diff --git a/jupyterhub/orm.py b/jupyterhub/orm.py
index 7f1dfe91..842567c9 100644
--- a/jupyterhub/orm.py
+++ b/jupyterhub/orm.py
@@ -663,8 +663,9 @@ def new_session_factory(url="sqlite:///:memory:",
 
     # check the db revision (will raise, pointing to `upgrade-db` if version doesn't match)
     check_db_revision(engine)
-    if mysql_large_prefix_check(engine):
-        add_row_format(Base)
+
+    if mysql_large_prefix_check(engine):  # if mysql is allows large indexes
+        add_row_format(Base)              # set format on the tables
     Base.metadata.create_all(engine)
 
     # We set expire_on_commit=False, since we don't actually need