatabaseFeatures.allows_group_by_selected_pks on MariaDB with ONLY_FULL_GROUP_BY sql mode.

Regression in 041551d716b69ee7c81199eee86a2d10a72e15ab.

[Mariusz Felisiak <felisiak.mariusz@…>]

In d9ba8fa:

[5.0.x] Fixed #34992 -- Fixed DatabaseFeatures.allows_group_by_selected_pks on MariaDB with ONLY_FULL_GROUP_BY sql mode.

Regression in 041551d716b69ee7c81199eee86a2d10a72e15ab.

Backport of 0257426fe1fe9d146fd5813f09d909917ff59360 from main

[Mariusz Felisiak <felisiak.mariusz@…>]

In 450d518d:

[4.2.x] Fixed #34992 -- Fixed DatabaseFeatures.allows_group_by_selected_pks on MariaDB with ONLY_FULL_GROUP_BY sql mode.

Regression in 041551d716b69ee7c81199eee86a2d10a72e15ab.

Backport of 0257426fe1fe9d146fd5813f09d909917ff59360 from main.