optimize students with no submissions report

fixes CNVS-10782

test plan
 - run students with no submissions report
 - it should run much faster

Change-Id: Ib57bac057e61a9ed7a0b2656b3f80c6efe134aa6
Reviewed-on: https://gerrit.instructure.com/29350
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

db/migrate/20140127203558_add_index_on_submissions_submitted_at.rb

@@ -0,0 +1,12 @@
+class AddIndexOnSubmissionsSubmittedAt < ActiveRecord::Migration
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def self.up
+    add_index :submissions, :submitted_at, algorithm: :concurrently
+  end
+
+  def self.down
+    remove_index :submissions, :submitted_at
+  end
+end

db/migrate/20140127204017_enrollments_on_root_account_and_course.rb

@@ -0,0 +1,13 @@
+class EnrollmentsOnRootAccountAndCourse < ActiveRecord::Migration
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def self.up
+    add_index :enrollments, [:root_account_id, :course_id],
+              algorithm: :concurrently
+  end
+
+  def self.down
+    remove_index :enrollments, [:root_account_id, :course_id]
+  end
+end

vendor/plugins/account_reports/lib/canvas/account_reports/student_reports.rb

@@ -67,9 +67,9 @@ module Canvas::AccountReports
       CSV.open(file, "w") do |csv|
 
         condition = [""]
-        condition.first << " AND submitted_at > ?"
+        condition.first << " AND s.submitted_at > ?"
         condition << start_at
-        condition.first << " AND submitted_at < ?"
+        condition.first << " AND s.submitted_at < ?"
         condition << end_at
 
         time_span_join = Pseudonym.send(:sanitize_sql, condition)
@@ -86,13 +86,12 @@ module Canvas::AccountReports
                  INNER JOIN pseudonyms p ON e.user_id = p.user_id
                    AND courses.root_account_id = p.account_id
                  INNER JOIN users u ON u.id = p.user_id").
-          where("NOT EXISTS (SELECT user_id
+          where("NOT EXISTS (SELECT s.user_id
                              FROM submissions s
                              INNER JOIN assignments a ON s.assignment_id = a.id
-                             INNER JOIN courses c ON a.context_id = c.id
                                AND a.context_type = 'Course'
+                               AND a.context_id = courses.id
                              WHERE s.user_id = p.user_id
-                             AND c.id = courses.id
                              #{time_span_join})")
 
         no_subs = add_term_scope(no_subs)