[jira] [Commented] (HIVE-6520) Skew Join optimization doesn't work if parent gets converted to MapJoin task

["Navis (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-6520?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13920505#comment-13920505 ] 

Navis commented on HIVE-6520:
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MapJoinOperator cannot handle skew join, which should know the total number of a join key. We can disable converting to MapJoin when it's for skewjoin. But If it can be converted MapJoin, it would be faster than doing it in classical skew join.

> Skew Join optimization doesn't work if parent gets converted to MapJoin task
> ----------------------------------------------------------------------------
>
>                 Key: HIVE-6520
>                 URL: https://issues.apache.org/jira/browse/HIVE-6520
>             Project: Hive
>          Issue Type: Bug
>    Affects Versions: 0.11.0
>            Reporter: Ankit Kamboj
>
> Skew join optimization (GenMRSkewJoinProcessor.java) assumes that its parent stage(that will create directory structure for skewed keys) will have a Reduce Join Operator. GenMRSkewJoinProcessor sets the "handleSkewJoin" flag only in that case.
> But it is possible that parent stage gets converted to MapJoin task (because of hive.auto.convert.join flag). In that case "handleSkewJoin" is not set for parent stage and it will not create directory structure for skewed keys in hdfs. This eventually leads to elimination of skew join conditional task (and its children) because the conditional task is not able to find the skewed key directories.
> Shouldn't the MapJoinOperator also handle skew join and create directory structure for skewed keys in addition to performing map join for the non-skewed keys?



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