Check for dangerous operations when adding foreign keys

[eiriklied]

If we add a models.ForeignKey from table foo to table bar, the Django automatically sets up an index on the new field on table foo, not using CONCURRENTLY. This means if we introduce a ForeignKey field, Postgres will hold a full AccessExclusiveLock at least on the referencing table foo while the index is built.

This is problematic especially if the foreign key field is added to large tables (even though the newly added field contains no data), making the full transaction slow, and especially if the referenced table is accessed a lot.

Example model file:

# Inside model Foo
field_name = models.ForeignKey(
    "Bar",
    related_name="from_field_name",
    blank=True,
    null=True,
    on_delete=models.PROTECT,
)

Example migration file:

class Migration(migrations.Migration):

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

ALTER TABLE "foo" ADD COLUMN "field_name_id" integer NULL CONSTRAINT "foo_field_name_id_e478f75e_fk_bar_" REFERENCES "bar"("id") DEFERRABLE INITIALLY DEFERRED; SET CONSTRAINTS "foo_field_name_id_e478f75e_fk_bar_" IMMEDIATE;
CREATE INDEX "foo_field_name_id_e478f75e" ON "foo" ("field_name_id");