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From "Lefty Leverenz (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-3562) Some limit can be pushed down to map stage
Date Thu, 02 Apr 2015 06:51:53 GMT

    [ https://issues.apache.org/jira/browse/HIVE-3562?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14392243#comment-14392243
] 

Lefty Leverenz commented on HIVE-3562:
--------------------------------------

Doc note & question:  This adds *hive.limit.pushdown.memory.usage* to HiveConf.java, so
it has been documented in the wiki with the description "The maximum memory to be used for
hash in RS operator for top K selection. The default value "-1" means no limit."

Does RS mean reduce sink?  (If so, I'll explain that in the description.)

* [Configuration Properties -- hive.limit.pushdown.memory.usage | https://cwiki.apache.org/confluence/display/Hive/Configuration+Properties#ConfigurationProperties-hive.limit.pushdown.memory.usage]

> 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
>             Fix For: 0.12.0
>
>         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, HIVE-3562.D5967.7.patch,
HIVE-3562.D5967.8.patch, HIVE-3562.D5967.9.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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