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https://issues.apache.org/jira/browse/HIVE-20406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16583149#comment-16583149
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Vineet Garg commented on HIVE-20406:
------------------------------------
COALESCE(null) is typed void and during constant folding
JavaVoidObjectInspector.getPrimitiveWritableObject is called which in turn
calls NullWritable.get() to get to the object (null here) value. For some
reason it ends up returning {{(null)}} instead of {{null}. Therefore coalesce
ends up returning this "supposedly" non null value.
> Nested Coalesce giving incorrect results
> ----------------------------------------
>
> Key: HIVE-20406
> URL: https://issues.apache.org/jira/browse/HIVE-20406
> Project: Hive
> Issue Type: Bug
> Components: CBO
> Affects Versions: 3.0.0
> Reporter: Riju Trivedi
> Assignee: Vineet Garg
> Priority: Critical
> Attachments: HIVE-20406.1.patch
>
>
> Below query is returning NULL instead of 'TEST'
> select coalesce(coalesce(null), 'TEST');
> {noformat}
> INFO : OK
> +-------+
> | _c0 |
> +-------+
> | NULL |
> +-------+
> {noformat}
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