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https://issues.apache.org/jira/browse/HIVE-4137?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13596690#comment-13596690
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Lianhui Wang commented on HIVE-4137:
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in addition. for bucketed/sorted tables, for single group by operator,it only
needs map-group by operator and doesnot have reduce-group by operator.
example:
select key,aggr() from T1 group by key.
now plan is
TS-SEL-GBY-RS-GBY-SEL-FS
but that can chang to following plan
TS-SEL-GBY-SEL-FS
> optimize group by followed by joins for bucketed/sorted tables
> --------------------------------------------------------------
>
> Key: HIVE-4137
> URL: https://issues.apache.org/jira/browse/HIVE-4137
> Project: Hive
> Issue Type: Improvement
> Components: Query Processor
> Reporter: Namit Jain
>
> Consider the following scenario:
> create table T1 (...) clustered by (key) sorted by (key) into 2 buckets;
> create table T2 (...) clustered by (key) sorted by (key) into 2 buckets;
> create table T3 (...) clustered by (key) sorted by (key) into 2 buckets;
> SET hive.enforce.sorting=true;
> SET hive.enforce.bucketing=true;
> insert overwrite table T3
> select ..
> from
> (select key, aggr() from T1 group by key) s1
> full outer join
> (select key, aggr() from T2 group by key) s2
> on s1.key=s2.ley;
> Ideally, this query can be performed in a single map-only job.
> Group By -> SortMerge Join.
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