Add check for making an existing column NOT NULL

[eiriklied]

In Postgres, setting NOT NULL on an existing column blocks reads and writes while every row is checked, which is problematic on large tables.

For reference, have a look here.

This might be a bit hard to check as the migration file will look something like below. But perhaps it makes sense to look at the generated SQL?

class Migration(migrations.Migration):

    operations = [
        migrations.AlterField(
            model_name='foo',
            name='field_name',
            field=models.ForeignKey(blank=True, on_delete=django.db.models.deletion.PROTECT, related_name='related_field_name', to='bar'),
        ),
    ]

While the generated SQL becomes like below (the interesting part is ALTER TABLE "foo" ALTER COLUMN "field_name_id" SET NOT NULL;)

BEGIN;
SET CONSTRAINTS "foo_field_name__f89f691c_fk_bar" IMMEDIATE; ALTER TABLE "foo" DROP CONSTRAINT "foo_field_name__f89f691c_fk_bar";
ALTER TABLE "foo" ALTER COLUMN "field_name_id" SET NOT NULL;
ALTER TABLE "foo" ADD CONSTRAINT "foo_field_name__f89f691c_fk_bar" FOREIGN KEY ("field_name_id") REFERENCES "bar" ("id") DEFERRABLE INITIALLY DEFERRED;
COMMIT;