[GitHub] [spark] sigmod commented on a diff in pull request #36909: [SPARK-39511][SQL] Push down limit 1 for right side of left semi/anti join if join condition is empty

[GitBox <gi...@apache.org>]

sigmod commented on code in PR #36909:
URL: https://github.com/apache/spark/pull/36909#discussion_r903350188


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sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala:
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@@ -753,6 +753,9 @@ object LimitPushDown extends Rule[LogicalPlan] {
     // Merge offset value and limit value into LocalLimit and pushes down LocalLimit through Offset.
     case LocalLimit(le, Offset(oe, grandChild)) =>
       Offset(oe, LocalLimit(Add(le, oe), grandChild))
+    // Push down local limit 1 if join type is LeftSemiOrAnti and join condition is empty.
+    case j @ Join(_, right, LeftSemiOrAnti(_), None, _) if !right.maxRows.exists(_ <= 1) =>

Review Comment:
   In this case, 
   when right.maxRows > 0
   - for SEMI join, can we eliminate the right side and the join entirely? 
   - for ANTI, can we rewrite it to an empty Relation?
   
   when when right.maxRows = 0
   - for SEMI, can we rewrite it to an empty Relation?
   - for ANTI join, can we eliminate the right side and the join entirely? 



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