Rachelint commented on PR #15591:
URL: https://github.com/apache/datafusion/pull/15591#issuecomment-5477626355

   > > So perhaps a simpler alternative is to add EmitTo::Last.
   > 
   > This is an interesting idea -- is your idea to keep batches managed by 
blocks internally, but not change the group_index API? And then EmitTo::Last ? 
Or were you imagining coping the EmitTo::Last rows to the output from the inner 
Vec allocation
   
   I want to try the latter, because:
   - I found the simple copying is not expansive as we think when I experiment 
other ideas.
   - According to the analysis of the coding agent, `Emit::Last` can be easily 
implemented for most `GroupValues` and `GroupsAccumulator` (compare with the 
totally blocked approach like this pr) 
   - Not like `Emit::First`, `Emit::Last` will be effective for most 
`GroupValues` and `GroupsAccumulator`(truncate + shrink)
   


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