Speed up external user statistics

app/controllers/external_users_controller.rb

@@ -14,9 +14,53 @@ class ExternalUsersController < ApplicationController
     authorize!
   end
 
+  def working_time_query
+    """
+    SELECT user_id,
+           exercise_id,
+           max(score) as maximum_score,
+           count(id) as runs,
+           sum(working_time_new) AS working_time
+    FROM
+      (SELECT user_id,
+              exercise_id,
+              score,
+              id,
+              CASE
+                  WHEN working_time >= '0:30:00' THEN '0'
+                  ELSE working_time
+              END AS working_time_new
+       FROM
+         (SELECT user_id,
+                 exercise_id,
+                 max(score) AS score,
+                 id,
+                 (created_at - lag(created_at) over (PARTITION BY user_id, exercise_id
+                                                     ORDER BY created_at)) AS working_time
+          FROM submissions
+          WHERE user_id = #{@user.id}
+            AND user_type = 'ExternalUser'
+          GROUP BY exercise_id,
+                   user_id,
+                   id) AS foo) AS bar
+    GROUP BY user_id,
+             exercise_id;
+    """
+  end
+
   def statistics
     @user = ExternalUser.find(params[:id])
     authorize!
+
+    statistics = {}
+
+    ActiveRecord::Base.connection.execute(working_time_query).each do |tuple|
+      statistics[tuple["exercise_id"].to_i] = tuple
+    end
+
+    render locals: {
+      statistics: statistics
+    }
   end
 
 end