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From "Brock Noland (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-4845) Correctness issue with MapJoins using the null safe operator
Date Tue, 16 Jul 2013 02:16:50 GMT

    [ https://issues.apache.org/jira/browse/HIVE-4845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13709372#comment-13709372
] 

Brock Noland commented on HIVE-4845:
------------------------------------

Hopefully this will be eclipsed by HIVE-4838, which has unit tests, but I always thing unit
tests are a fair request. Will do :)
                
> Correctness issue with MapJoins using the null safe operator
> ------------------------------------------------------------
>
>                 Key: HIVE-4845
>                 URL: https://issues.apache.org/jira/browse/HIVE-4845
>             Project: Hive
>          Issue Type: Bug
>            Reporter: Brock Noland
>            Assignee: Brock Noland
>            Priority: Critical
>         Attachments: HIVE-4845.patch, HIVE-4845.patch
>
>
> I found a correctness issue while working on HIVE-4838. The following query from join_nullsafe.q
gives different results depending on if it's executed map-side or reduce-side:
> {noformat}
> SELECT /*+ MAPJOIN(a) */ * FROM smb_input1 a JOIN smb_input1 b ON a.key <=> b.key
AND a.value <=> b.value ORDER BY a.key, a.value, b.key, b.value;
> {noformat}
> For that query, on the map side, rows which should be joined are not. For example, the
reduce side outputs this row:
> {noformat}
> a.key   a.value   b.key   b.value
> 148     NULL      148     NULL
> {noformat}
> which makes sense since a.key is equal to b.key and a.value is equal to b.value but the
current map-side code omits this row. The reason is that MapJoinDoubleKey is used for the
map-side join which doesn't properly compare null values.

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