julianhyde commented on issue #1469: [CALCITE-3367] Add AntiJoinRule to convert 
project-filter-join-aggreg…
URL: https://github.com/apache/calcite/pull/1469#issuecomment-534833835
 
 
   Semi-join may have a lower cost implementation than a regular Join. It’s 
worth knowing that you have one.
   
   Also, because it projects fewer columns, it may match materialized views 
that an equivalent ordinary join would not. 
   
   Lastly, if your plan is using a semi-join operator you can safely weaken its 
input constraint from must-be-unique to should-be-unique. 

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