[GitHub] [arrow-datafusion] liukun4515 commented on issue #6261: RFC: Add `result type` for Expr

["liukun4515 (via GitHub)" <gi...@apache.org>]

liukun4515 commented on issue #6261:
URL: https://github.com/apache/arrow-datafusion/issues/6261#issuecomment-1541486489

   agree with @jackwener 
   
   I think  type of expr should not be changed in the physical phase and all of type coercion should be done in the logical phase, but in current codebase, we can't do this.
   
   The current implementation of the logical expr is `enum` and each type of logical lacks the result type , which is not friendly to achieve above thought.
   
   In other SQL system, the logical expr is import part, each expr should has its input type and output type, and the output type need to be calculated from input types with operation. Once the common type and result are calculated in the logical phase, any changes for the type of expr should be forbidden.
   
   As the comments pointed by @jackwener, some expr can't be evaluated many times. For example,
   
   `old_expr = Add(left : decimal(10,2),right: decimal(10,2))`
   after evaluating the first time:
   the common type is decimal(11,2)
   left casted from decimal(10,2) to decimal(11,2)
   right casted from decimal(10,2) to decimal(11,2)
   the result type is decimal(11,2)
   
   and will get the `new_expr = Add(left: cast(decimal(10,2) to decimal(11,2)), right:cast(decimal(10,2) to decimal(11,2))`
   If we evaluate the `new_expr` again, the new result will be generated.
   
   @alamb 
   
   
   
   


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