src/Service/DB/Select.php
public function iterator()
    {
        $res = $this->execute();

        while ($row = $res->fetch()) {
            yield $this->process($row);
        }
    }
public function iterator()
    {
        $res = $this->execute();

        while ($row = $res->fetch()) {
            yield $this->process($row);
        }
    }
以iterator的形式返回查询结果
通过遍历iterator的方式处理查询结果，避免过大的内存占用.