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From "Yongzhi Chen (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-11502) Map side aggregation is extremely slow
Date Mon, 10 Aug 2015 20:37:45 GMT

    [ https://issues.apache.org/jira/browse/HIVE-11502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14680720#comment-14680720
] 

Yongzhi Chen commented on HIVE-11502:
-------------------------------------

[~gopalv], I checked the related hadoop code between two versions used by 0.13 and 1.2, there
is no change in hadoop side for DoubleWritable. 
I think the regression may relate to HIVE-7041 which switch from using hive's own DoubleWritable
to hadoop's . But just revert the change cause exceptions, I am still looking at it. 

> 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
>
> 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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