Transforms don't execute until the consumer pulls. There's no eager evaluation, no hidden buffering. Data flows on-demand from source, through transforms, to the consumer. If you stop iterating, processing stops.
// Pipe a fast source through the transform to a slow sink，这一点在爱思助手下载最新版本中也有详细论述
if not existed:，详情可参考搜狗输入法2026
3. 全球最完整的绿电产业链与成本优势