manuzhang opened a new pull request #29540:
URL: https://github.com/apache/spark/pull/29540


   ### What changes were proposed in this pull request?
   
   When the minimum number of coalesced partitions is not set in AQE, we do not 
fall back to default parallelism but coalesce partitions towards the 
`advisoryTargetSize`.
   
   
   ### Why are the changes needed?
   
   The default parallelism is usually the number of total executors cores which 
might be different across different runs of the same job when dynamic 
allocation is enabled. It results in uncertainty of the number of tasks after 
coalescing and leads to many small output files in some cases. The behavior is 
complex and hard to reason about.
   
   Hence, I'm proposing not falling back to the default parallelism but 
coalescing towards the `advisoryTargetSize` when the minimum number of 
coalesced partitions is not set.
   
   ### Does this PR introduce _any_ user-facing change?
   No.
   
   
   ### How was this patch tested?
   Existing Tests.
   


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