ujjwaltwri commented on issue #19171:
URL: https://github.com/apache/datafusion/issues/19171#issuecomment-3623138014

   Hi! I’m currently working on a fix for this.
   The root cause is that ConcatFunc does not override return_field_from_args, 
so the planner always treats the result as nullable. According to SQL semantics 
(and existing DataFusion behavior at execution time), concat should only be 
nullable when all input argument fields are nullable.
   I’m implementing return_field_from_args for ConcatFunc so that:
   The return DataType is derived using the existing return_type logic, and
   The return Field.nullable is args.iter().all(|f| f.is_nullable()).
   This brings schema-time nullability in line with runtime behavior and other 
SQL engines.
   A PR will be opened shortly after I finish local testing.


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