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From "Xuefu Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-11502) Map side aggregation is extremely slow
Date Wed, 19 Aug 2015 14:34:45 GMT

    [ https://issues.apache.org/jira/browse/HIVE-11502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14703102#comment-14703102
] 

Xuefu Zhang commented on HIVE-11502:
------------------------------------

For my understanding, it's interesting to know why changing in ListKeyWrapper's hashcode solves
the problem. I originally thought the problem is with hashcode for DoubleWritable. Any explanation
would be appreciated.

> Map side aggregation is extremely slow
> --------------------------------------
>
>                 Key: HIVE-11502
>                 URL: https://issues.apache.org/jira/browse/HIVE-11502
>             Project: Hive
>          Issue Type: Bug
>          Components: Logical Optimizer, Physical Optimizer
>    Affects Versions: 1.2.0
>            Reporter: Yongzhi Chen
>            Assignee: Yongzhi Chen
>         Attachments: HIVE-11502.1.patch, HIVE-11502.2.patch, HIVE-11502.3.patch
>
>
> For the query as following:
> {noformat}
> create table tbl2 as 
> select col1, max(col2) as col2 
> from tbl1 group by col1;
> {noformat}
> If the column for group by has many different values (for example 400000) and it is in
type double, the map side aggregation is very slow. I ran the query which took more than 3
hours , after 3 hours, I have to kill the query.
> The same query can finish in 7 seconds, if I turn off map side aggregation by:
> {noformat}
> set hive.map.aggr = false;
> {noformat}



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