hive-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Namit Jain (JIRA)" <>
Subject [jira] Commented: (HIVE-908) optimize limit
Date Thu, 29 Oct 2009 21:57:59 GMT


Namit Jain commented on HIVE-908:

In general, if the limit is happening at the reducer, it is not much of a problem, since the
number of reducers are usually not that large. 
There is already a limit operator  - we can work on pushing it up as well, but both these
approaches seem independent.

> optimize limit
> --------------
>                 Key: HIVE-908
>                 URL:
>             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

This message is automatically generated by JIRA.
You can reply to this email to add a comment to the issue online.

View raw message