osipovartem commented on issue #12907:
URL: https://github.com/apache/datafusion/issues/12907#issuecomment-5558807571

   I would like to pick up the UNPIVOT half as a focused first PR. I maintain a 
downstream implementation, but rather than revive the previous 1,000+ line 
combined change, I plan to split it into:
   
   1. a reusable `LogicalPlanBuilder::unpivot` transformation,
   2. a thin DataFrame API wrapper, and
   3. SQL `TableFactor::Unpivot` lowering with focused tests.
   
   The implementation will use ordinary projections + `UNION ALL`, avoid 
dependencies on scalar UDFs, cover INCLUDE/EXCLUDE NULLS, aliases, type 
coercion, and validation, and leave PIVOT for a separate follow-up. I will 
preserve attribution to the prior work where code is reused.


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