```ruby
class AddRunUntaggedToCiRunner < ActiveRecord::Migration
  ##
  # Downtime expected!
  #
  # This migration will cause downtime due to exclusive lock
  # caused by the default value.
  #
  def change
    add_column :ci_runners, :run_untagged, :boolean, default: true, null: false
  end
end
```

Use migration helper to prevent downtime migration
user
```ruby
class AddRunUntaggedToCiRunner < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers
  disable_ddl_transaction!

  def up
    add_column_with_default(:ci_runners, :run_untagged, :boolean,
                            default: true, allow_null: false)
  end

  def down
    remove_column(:ci_runners, :run_untagged)
  end
end
```
assistant