Index issues on sent_notifications

Add index to sent_notifications table to prevent statement timeouts

changelogs/unreleased/issue_11391_2.yml

+---
+title: Index issues on sent_notifications table
+merge_request: 27034
+author:
+type: performance

db/post_migrate/20200311192351_add_index_on_noteable_type_and_noteable_id_to_sent_notifications.rb

+# frozen_string_literal: true
+
+class AddIndexOnNoteableTypeAndNoteableIdToSentNotifications < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_sent_notifications_on_noteable_type_noteable_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :sent_notifications,
+      [:noteable_id],
+      name: INDEX_NAME,
+      where: "noteable_type = 'Issue'"
+  end
+
+  def down
+    remove_concurrent_index_by_name :sent_notifications, INDEX_NAME
+  end
+end

db/schema.rb

@@ -3919,6 +3919,7 @@ ActiveRecord::Schema.define(version: 2020_03_13_123934) do
     t.string "note_type"
     t.text "position"
     t.string "in_reply_to_discussion_id"
+    t.index ["noteable_id"], name: "index_sent_notifications_on_noteable_type_noteable_id", where: "((noteable_type)::text = 'Issue'::text)"
     t.index ["reply_key"], name: "index_sent_notifications_on_reply_key", unique: true
   end