change backfill migration to just drop/add the column with default in…

…stead

it's much faster with postgres >= 11

Change-Id: Iba90373eaf6622c7b0e048bf898a9eea0fa15ac6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/260034
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

db/migrate/20201027182857_add_redo_request_to_submissions.rb → db/migrate/20210127005857_add_redo_request_to_submissions.rb

@@ -17,16 +17,15 @@
 # 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 AddRedoRequestToSubmissions < ActiveRecord::Migration[5.2]
   tag :predeploy
 
-  def change
-    add_column :submissions, :redo_request, :boolean
-    change_column_default(:submissions, :redo_request, false)
+  def up
+    remove_column :submissions, :redo_request, if_exists: true
+    add_column :submissions, :redo_request, :boolean, default: false, null: false
   end
 
   def down
-    remove_column :submissions, :redo_request, :boolean
+    change_column_null(:submissions, :redo_request, true)
   end
 end