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From "Hyukjin Kwon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SPARK-6072) Enable hash joins for null-safe equality predicates
Date Thu, 23 Feb 2017 14:00:48 GMT

    [ https://issues.apache.org/jira/browse/SPARK-6072?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15880456#comment-15880456
] 

Hyukjin Kwon commented on SPARK-6072:
-------------------------------------

Hi [~dimazhiyanov], could you confirm ^ please?

> Enable hash joins for null-safe equality predicates
> ---------------------------------------------------
>
>                 Key: SPARK-6072
>                 URL: https://issues.apache.org/jira/browse/SPARK-6072
>             Project: Spark
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 1.2.1
>            Reporter: Dima Zhiyanov
>
> Currently joins such as 
> A join B on A.x = B.x AND A.y <=> B.y
> are evaluated as hash join on just x followed by filter on y. 
> This causes a skew problem (very long join) when a particular value of x has a high cardinality
even though (x, y) is evenly distributed
> Can we implement is as a hash join on (X, Option(Y))? This will eliminate the skew in
this case
> Imagine a join:
>  People as p1 join People as p2 on p1.name = p2.name and p1.address <=> p2.address
> (very small percentage of people has unknown address)
> This causes a skewed join on popular names such as "Mary Brown" if we hash on names alone,
but will not cause a skew if we hash on (Name, Option(Address))



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