Hi Zhanghao
AFAIK, you might to see the `StreamingJobGraphGenerator` not the
`JobGraphGenerator` which is only used by the old flink stream sql stack.
>From comment of the `StreamingJobGraphGenerator::isChainableInput` the `an
union operator` does not support chain currently.
Best,
Guowei
On Wed, Apr 6, 2022 at 12:11 AM Zhanghao Chen <zhanghao.c...@outlook.com>
wrote:
> Dear all,
>
> I was recently investigating why the chaining behavior of a Flink SQL job
> containing union ops is a bit surprising. The SQL, simplified to the
> extreme, is as below:
>
> *CREATE TABLE datagen_source (word VARCHAR)*
> * WITH ('connector' = 'datagen', 'rows-per-second' = '5');*
>
> *CREATE TABLE blackhole_sink (word VARCHAR)*
> * WITH ('connector' = 'blackhole');*
>
> *INSERT INTO blackhole_sink*
> *SELECT word*
> *FROM (*
> * SELECT word*
> * FROM datagen_source*
> * WHERE word = '1'*
> * UNION ALL*
> * SELECT word*
> * FROM datagen_source*
> * WHERE word = '1'*
> * )*
>
> With all the operators having the same parallelism, I thought all the ops
> should be chained, but it turns out that the sink is not chained. I found
> the following comment in the code piece for checking the eligibility of
> chaining in JobGraphGenerator::createSingleInputVertex:
> "*first op after union is stand-alone, because union is merged*" that
> could be relevant, but I'm not sure what it means.
>
> Could anyone enlighten me how to understand this?
>
> Best,
> Zhanghao Chen
>
