Django: Does unique_together imply db_index=True in the same way that ForeignKey does?
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27-10-2019 - |
문제
A field on a model, foo = models.ForeignKey(Foo)
will automatically add a database index for the column, in order to make look-ups faster. That's good and well, but Django's docs don't state whether the fields in a model-meta's unique_together
receive the same treatment. I happen to have a model in which one char field which is listed in unique_together
requires an index for quick lookups. I know that it won't hurt anything to add a duplicate db_index=True
in the field definition, but I'm curious.
해결책
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
다른 팁
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.
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.
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'
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.