code: public function down()
      {
          $authManager = $this->getAuthManager();
  
          $this->dropIndex('{{%idx-auth_assignment-user_id}}', $authManager->assignmentTable);
          $this->createIndex('auth_assignment_user_id_idx', $authManager->assignmentTable, 'user_id');
  
  
          $this->dropIndex('{{%idx-auth_item-type}}', $authManager->itemTable);
          $this->createIndex('idx-auth_item-type', $authManager->itemTable, 'type');
      }
docs: {@inheritdoc}