zml1206 commented on PR #5750:
URL: 
https://github.com/apache/incubator-gluten/pull/5750#issuecomment-2128390643

   > > > > Background history: 
[SPARK-36612](https://issues.apache.org/jira/browse/SPARK-36612) support left 
outer join build left or right outer join build right in shuffled hash join, it 
first took effect in spark 3.5.
   > > > 
   > > > 
   > > > @zml1206
   > > > Then how does this PR work with lower Spark version?
   > > 
   > > 
   > > If there is no fallback, all versions of velox bakcend support using 
small table to build hashmap. If there is a fallback, it will remain consistent 
with vanilla spark, it is supported after 3.5 and not supported before 3.5. 
@zhztheplayer
   > 
   > Does that mean we don't actually need strategy `JoinSelectionOverrides` to 
alter join build side on vanilla query plan ? Can we completely remove that 
strategy?
   
   First of all, the judgment of JoinSelectionOverrides and Spark Join 
Selection are not the same. I am not sure why JoinSelectionOverrides does not 
have the judgment of size. Secondly, clickhouse backend has special processing.


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