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https://issues.apache.org/jira/browse/HIVE-5093?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13740258#comment-13740258
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Gopal V commented on HIVE-5093:
-------------------------------
Quick test-results
{code}
hive> set min.num.spills.for.combine=0;
hive> set hive.exec.combiner=false;
hive> select cd_credit_rating, cd_education_status from customer_demographics
order by cd_credit_rating limit 10;
Job 0: Map: 1 Reduce: 1 Cumulative CPU: 14.92 sec HDFS Read: 18709 HDFS
Write: 130 SUCCESS
Reduce input groups 4
Reduce input records 480200
{code}
With the optimizer on
{code}
hive> set hive.exec.combiner=true;
hive> select cd_credit_rating, cd_education_status from customer_demographics
order by cd_credit_rating limit 10;
Job 0: Map: 1 Reduce: 1 Cumulative CPU: 9.39 sec HDFS Read: 18709 HDFS
Write: 130 SUCCESS
Reduce input groups 1
Reduce input records 10
{code}
That is a massive reduction in reduce records for minimal complexity (the
spills per-combiner is set to zero, to always run it).
On query27, we went from 816036 bytes shuffled to 9006 bytes shuffled over-all.
> Use a combiner for LIMIT with GROUP BY and ORDER BY operators
> -------------------------------------------------------------
>
> Key: HIVE-5093
> URL: https://issues.apache.org/jira/browse/HIVE-5093
> Project: Hive
> Issue Type: Bug
> Components: Query Processor
> Affects Versions: 0.12.0
> Reporter: Gopal V
> Assignee: Gopal V
>
> Operator trees of the following structure can have a memory friendly combiner
> put in place after the sort-phase
> "GBY-LIM" and "OBY-LIM"
> This will cut down on I/O when spilling to disk and particularly during the
> merge phase of the reducer.
> There are two possible combiners - LimitNKeysCombiner and
> LimitNValuesCombiner.
> The first one would be ideal for the GROUP-BY case, while the latter would
> more useful for the ORDER-BY case.
> The combiners are still relevant even if there are 1:1 forward operators on
> the reducer side and for small data items, the MR base layer does not run the
> combiners at all.
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