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https://issues.apache.org/jira/browse/HADOOP-4139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12630255#action_12630255
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Ashish Thusoo commented on HADOOP-4139:
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+1
looks good to me.
> [Hive] multi group by statement is not optimized
> ------------------------------------------------
>
> Key: HADOOP-4139
> URL: https://issues.apache.org/jira/browse/HADOOP-4139
> Project: Hadoop Core
> Issue Type: Bug
> Components: contrib/hive
> Reporter: Namit Jain
> Assignee: Namit Jain
> Attachments: patch1, patch3
>
>
> A simple multi-group by statement is not optimized. A simple statement like:
> FROM SRC
> INSERT OVERWRITE TABLE DEST1 SELECT SRC.key, count(distinct
> SUBSTR(SRC.value,4)) GROUP BY SRC.key
> INSERT OVERWRITE TABLE DEST2 SELECT SRC.key, count(distinct
> SUBSTR(SRC.value,4)) GROUP BY SRC.key;
> results in making 2 copies of the data (SRC). Instead, the data can be first
> partially aggregated on the distinct value and then aggregated.
> The first step can be common to all group bys.
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