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https://issues.apache.org/jira/browse/HIVE-11502?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14679402#comment-14679402
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Zheng Shao commented on HIVE-11502:
-----------------------------------
Seems like that the new version of Hive introduced KeyWrapperFactory which
wraps keys for HashMap so that all kinds of objects can be used as HashMap
keys. This should not be necessary if the key objects are already capable of
being HashMap keys (like Java Primitive Objects and Writable Objects) where
hashVode() and equals() are well
> 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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