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