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https://issues.apache.org/jira/browse/HIVE-3562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13748676#comment-13748676
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Ashutosh Chauhan commented on HIVE-3562:
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[~navis] It occurred to me that this optimization will become very powerful in
combination with HIVE-4002 Imagine a case where there is a limit which can be
pushed up in front of last RS. Than mappers will output very little data and
with HIVE-4002 we can eliminate reducer altogether. Though these two
optimizations cannot occur simultaneously in their current form since RSHash is
implemented inside RS. We need to reimplement RSHash in FS. Alternative
approach could be to implement RSHash as an operator which can than be put in
front of either RS or FS. What do you think?
> 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, HIVE-3562.D5967.2.patch,
> HIVE-3562.D5967.3.patch, HIVE-3562.D5967.4.patch, HIVE-3562.D5967.5.patch,
> HIVE-3562.D5967.6.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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