more faster index on notifications table (#6108)

db/migrate/20171226094803_more_faster_index_on_notifications.rb

@@ -0,0 +1,7 @@
+class MoreFasterIndexOnNotifications < ActiveRecord::Migration[5.1]
+  def change
+    commit_db_transaction
+    add_index :notifications, [:account_id, :id], order: { id: :desc }, algorithm: :concurrently
+    remove_index :notifications, name: :index_notifications_on_id_and_account_id_and_activity_type
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20171212195226) do
+ActiveRecord::Schema.define(version: 20171226094803) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -259,8 +259,8 @@ ActiveRecord::Schema.define(version: 20171212195226) do
     t.bigint "account_id"
     t.bigint "from_account_id"
     t.index ["account_id", "activity_id", "activity_type"], name: "account_activity", unique: true
+    t.index ["account_id", "id"], name: "index_notifications_on_account_id_and_id", order: { id: :desc }
     t.index ["activity_id", "activity_type"], name: "index_notifications_on_activity_id_and_activity_type"
-    t.index ["id", "account_id", "activity_type"], name: "index_notifications_on_id_and_account_id_and_activity_type", order: { id: :desc }
   end
 
   create_table "oauth_access_grants", force: :cascade do |t|