[jira] Commented: (HIVE-908) optimize limit

["He Yongqiang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-908?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12771624#action_12771624 ] 

He Yongqiang commented on HIVE-908:
-----------------------------------

I have seen this before, so i guess we may already have one ticket. Will try to find out.

> optimize limit
> --------------
>
>                 Key: HIVE-908
>                 URL: https://issues.apache.org/jira/browse/HIVE-908
>             Project: Hadoop Hive
>          Issue Type: Improvement
>          Components: Query Processor
>            Reporter: Namit Jain
>             Fix For: 0.5.0
>
>
> If there is a limit, all the mappers have to finish and create 'limit' number of rows - this can be pretty expensive for a large file.
> The following optimizations can be performed in this area:
> 1. Start fewer mappers if there is a limit - before submitting a job, the compiler knows that there is a limit - so, it might be useful to increase the split size, thereby reducing the number of mappers.
> 2. A counter is maintained for the total outputs rows - the mappers can look at those counters and decide to exit instead of emitting 'limit' number of rows themselves.
> 2. may lead to some bugs because of bugs in counters, but 1. should definitely help

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