Hand-optimize some queries

src/pretix/base/migrations/0080_auto_20171016_1553.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.5 on 2017-10-16 15:53
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0079_auto_20171010_2117'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='cartposition',
+            name='cart_id',
+            field=models.CharField(blank=True, db_index=True, max_length=255, null=True, verbose_name='Cart ID (e.g. session key)'),
+        ),
+    ]