simonvandel commented on PR #23548:
URL: https://github.com/apache/datafusion/pull/23548#issuecomment-4990958946

   Thanks @alamb 
   After merging in your fix, CI started to fail because the `intersect()` test 
in datafusion/optimizer/tests/optimizer_integration.rs that 
https://github.com/apache/datafusion/pull/22903 improved, regressed. I think 
this happens because we are not adequately tracking that functional 
dependencies through group-by expressions. I **think** that we could mark 
**nullable = true** here 
https://github.com/apache/datafusion/blob/a287c0a959310a6a52cafaf9492adc65d0755037/datafusion/common/src/functional_dependencies.rs#L503
 since a group-by considers Nulls equal so there can never be more than 1 null 
value after the group-by, but not 100% sure.
   
   I pushed 
https://github.com/apache/datafusion/pull/23548/commits/9601db86a9f052fbea3dbd5eed8f43ca0dae28da
 to restore the snapshot to before 
https://github.com/apache/datafusion/pull/22903.
   Would you be OK to merge the PR as-is now, although it regresses the 
optimization that https://github.com/apache/datafusion/pull/22903 added? 
Perhaps we could add a follow-up issue for better group-by tracking to unlock 
the regression of the https://github.com/apache/datafusion/pull/22903 
optimization


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