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