[I] [C++][Compute] Inaccurate calculation results will be produced when the 'cases' arrays of 'case_when' have different Decimal types [arrow]

["ZhangHuiGui (via GitHub)" <gi...@apache.org>]

ZhangHuiGui opened a new issue, #41336:
URL: https://github.com/apache/arrow/issues/41336

   ### Describe the bug, including details regarding any error messages, version, and platform.
   
   It's an interesting problem we find recently in `case_when` kernel.
   
   For the call function:
   `CaseWhen({MakeStruct({cond1, cond2, cond3}), {arr1, arr2, arr3, arr4}})`
   will produce the `case_when` logic like below:
   ```shell
   cond1 --> arr1 decimal(9,7),
   cond2 --> arr2 decimal(6,2),
   cond3 --> arr3 decimal(3, 2),
   arr4 decimal(2,1)
   ```
   
   The `case_when` will use the `LastType` which is `decimal(2,1)` as it's output type.
   https://github.com/apache/arrow/blob/79799e59b1a0c7bde8b85ea955593b2d63d2a46c/cpp/src/arrow/compute/kernels/scalar_if_else.cc#L2683-L2685.
   
   That means for the arr1/arr2/arr3's output results, the precision will be set to `decimal(2,1)`, obviously it's inaccurate in fact especially when we the `case_when` as an expression combining with aggregate's `sum/avg...`.
   
   laotan332 [qinpengxiang@outlook.com](mailto:qinpengxiang@outlook.com)
   
   
   
   ### Component(s)
   
   C++


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