Github user xuanyuanking commented on the issue:
https://github.com/apache/spark/pull/19773
@maropu @dongjoon-hyun Great thanks for your guidance !
```
Apache Spark already supports changing column types as a part of schema
evolution. Especially, ORC vectorized reader support upcasting although it's
not the same with canCast.
For the detail support Spark coverage, see SPARK-23007. It covered all
built-in data source at that time.
```
Great thanks, I'll study these background soon.
```
Please note that every data sources have different capability. So, this PR
needs to prevent ALTER TABLE CHANGE COLUMN for those data sources case-by-case.
And, we need corresponding test cases.
```
Got it, I'll keep following the cases in this PR, I roughly split these
into 4 tasks and update the description of this PR firstly. I'll pay attention
to the corresponding test cases in each task.
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