Django: Est-ce que unique_together implique db_index = True de la même manière que le fait ForeignKey?

Question

A champ sur un modèle, foo = models.ForeignKey(Foo) ajoutera automatiquement un index de base de données pour la colonne, afin de faire paraître-ups plus rapidement. C'est bon et bien, mais les docs de Django ne le font pas état si les champs dans le unique_together d'un modèle méta reçoivent le même traitement. J'arrive d'avoir un modèle dans lequel un champ char qui figure dans unique_together nécessite un index pour les recherches rapides. Je sais que ce sera tout pas mal d'ajouter un db_index=True double dans la définition du champ, mais je suis curieux.

Answer 1

unique_together does not automatically add indexes for each field included in the list.

The new versions of Django suggest using Index & constraint meta options instead:

https://docs.djangoproject.com/en/2.2/ref/models/options/#unique-together

https://docs.djangoproject.com/en/2.2/ref/models/options/#index-together

Answer 2

For anyone coming here wondering if they need an index_together in addition to unique_together to get the index's performance benefit, the answer for Postgres is no, they are functionally the same.

Answer 3

If unique_together does add an index, it will be a multiple column index.

If you want one of the columns to be indexed individually, I believe you need to specify db_index=True in the field definition.

Answer 4

In Django 1.5 and higher, you can use the {Model}.Meta.index_together class attribute. If you had two fields named foo and bar, you would add:

class Meta(object):
    index_together = unique_together = [
        ['foo', 'bar']
    ]

If you have only one set of unique fields, you can use a one-dimensional iterable for unique_together. However, the documentation does not indicate that the same applies to index_together.

This would also be okay:

class Meta(object):
    unique_together = 'foo', 'bar'
    index_together = [
        ['foo', 'bar']
    ]

This, however, is NOT supported by the documentation:

class Meta(object):
    unique_together = 'foo', 'bar'
    index_together = 'foo', 'bar'

Answer 5

According to the docs, it will only enforce uniqueness on database level. I think generally making a field unique does not imply it has an index. Though you could also simply check on db level if the index exists. Everything indicates though it does not.