viirya opened a new pull request #27499: [SPARK-30590][SQL] Untyped select API 
cannot take typed column expression
URL: https://github.com/apache/spark/pull/27499
 
 
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   ### What changes were proposed in this pull request?
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   This patch proposes to throw analysis exception if untyped `Dataset.select` 
takes typed column expression.
   
   This patch also proposes to make `Dataset.selectUntyped` as public API for 
selecting multiple typed column expressions.
   
   ### Why are the changes needed?
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   `Dataset` provides few typed `select` helper functions to select typed 
column expressions. The maximum number of typed columns supported is 5. If 
wanting to select more than 5 typed columns, it silently calls untyped 
`Dataset.select` can causes weird unresolved error.
   
   We should explicitly let users know that they are incorrectly calling 
untyped `select` with typed columns.
   
   Because typed `Dataset.select` cannot be used to select more than 5 typed 
columns, this also opens `selectUntyped`.
   
   ### Does this PR introduce any user-facing change?
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   Yes. If users intentionally or unintentionally call `Dataset.select` API 
with typed column, an analysis exception will be thrown. Users can use 
`selectUntyped` API to select multiple typed columns.
   
   ### How was this patch tested?
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   Unit tests.
   

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