Django migrations: create a partial index and set `db_index=True` without creating a full index?

Question

I'm adding a NULL column to a large table, and I'd like to make sure that it has db_index=True set in the model definition:

class MyLargeModel(Model):
    new_field = IntegerField(null=True, default=None, db_index=True)

But for the actual index I'd like to use a partial index:

migrations.RunSQL("""
    CREATE INDEX my_index
    ON mylargemodel (new_field)
    WHERE new_field IS NOT NULL
""")

However, by default, this will mean Django's migrations will create an index (which will be very slow), then I'll need to manually drop that index and create my own.

How can I create a migration which will tell Django Migrations that db_index=True is set without creating the index in the database?

Why do you need to set db_index=True when you create your own index? — knbk
– knbk, Commented Jan 10, 2017 at 19:22
I'd like to keep db_index=True so that future developers (and future me) will know it's indexed and won't need to dig around to wonder what's going on. I know I could add a comment… but I'd rather have the explicit attribute. — David Wolever
– David Wolever, Commented Jan 10, 2017 at 20:16
You might find the class based indexes feature useful in the upcoming Django 1.11. — Alasdair
– Alasdair, Commented Jan 10, 2017 at 21:02

2ps · Accepted Answer · 2017-01-11 05:50:03Z

Add new_field to MyLargeModel without db_index=True and save
run makemigrations to generate a migration that will add new_field without the index.
Update new_field in the MyLargeModel definition and add back db_index=True
run makemigrations to generate the standard migration to add the index to MyLargeModel.new_field

Update the migration from (4) by adding the following custom class to the top of the file:

from django.db import migrations, models

class NoopAlterField(migrations.AlterField):
    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        pass

Update the migration from (4) by changing the call from migration.AlterField to our custom NoopAlterField

operations = [
    NoopAlterField(
        model_name='mylargemodel',
        name='new_field',
        field=models.IntegerField(...),
    ),
]

As you can guess all that this does is creates a NoopAlterField migration handler that will update the internal schema tracker but forego the actual database step of creating the index.

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