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https://issues.apache.org/jira/browse/HIVE-3562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13500165#comment-13500165
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Sivaramakrishnan Narayanan commented on HIVE-3562:
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Apologies, you can use a heap to maintain a top-k as opposed to an array or a
linked list.
You may also want to consider the case where the top-k do not fit in memory.
One possibility would be to employ this optimization only if K is less than
some threshold.
This approach has the advantage that it is a Hive-only change and does not
depend on a Hadoop change. That is a pretty big plus.
> Some limit can be pushed down to map stage
> ------------------------------------------
>
> Key: HIVE-3562
> URL: https://issues.apache.org/jira/browse/HIVE-3562
> Project: Hive
> Issue Type: Bug
> Reporter: Navis
> Assignee: Navis
> Priority: Trivial
> Attachments: HIVE-3562.D5967.1.patch
>
>
> Queries with limit clause (with reasonable number), for example
> {noformat}
> select * from src order by key limit 10;
> {noformat}
> makes operator tree,
> TS-SEL-RS-EXT-LIMIT-FS
> But LIMIT can be partially calculated in RS, reducing size of shuffling.
> TS-SEL-RS(TOP-N)-EXT-LIMIT-FS
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