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https://issues.apache.org/jira/browse/HIVE-11502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14693556#comment-14693556
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Yongzhi Chen commented on HIVE-11502:
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The basic idea of the fix is in LazyDouble, I separate hashcode for internal
use from hashcode for hdfs.
The aggregation hashmap use LazyDouble, and I think hadoop use the value by
DoubleWritable object which is an instance variable in
LazyDouble (data). The change let LazyDouble calculate its own hashcode instead
of blindly use data's.
I do not see risk here. You can assume the LazyDouble as a vectorized object
which only has one element in it.
Please correct me, if you find anything is not right.
Thanks
> 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
>
>
> 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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