alamb commented on issue #24768:
URL: https://github.com/apache/datafusion/issues/24768#issuecomment-5590296831

   > Type 1 can be optimized in a similar way with a dynamic filter, without 
adding too much extra complexity inside the operator. To implement type 2, we 
might need to maintain two parallel HJ implementations (order-preserving vs. 
non-order-preserving) plus extra optimizer rules, which is a lower ROI for the 
engineering effort involved.
   
   I agree we would probably need two implementations (or a flag that said 
order must be preserved, and then continue to error if the hash table exceeded 
the memory size). I think it would be hard to remove functionality that 
DataFusion already has (and make some queries slower) -- the good news is I 
think it would be fine to keep te existing limitations on HashJoin for the 
order preserving case (e.g. can't spill)


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