add indexes for discussion_topics searching

fixes COMMS-745

Test Plan:
- run migrations
- discussion_topic searching still works

Change-Id: I9aee3e1ca502385911b2959c3e53f013ef727bbf
Reviewed-on: https://gerrit.instructure.com/139112
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
QA-Review: Venk Natarajan <vnatarajan@instructure.com>
Product-Review: Steven Burnett <sburnett@instructure.com>

db/migrate/20180125172528_add_gist_indexes_for_discussion_topic_search.rb

@@ -0,0 +1,30 @@
+#
+# Copyright (C) 2013 - 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 AddGistIndexesForDiscussionTopicSearch < ActiveRecord::Migration[5.0]
+  disable_ddl_transaction!
+  tag :predeploy
+  def self.up
+    if (schema = connection.extension_installed?(:pg_trgm))
+      add_index :discussion_topics, "LOWER(title) #{schema}.gist_trgm_ops", name: "index_trgm_discussion_topics_title", using: :gist, algorithm: :concurrently
+    end
+  end
+
+  def self.down
+    remove_index :discussion_topics, name: 'index_trgm_discussion_topics_title'
+  end
+end