[GitHub] [spark] emanuelebardelli commented on issue #24780: [SPARK-26388][SQL][WIP]_implement_hive_ddl_alter_table_replace_columns

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emanuelebardelli commented on issue #24780: [SPARK-26388][SQL][WIP]_implement_hive_ddl_alter_table_replace_columns
URL: https://github.com/apache/spark/pull/24780#issuecomment-499401355
 
 
   @thesuperzapper 
   
   yes, the idea is to implement `replace columns` fully, but it has to be done in a "spark-safe" way. 
   at the moment, even the core functionality required for this operation (ie: dropping columns) is not allowed and still being questioned (see above conversation).
   
   in relation to your use cases:
   
   1. yes, can fully replace a column
   -- i have not tested with comments, when there's an agreement if this PR is actually viable, i'll add them
   
   2. yes
   
   3. yes and no : by default it is possibile to change the type of a column while replacing it. however, i have specifically disabled this option in an attempt to make the replace columns operation safer (8b6da23).
   -- i have not tested complex data types (ie: `STRUCT`). when there's an agreement if this PR is actually viable, i'll add them 

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