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From "Ashutosh Chauhan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-3562) Some limit can be pushed down to map stage
Date Fri, 23 Aug 2013 16:18:53 GMT

    [ https://issues.apache.org/jira/browse/HIVE-3562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13748676#comment-13748676
] 

Ashutosh Chauhan commented on HIVE-3562:
----------------------------------------

[~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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