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From chenghao-intel <...@git.apache.org>
Subject [GitHub] spark pull request: [SPARK-7269] [SQL] [WIP] Refactor the class At...
Date Fri, 12 Jun 2015 00:45:40 GMT
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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