pepijnve commented on PR #23522:
URL: https://github.com/apache/datafusion/pull/23522#issuecomment-4982566176

   > I'm worried about dropping the FIFO semantics (even if they were broken). 
Consider a plan like `Projection(inner=Repartition(inner=Sort, 
preserve_order=true)))`: if `RepartitionExec` does _not_ actually preserve 
order under spilling then the sort order will be lost and the results will be 
wrong.
   
   I'm probably missing a bit too much context. Could you help me define the 
exact order guarantee that's being promised by the spill pool? Single writer 
remains FIFO, but in the presence of multiple concurrent writers, what order 
can you actually guarantee for the reader? The best you can do I think is 
guarantee that the relative order of the batches per writer is retained. Is 
that sufficient?


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