Under what circumstances will Flink combine operator chains to form operator chains?

• The parallelism of upstream and downstream is the same
• The penetration of downstream nodes is 1 （ That is to say, the downstream node has no input from other nodes ）
• The upstream and downstream nodes are all in the same slot group in
• The data partition between two nodes is forward（ Refer to understand the partition of data flow ） Otherwise it's all all_to_all
• Of upstream and downstream nodes chain Strategies for ALWAYS