Github user gatorsmile commented on the pull request:
https://github.com/apache/spark/pull/10678#issuecomment-171183014
In this update, the code changes are trying to extend the existing support
for the following scenario: ```sort by attributes that are not present in the
SELECT clause```
Now, between the operator that can resolve the sorting columns and the top
`Sort` operator, it can have one or multiple different operators of
`Aggregate`, `Project`, `Subquery` and `Window`. The reason why we just support
these four types is that only these four types we can add sorting attributes
into their project list. In the future, we can easily extend the solution by
supporting more types.
I am not sure if we should change the collection we used in this tail
recursion function? Now, it is `Seq`. If the plan tree is huge, the performance
could be not good. Thanks!
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