add various performance indexes

test plan:
* migrations run

closes #CNVS-47544 #CNVS-47543 #CNVS-47540 #CNVS-47538

Change-Id: I5e96ba4ee7c99597b1e4449d6d132e3c9bc5333b
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/216620
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins

db/migrate/20191111150403_add_misc_performance_indexes.rb

@@ -0,0 +1,37 @@
+#
+# Copyright (C) 2019 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+class AddMiscPerformanceIndexes < ActiveRecord::Migration[5.2]
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def change
+    add_index :discussion_entries, [:user_id, :discussion_topic_id], algorithm: :concurrently,
+      where: "workflow_state <> 'deleted'", name: "index_discussion_entries_active_on_user_id_and_topic"
+
+    add_index :conversation_participants, [:user_id], algorithm: :concurrently,
+      where: "workflow_state = 'unread'", name: "index_conversation_participants_unread_on_user_id"
+
+    add_index :assignments, [:duplication_started_at], algorithm: :concurrently,
+      where: "workflow_state = 'migrating' AND duplication_started_at IS NOT NULL",
+      name: "index_assignments_duplicating_on_started_at"
+
+    add_index :submissions, [:user_id], algorithm: :concurrently,
+      where: "(score IS NOT NULL AND workflow_state = 'graded') OR excused = TRUE",
+      name: "index_submissions_graded_or_excused_on_user_id"
+  end
+end