diff --git a/app/models/exercise.rb b/app/models/exercise.rb
index 94210fd0..21ab412e 100644
--- a/app/models/exercise.rb
+++ b/app/models/exercise.rb
@@ -82,23 +82,111 @@ class Exercise < ActiveRecord::Base
   def get_quantiles(quantiles)
     quantiles_str = "[" + quantiles.join(",") + "]"
     result = self.class.connection.execute("""
-      SELECT unnest(PERCENTILE_CONT(ARRAY#{quantiles_str}) WITHIN GROUP (ORDER BY working_time))
-      FROM
-          (
-            SELECT user_id,
-                   sum(working_time_new) AS working_time
-            FROM
-              (SELECT user_id,
-                      CASE WHEN working_time >= '0:30:00' THEN '0' ELSE working_time END AS working_time_new
-               FROM
-                  (SELECT user_id,
-                          id,
-                          (created_at - lag(created_at) OVER (PARTITION BY user_id, exercise_id
-                                                              ORDER BY created_at)) AS working_time
-                  FROM submissions
-                  WHERE exercise_id=#{self.id} AND user_type = 'ExternalUser') AS foo) AS bar
-            GROUP BY user_id
-      ) AS foo
+            WITH working_time AS
+      (
+               SELECT   user_id,
+                        id,
+                        exercise_id,
+                        Max(score)                                                                                  AS max_score,
+                        (created_at - Lag(created_at) OVER (partition BY user_id, exercise_id ORDER BY created_at)) AS working_time
+               FROM     submissions
+               WHERE    exercise_id = #{id}
+               AND      user_type = 'ExternalUser'
+               GROUP BY user_id,
+                        id,
+                        exercise_id), max_points AS
+      (
+               SELECT   context_id  AS ex_id,
+                        Sum(weight) AS max_points
+               FROM     files
+               WHERE    context_type = 'Exercise'
+               AND      context_id = #{id}
+               AND      role = 'teacher_defined_test'
+               GROUP BY context_id),
+      -- filter for rows containing max points
+      time_max_score AS
+      (
+             SELECT *
+             FROM   working_time W1,
+                    max_points MS
+             WHERE  w1.exercise_id = ex_id
+             AND    w1.max_score = ms.max_points),
+      -- find row containing the first time max points
+      first_time_max_score AS
+      (
+             SELECT id,
+                    user_id,
+                    exercise_id,
+                    max_score,
+                    working_time,
+                    rn
+             FROM   (
+                             SELECT   id,
+                                      user_id,
+                                      exercise_id,
+                                      max_score,
+                                      working_time,
+                                      Row_number() OVER(partition BY user_id, exercise_id ORDER BY id ASC) AS rn
+                             FROM     time_max_score) T
+             WHERE  rn = 1), times_until_max_points AS
+      (
+             SELECT w.id,
+                    w.user_id,
+                    w.exercise_id,
+                    w.max_score,
+                    w.working_time,
+                    m.id AS reachedmax_at
+             FROM   working_time W,
+                    first_time_max_score M
+             WHERE  w.user_id = m.user_id
+             AND    w.exercise_id = m.exercise_id
+             AND    w.id <= m.id),
+      -- if user never makes it to max points, take all times
+      all_working_times_until_max AS (
+      (
+             SELECT id,
+                    user_id,
+                    exercise_id,
+                    max_score,
+                    working_time
+             FROM   times_until_max_points)
+      UNION ALL
+                (
+                       SELECT id,
+                              user_id,
+                              exercise_id,
+                              max_score,
+                              working_time
+                       FROM   working_time W1
+                       WHERE  NOT EXISTS
+                              (
+                                     SELECT 1
+                                     FROM   first_time_max_score F
+                                     WHERE  f.user_id = w1.user_id
+                                     AND    f.exercise_id = w1.exercise_id))), filtered_times_until_max AS
+      (
+             SELECT user_id,
+                    exercise_id,
+                    max_score,
+                    CASE
+                           WHEN working_time >= '0:30:00' THEN '0'
+                           ELSE working_time
+                    END AS working_time_new
+             FROM   all_working_times_until_max ), result AS
+      (
+               SELECT   e.external_id AS external_user_id,
+                        f.user_id,
+                        exercise_id,
+                        Max(max_score)        AS max_score,
+                        Sum(working_time_new) AS working_time
+               FROM     filtered_times_until_max f,
+                        external_users e
+               WHERE    f.user_id = e.id
+               GROUP BY e.external_id,
+                        f.user_id,
+                        exercise_id )
+      SELECT   unnest(percentile_cont(array#{quantiles_str}) within GROUP (ORDER BY working_time))
+      FROM     result
     """)
     if result.count > 0
       quantiles.each_with_index.map{|q,i| Time.parse(result[i]["unnest"]).seconds_since_midnight}