Github user chenghao-intel commented on the pull request:
https://github.com/apache/spark/pull/6587#issuecomment-111315693
@marmbrus thanks for explanation.
I am not so sure what kind of bugs you've seen, but as you know we have
lots of places use the `Set[Expression]` and `Map[Expression]`, those
scala/java collections are naturely rely on the `.hashCode()` and `equals()`
method, not the `semanticEquals`, and definitely will cause weird bugs for the
developers who is new to Catalyst. I believe that will keep happening.
For performance concern, I don't think that's a issue! If the user code
return a different instance reference from the transformation rule, why should
we call its `equal()` method and then decide whether the substitution takes,
why not just return the same instance if the user code doesn't want to change
anything? One scenario that I can assume is the deep copy a `TreeNode` for
further analysis without change the original one, as the `equals()` will be
considered in the substitution, I am not sure what's the easiest way to do this.
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