protected function addIndex($name, $type, $column)
    {
        if (isset($this->indexes[$name])) {
            $this->indexes[$name]['columns'][] = $column;
        }
        else {
            $this->indexes[$name] = [
                'name' => $name,
                'type' => $type,
                'columns' => [
                    $column
                ]
            ];
        }
    }