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https://issues.apache.org/jira/browse/HIVE-11502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14681015#comment-14681015
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Yongzhi Chen commented on HIVE-11502:
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[~gopalv], thanks for the workaround. But I am afraid some users do not want to
change their input format. And this HashMap may affect mapjoin too. We help a
user workaround this map side aggregation issue by set hive.map.aggr = false;
After that, the simple group test case has very good performance, but a more
complicated join query with group by as subquery stuck on mapjoin. So we have
to let the user turn off mapjoin by set hive.auto.convert.join=false; The
performance hit by this bug is really outstanding. Without workaround, none of
the query can finish in several hours. So I think we have to fix 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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