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https://issues.apache.org/jira/browse/HIVE-17043?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16624240#comment-16624240
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Vineet Garg commented on HIVE-17043:
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[~jcamachorodriguez] I introduced new logic to compute unique keys based on
statistics. Now {{RelMdUniqueKeys}} is only used for computing keys based on
constraints.
> Remove non unique columns from group by keys if not referenced later
> --------------------------------------------------------------------
>
> Key: HIVE-17043
> URL: https://issues.apache.org/jira/browse/HIVE-17043
> Project: Hive
> Issue Type: Sub-task
> Components: Logical Optimizer
> Affects Versions: 3.0.0
> Reporter: Ashutosh Chauhan
> Assignee: Vineet Garg
> Priority: Major
> Attachments: HIVE-17043.1.patch, HIVE-17043.2.patch,
> HIVE-17043.3.patch, HIVE-17043.4.patch, HIVE-17043.5.patch
>
>
> Group by keys may be a mix of unique (or primary) keys and regular columns.
> In such cases presence of regular column won't alter cardinality of groups.
> So, if regular columns are not referenced later, they can be dropped from
> group by keys. Depending on operator tree may result in those columns not
> being read at all from disk in best case. In worst case, we will avoid
> shuffling and sorting regular columns from mapper to reducer, which still
> could be substantial CPU and network savings.
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