Optimize SQL queries in order list and order search

src/pretix/base/migrations/0107_auto_20190129_1337.py

@@ -0,0 +1,21 @@
+# Generated by Django 2.1.5 on 2019-01-29 13:37
+
+from django.db import migrations, models
+import django.db.models.deletion
+import jsonfallback.fields
+import pretix.base.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0106_auto_20190118_1527'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='order',
+            name='datetime',
+            field=models.DateTimeField(db_index=True, verbose_name='Date'),
+        ),
+    ]