Does Django automatically create indexes?

Question

I'm working on a Django project using Postgresql as an attached database. I've noticed that some queries took a very long time and even found that I had over a dozen indexes on many different fields in my database, some that I've deemed unnecessary.

My plan is to remove the ones I do not need but I'm interested to know if Django does this in a "smart" way when creating models or there is some default mechanism in place for creating indexes that I am unaware of.

Abdul Aziz Barkat · Accepted Answer · 2021-03-05 04:32:04Z

17

Django does create indexes automatically for some fields. For example it is stated in the documentation for Foreign Keys:

A database index is automatically created on the ForeignKey. You can disable this by setting db_index to False.

Also when you set unique to True on a field:

Note that when unique is True, you don’t need to specify db_index, because unique implies the creation of an index.

Also on a SlugField:

Implies setting Field.db_index to True.

On a field level you can control whether a field has an index or not by passing the db_index argument to the field.

You can also specify the indexes option in the models Meta:

class MyModel(model.Model):
    ...
    class Meta:
        indexes = [
            models.Index(fields=['field1', 'field2'], name='some_idx'),
        ]

answered Mar 5, 2021 at 4:32

Abdul Aziz Barkat

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4 Comments

Thatcher Thornberry Over a year ago

Is it best practice to have an index for each foreign key in my database?

Abdul Aziz Barkat Over a year ago

@ThatcherThornberry that totally depends on your need. If you see the documentation on Foreign Keys that I have linked it also states that "You may want to avoid the overhead of an index if you are creating a foreign key for consistency rather than joins, or if you will be creating an alternative index like a partial or multiple column index."

Deepanshu Mehta Over a year ago

is this meta option will create indexing on individual fields or joint index ?

Abdul Aziz Barkat Over a year ago

@DeepanshuMehta there will be an index created for each instance of Index in the list indexes. So for the given example it creates one multi-column index on the fields 'field1' and 'field2'. You can read the linked documentation (which also further links to Model index reference) for more info.

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