Fixing MySQL GROUP_CONCAT() Issues

The group_concat_max_len value needs to be increased for some courses. This value is specific to MySQL (as opposed to SQLite run locally), so the DB vendor must be compared to ensure the new code only runs for MySQL databases.

analytics_data_api/v0/views/courses.py

@@ -645,7 +645,14 @@ FROM answer_distribution
 WHERE course_id = %s
 GROUP BY module_id;
         """
-        with connections[settings.ANALYTICS_DATABASE].cursor() as cursor:
+        connection = connections[settings.ANALYTICS_DATABASE]
+        with connection.cursor() as cursor:
+            if connection.vendor == 'mysql':
+                # The default value of group_concat_max_len, 1024, is too low for some course data. Increase this value
+                # to its maximum possible value. For more information see
+                # http://code.openark.org/blog/mysql/those-oversized-undersized-variables-defaults.
+                cursor.execute("SET @@group_concat_max_len = @@max_allowed_packet;")
+
             cursor.execute(sql, [self.course_id])
             rows = dictfetchall(cursor)