Make user stats in exercise statistics faster

app/controllers/exercises_controller.rb

@@ -155,7 +155,15 @@ class ExercisesController < ApplicationController
     if(@external_user)
       render 'exercises/external_users/statistics'
     else
-      render 'exercises/statistics'
+      user_statistics = {}
+      query = "SELECT user_id, MAX(score) AS maximum_score, COUNT(id) AS runs
+              FROM submissions WHERE exercise_id = 101 GROUP BY user_id;"
+      ActiveRecord::Base.connection.execute(query).each do |tuple|
+        user_statistics[tuple["user_id"].to_i] = tuple
+      end
+      render locals: {
+        user_statistics: user_statistics
+      }
     end
   end
 

app/views/exercises/statistics.html.slim

@@ -22,8 +22,7 @@ h1 = @exercise
       tbody
         - @exercise.send(symbol).distinct().each do |user|
           tr
-            - submissions = @exercise.submissions.where('user_id=?', user.id)
             td = link_to_if symbol==:external_users, "#{user.name} (#{user.email})", {controller: "exercises", action: "statistics", external_user_id: user.id, id: @exercise.id}
-            td = submissions.maximum('score') or 0
-            td = submissions.count('id')
+            td = user_statistics[user.id]['maximum_score'] or 0
+            td = user_statistics[user.id]['runs']
             td = @exercise.average_working_time_for(user.id) or 0