Jihoon Son created SPARK-51772:
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             Summary: Use of `copy()` instead of `withNewChildren` to create a 
copy of `TreeNode` with new children
                 Key: SPARK-51772
                 URL: https://issues.apache.org/jira/browse/SPARK-51772
             Project: Spark
          Issue Type: Bug
          Components: Optimizer
    Affects Versions: 4.1.0
            Reporter: Jihoon Son


The `TreeNode` class provides `withNewChildren()` function to create a copy of 
the node with new children. `withNewChildren()` does not only perform the copy 
of the node, but also updates the origin and copies the tags. However, I am 
seeing direct uses of `copy()` instead of `withNewChildren()` such as 
[here|[https://github.com/apache/spark/blob/master/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala#L1850].]
 I actually see quite many cases of this pattern. When I searched for the 
pattern `.copy(child = `, I get 115 cases. `.copy(children = ` adds another 18 
cases. I'm not sure whether this is intended or not. 

The reason I started looking into this is that I am developing something that 
relies on the tags in the `LogicalPlan`, and I realized that the tags are 
missing sometimes after some optimizations. I wonder whether the tagging is not 
a stable API for `LogicalPlan`.



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