milenkovicm commented on PR #1722:
URL: 
https://github.com/apache/datafusion-ballista/pull/1722#issuecomment-4482812798

   > Wasted memory: a downstream aggregate stays Linear (hash table, 
O(distinct-groups) memory) even when an upstream stage completes and grants 
ordering on the group-by columns — where Sorted (streaming, O(1) memory) would 
suffice.
   
   In which cases will this happen? When can w guarantee preserving sorting 
across shuffles ?
   
   
   > Stale correctness assumption: after a subtree rewrite, an aggregate may 
hold a Sorted claim when its input is no longer ordered.
   
   Can you please give an example when would this claim  be true? 


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