Github user yhuai commented on the issue:
https://github.com/apache/spark/pull/15575
Our planner decides if to add an `ShuffleExchange` by consider
`outputPartitioning` and `requiredDistribution` together. If the
`outputPartitioning` of the child does not satisfy the `requiredDistribution`
of the parent, we will add a `ShuffleExchange` operator.
When we say `child`, it is literally the child of a node. It does not
matter if the child is a `ShuffleExchange` or not. For a node, when its
`outputPartitioning` is `child. outputPartitioning`, this node does not shuffle
data.
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