add index specifically supporting Submission.needs_grading

Change-Id: I9718a6f931f586b43c45f5505d0c6877eb3605ac
Reviewed-on: https://gerrit.instructure.com/124526
Reviewed-by: Tyler Pickett <tpickett@instructure.com>
Tested-by: Jenkins
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

app/models/submission.rb

@@ -214,8 +214,8 @@ class Submission < ActiveRecord::Base
   end
 
   # see #needs_grading?
-  # When changing these conditions, consider updating index_submissions_on_assignment_id
-  # to maintain performance.
+  # When changing these conditions, update index_submissions_needs_grading to
+  # maintain performance.
   def self.needs_grading_conditions
     conditions = <<-SQL
       submissions.submission_type IS NOT NULL

db/migrate/20170830204122_add_submissions_needs_grading_index.rb

@@ -0,0 +1,18 @@
+class AddSubmissionsNeedsGradingIndex < ActiveRecord::Migration[5.0]
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def change
+    # see Submission.needs_grading; duplicated instead of called directly so the migration doesn't
+    # change even if the query does
+    add_index :submissions, :assignment_id, name: 'index_submissions_needs_grading', algorithm: :concurrently, where: <<-SQL
+      submissions.submission_type IS NOT NULL
+      AND (submissions.excused = 'f' OR submissions.excused IS NULL)
+      AND (submissions.workflow_state = 'pending_review'
+        OR (submissions.workflow_state IN ('submitted', 'graded')
+          AND (submissions.score IS NULL OR NOT submissions.grade_matches_current_submission)
+        )
+      )
+    SQL
+  end
+end