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https://issues.apache.org/jira/browse/SPARK-34514?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Yuming Wang reassigned SPARK-34514:
-----------------------------------
Assignee: Cheng Su (was: Apache Spark)
> Push down limit for LEFT SEMI and LEFT ANTI join
> ------------------------------------------------
>
> Key: SPARK-34514
> URL: https://issues.apache.org/jira/browse/SPARK-34514
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 3.1.0
> Reporter: Cheng Su
> Assignee: Cheng Su
> Priority: Trivial
> Fix For: 3.2.0
>
>
> I found out during code review of
> [https://github.com/apache/spark/pull/31567|https://github.com/apache/spark/pull/31567,](
> [https://github.com/apache/spark/pull/31567#discussion_r577379572] ), where
> we can push down limit to the left side of LEFT SEMI and LEFT ANTI join, if
> the join condition is empty.
> Why it's safe to push down limit:
> The semantics of LEFT SEMI join without condition:
> (1). if right side is non-empty, output all rows from left side.
> (2). if right side is empty, output nothing.
>
> The semantics of LEFT ANTI join without condition:
> (1). if right side is non-empty, output nothing.
> (2). if right side is empty, output all rows from left side.
>
> With the semantics of output all rows from left side or nothing (all or
> nothing), it's safe to push down limit to left side.
> NOTE: LEFT SEMI / LEFT ANTI join with non-empty condition is not safe for
> limit push down, because output can be a portion of left side rows.
>
> Physical operator for LEFT SEMI / LEFT ANTI join without condition -
> [https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/joins/BroadcastNestedLoopJoinExec.scala#L200-L204]
> .
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