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https://issues.apache.org/jira/browse/HIVE-942?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13404730#comment-13404730
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Lianhui Wang commented on HIVE-942:
-----------------------------------
i think in HIVE-931 ,the group by keys must be the same with the sort keys.
but in the case that the group by keys contain the sort keys, it may be
complete it to use the hash table on the mapper.
for example：
t is a bucket table, sort by c1,c2.
sql: select t.c1,t.c2,t.c3.sum(t.c4) from t group by t.c1,t.c2,t.c3.
i think generally that only use the hash table on the mapper.so do not do
anything on the reducer.
> use bucketing for group by
> --------------------------
>
> Key: HIVE-942
> URL: https://issues.apache.org/jira/browse/HIVE-942
> Project: Hive
> Issue Type: New Feature
> Components: Query Processor
> Reporter: Namit Jain
>
> Group by on a bucketed column can be completely performed on the mapper if
> the split can be adjusted to span the key boundary.
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