pepijnve commented on PR #17813:
URL: https://github.com/apache/datafusion/pull/17813#issuecomment-3499267271

   The test failures are now cause by the optimizer rewriting queries like
   
   ```
   SELECT ALL (+ (+ CASE - 54 WHEN COUNT(*) + - COUNT(*) THEN NULL ELSE - 46 + 
- 59 END)) * COUNT(*);
   ```
   
   In the original version the case nullability logic can't evaluate the 'when' 
expressions due the presence of `count(*)`. This causes the expression to be 
marked as nullable.
   
   The optimiser ends up eliminating the `count(*)` and eventually the case 
nullability logic does say the expression is not nullable.
   
   Since `<TYPE> NOT NULL` is strict subset of `<TYPE>`, I'm wondering if that 
schema change should actually be allowed to pass.


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