Github user HyukjinKwon commented on the issue:

    https://github.com/apache/spark/pull/14124
  
    Thanks for feedback @cloud-fan !
    
    If the user-given schema is wrong, it is handled differently for each 
datasource specific.
    
    - For JSON and CSV
      it is kind of permissive generally (for example, compatibility among 
numeric types).
    
    - For ORC and Parquet
      Generally it is strict to types. So they don't allow the compatibility 
(except for very few cases, e.g. for parquet, 
https://github.com/apache/spark/pull/14272 and 
https://github.com/apache/spark/pull/14278)
    
    I think so. Should we disallow specifying schemas for these? 
    - For JDBC
      it does not take user-given schema since it does not implement 
`SchemaRelationProvider`.
    



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