dongjoon-hyun edited a comment on pull request #29172:
URL: https://github.com/apache/spark/pull/29172#issuecomment-661616433


   For @HyukjinKwon  comments,
   - `CaseInsensitiveMap` is already modifying `originalMap` at `-` operation. 
Technically, `originalMap` is immutable, but  `+` operation can modify it too 
in the same reason. In short, both `-` and `+` operation should return the 
desirable map object for case-insensitivity. Also, this PR makes the class more 
consistent.
   - In general, `DataFrameReader` 
[example](https://github.com/apache/spark/pull/29172#issuecomment-661604611) is 
orthogonal to this PR because this PR is focusing on `CaseInsensitiveMap` only.
   - For the example, I believe it's not a good practice. For that alternative 
approach, we need to add an implicit assumption into the Apache Spark 
code-base. And, it'll be fragile when we add a new code.
       - We should warn that `.toMap` should not be used always.
       - All concatenation should start with 
`collection.immutable.ListMap.empty[String, String]` always.
   


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