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From "Ning Zhang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-741) NULL is not handled correctly in join
Date Sun, 15 Aug 2010 21:38:17 GMT

    [ https://issues.apache.org/jira/browse/HIVE-741?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12898756#action_12898756
] 

Ning Zhang commented on HIVE-741:
---------------------------------

@Amareshwar, currently we already distinguish different join types with different functions
(take a look at CommonJoinOperator.joinObjects()). I look forward to seeing your proposal
to avoid grouping null-keyed rows.

@Ted, I agree with Amareshwar and John that we cannot avoid rows (or the value part of the
key-value pairs) with null as a key. However you have a point in that if we know the join
operator does not involve outer join at all (we already have a flag noOuterJoin in JoinDesc),
then we could avoid sending rows will null keys from the mappers to the reducers. This will
save bandwidth as well as processing time. Could you open another JIRA and be able to submit
a patch?


> NULL is not handled correctly in join
> -------------------------------------
>
>                 Key: HIVE-741
>                 URL: https://issues.apache.org/jira/browse/HIVE-741
>             Project: Hadoop Hive
>          Issue Type: Bug
>            Reporter: Ning Zhang
>            Assignee: Amareshwari Sriramadasu
>
> With the following data in table input4_cb:
> Key        Value
> ------       --------
> NULL     325
> 18          NULL
> The following query:
> {code}
> select * from input4_cb a join input4_cb b on a.key = b.value;
> {code}
> returns the following result:
> NULL    325    18   NULL
> The correct result should be empty set.
> When 'null' is replaced by '' it works.

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