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