protected function _setDefault()
    {
        if ($this->_default === null) {
            $this->_table->alter(
                sprintf('ALTER COLUMN %s DROP DEFAULT', $this->_database->prepareIdentifier($this->_name))
            );
        } else {
            $this->_table->alter(
                sprintf(
                    'ALTER COLUMN %s SET DEFAULT %s',
                    $this->_database->prepareIdentifier($this->_name),
                    $this->_database->prepareValue($this->_default, $this->_datatype)
                )
            );
        }
    }