public function getSchema()
    {
        if (!isset($this->schema)) {
            $class = '\\Wslim\\Db\\Schema\\' . $this->options['adapter'] . 'Schema';
            if (class_exists($class)) {
                $this->schema = new $class($this->getAdapter());
            } else {
                throw new DbException('Schema is not exist:' . $class);
            }
        }
        return $this->schema;
    }