Merge pull request #17879 from edx/bmedx/remove_default_site_from_attributes

Exclude the default site from UserAttribute 'created_on_site'

common/djangoapps/student/helpers.py

@@ -402,9 +402,12 @@ def generate_activation_email_context(user, registration):
 def create_or_set_user_attribute_created_on_site(user, site):
     """
     Create or Set UserAttribute indicating the microsite site the user account was created on.
-    User maybe created on 'courses.edx.org', or a white-label site
+    User maybe created on 'courses.edx.org', or a white-label site. Due to the very high
+    traffic on this table we now ignore the default site (eg. 'courses.edx.org') and
+    code which comsumes this attribute should assume a 'created_on_site' which doesn't exist
+    belongs to the default site.
     """
-    if site:
+    if site and site.id != settings.SITE_ID:
         UserAttribute.set_user_attribute(user, 'created_on_site', site.domain)