Add a (user_id, role_id) index to dcc_role_users table.

common/djangoapps/django_comment_common/migrations/0008_role_user_index.py

@@ -0,0 +1,17 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_comment_common', '0007_discussionsidmapping'),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            'CREATE INDEX dcc_role_users_user_role_idx ON django_comment_client_role_users(user_id, role_id);'
+        ),
+    ]