Does Django index Autofield / ID keys in PostgreSQL?

Question

Django's docs state that id fields created with AutoField are indexed:

id is indexed by the database and is guaranteed to be unique.

Similarly it applies an index to every FK relationship.

However, in PostgreSQL whilst FKs appear to be indexed, IDs are not. Here's an example:

class TestModelBase(models.Model):
    name = models.CharField(max_length=50)
    fkfield = models.ForeignKey(TestModelFK, blank=True, null=True,
                                on_delete=models.CASCADE)
    m2mfield = models.ManyToManyField(TestModelM2M, related_name='base_m2m')

This model appears to apply the fkfield index, but not the id autofield. From PGAdmin below:

Am I missing something?

Alasdair · Accepted Answer · 2020-07-30 12:01:04Z

5

PostgreSQL automatically creates indexes for primary keys. From the docs:

Adding a primary key will automatically create a unique B-tree index on the column or group of columns listed in the primary key, and will force the column(s) to be marked NOT NULL.

It appears that PGAdmin does not show those indexes. This mailing list thread is the best source I could find.

answered Jul 30, 2020 at 12:01

Alasdair

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2 Comments

alias51 Over a year ago

Thanks. Interesting that django + postgres will still let you manually create an index on a primary key (e.g. indexes = [models.Index(fields=['id'], name='%(class)s_id_idx'),] ). I would have thought it would reject the duplicate?

Alasdair Over a year ago

Postgres allows multiple indexes on the same field so Django should as well. It might be useful if Django had a system check to warn about it.

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