optimize planner wiki page query

it's far more common for wiki pages to not be linked to wiki pages, so only look up their
assignment visibilities if necessary

Change-Id: Idf3f05e2c5e59a2163bd692497160c2388551be2
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/246533
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: James Williams <jamesw@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

app/models/wiki_page.rb

@@ -81,8 +81,7 @@ class WikiPage < ActiveRecord::Base
   end
 
   scope :visible_to_user, -> (user_id) do
-    joins(sanitize_sql(["LEFT JOIN #{AssignmentStudentVisibility.quoted_table_name} as asv on wiki_pages.assignment_id = asv.assignment_id AND asv.user_id = ?", user_id])).
-      where("wiki_pages.assignment_id IS NULL OR asv IS NOT NULL")
+    where("wiki_pages.assignment_id IS NULL OR EXISTS (SELECT 1 FROM #{AssignmentStudentVisibility.quoted_table_name} asv WHERE wiki_pages.assignment_id = asv.assignment_id AND asv.user_id = ?)", user_id)
   end
 
   TITLE_LENGTH = 255

db/migrate/20200831150747_add_index_on_wiki_pages_for_planner.rb

@@ -0,0 +1,26 @@
+#
+# Copyright (C) 2020 - 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 AddIndexOnWikiPagesForPlanner < ActiveRecord::Migration[5.2]
+  tag :predeploy
+  disable_ddl_transaction!
+
+  def change
+    add_index :wiki_pages, [:wiki_id, :todo_date], where: "todo_date IS NOT NULL",
+              algorithm: :concurrently, if_not_exists: true
+  end
+end