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From "Namit Jain (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4139) [Hive] multi group by statement is not optimized
Date Wed, 10 Sep 2008 16:39:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4139?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12629861#action_12629861
] 

Namit Jain commented on HADOOP-4139:
------------------------------------

1. I will change the test.
2. The check is done before the plan is generated.
3. I will write comments/algrithm


> [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
>
>
> 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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