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https://issues.apache.org/jira/browse/HIVE-5093?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13743866#comment-13743866
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Gopal V commented on HIVE-5093:
-------------------------------
[~thejas] You are right, the systems slows down & adds another 3 seconds when
the combiner is not actually doing anything useful (i.e limit > row count).
But the use-case of LIMIT <human-number> queries used in analytics is common
enough for this to be worthwhile.
If this is saving time, the next step is to use a O(n + k*lg(k)) quick-select
with a limit-k instead of quick-sort as the map sort class & get away from
sorting it as well.
Particularly if k is so much smaller than n, which is an easy decision at the
partition phase in the indexed sorter.
> 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
> Attachments: HIVE-5093-WIP-01.patch
>
>
> 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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