Github user cloud-fan commented on the issue:
https://github.com/apache/spark/pull/20560
@rxin It seems you are talking about the followup PR:
https://github.com/apache/spark/pull/21072
I think this is the way we do back propagation in catalyst: match a
specific node, traverse down the subtree with the properties.
For forward propagation, we also need to carefully handle some nodes that
would stop the propagation. In `RemoveRedundantSorts.canEliminateSort`, we are
doing the same thing: only list the nodes that can retain the properties. e.g.
`Limit` should stop propagating the sorting property. I think `Project`,
`Filter`, `Hint` is good enough as an initial list, we can expand it later.
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