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From "Joydeep Sen Sarma (JIRA)" <>
Subject [jira] Commented: (HIVE-105) estimate number of required reducers and other map-reduce parameters automatically
Date Wed, 21 Jan 2009 01:09:59 GMT


Joydeep Sen Sarma commented on HIVE-105:

> user would need to override this value in the session or hive-default/site.xml. 

ok that makes sense. should we change the hive-default.xml in the standard distribution as
well then to set this to <0.

> I think the issue of what kind of plan we choose is there irrespective of whether we
use query hints or query specific parameters, no?

Agreed - which is why i don't see how we can specify a plan specific parameter (number of
reducers) without control over the kind of plan first - no? but i guess this is a moot point
if we are going to work on query hinting infrastructure first.

for the record - would very much prefer the (query level) configuration way of doing things.
i don't think we should use unnecessary language constructs. people already find the language
hard to get right. configuration variables are easy to understand and use. syntax is not complicated.
the map side aggregation is implemented as a configuration variable and is perfectly easy
to understand and use. if it had to be specified inline in the query - my guess is that many
more syntax errors from first time users would result. 

is there a difference functionality wise between the two approaches?

> estimate number of required reducers and other map-reduce parameters automatically
> ----------------------------------------------------------------------------------
>                 Key: HIVE-105
>                 URL:
>             Project: Hadoop Hive
>          Issue Type: Improvement
>          Components: Query Processor
>            Reporter: Joydeep Sen Sarma
>            Assignee: Zheng Shao
> currently users have to specify number of reducers. In a multi-user environment - we
generally ask users to be prudent in selecting number of reducers (since they are long running
and block other users). Also - large number of reducers produce large number of output files
- which puts pressure on namenode resources.
> there are other map-reduce parameters - for example the min split size and the proposed
use of combinefileinputformat that are also fairly tricky for the user to determine (since
they depend on map side selectivity and cluster size). This will become totally critical when
there is integration with BI tools since there will be no opportunity to optimize job settings
and there will be a wide variety of jobs.
> This jira calls for automating the selection of such parameters - possibly by a best
effort at estimating map side selectivity/output size using sampling and determining such
parameters from there.

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