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https://issues.apache.org/jira/browse/HIVE-4845?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13709385#comment-13709385
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Brock Noland commented on HIVE-4845:
------------------------------------
Hey, sorry for being unclear. So this patch, HIVE-4845 fixes the immediate
correctness issue as you requested over in
[HIVE-4838|https://issues.apache.org/jira/browse/HIVE-4838?focusedCommentId=13707205&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13707205]
where as HIVE-4838 is the refactor of the MapJoin code to make it more
readable and testable.
> 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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