zhuqi-lucas commented on issue #21381:
URL: https://github.com/apache/datafusion/issues/21381#issuecomment-4190420957

   Thanks @2010YOUY01  for the context! This issue is specifically about the 
case after SortExec is eliminated via sort pushdown (#21182). When sort 
elimination removes SortExec entirely, there is no Stage 1 merge. SPM becomes 
the only merge operator, reading directly from I/O-bound DataSourceExec 
partitions.
   
   In that scenario, a parallel merge within SPM itself (splitting N input 
streams into groups and merging in parallel) could help. The DuckDB parallel 
k-way merge link is great — thanks for sharing!


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