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https://issues.apache.org/jira/browse/SPARK-21795?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Sean Owen resolved SPARK-21795.
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Resolution: Duplicate
> Broadcast hint ignored when dataframe is cached
> -----------------------------------------------
>
> Key: SPARK-21795
> URL: https://issues.apache.org/jira/browse/SPARK-21795
> Project: Spark
> Issue Type: Question
> Components: Documentation, SQL
> Affects Versions: 2.2.0
> Reporter: Lior Chaga
> Priority: Minor
>
> Not sure if it's a bug or by design, but if a DF is cached, the broadcast
> hint is ignored, and spark uses SortMergeJoin.
> {code}
> val largeDf = ...
> val smalDf = ...
> smallDf = smallDf.cache
> largeDf.join(broadcast(smallDf))
> {code}
> It make sense there's no need to use cache when using broadcast join,
> however, I wonder if it's the correct behavior for spark to ignore the
> broadcast hint just because the DF is cached. Consider a case when a DF
> should be cached for several queries, and on different queries it should be
> broadcasted.
> If this is the correct behavior, at least it's worth documenting that cached
> DF cannot be broadcasted.
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