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From "alex gemini (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-3086) Skewed Join Optimization
Date Wed, 27 Jun 2012 03:56:43 GMT

    [ https://issues.apache.org/jira/browse/HIVE-3086?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13401945#comment-13401945
] 

alex gemini commented on HIVE-3086:
-----------------------------------

maybe what we want is dynamic change our partition key base on our hints,for examples:
select /**+ partitions logs(userid,timestamps),users(id) */ count(userid),to_date(timestamps,'YYYYMMDD'),age
from logs where timestamps > 2011-12-01 and timestamps < 2011-12-31 and age<25 and
age>18.
this time we will partition logs by userid and timestamps . so for records in 2011-12-24 it
will hash to six reduce instead of one, each reduce will process same amout of records.
another query example:
select /**+ partitions logs(userid),users(id,age) */ count(userid),to_date(timestamps,'YYYYMMDD'),age
from logs where timestamps > 2011-01-01 and timestamps < 2011-12-31 and age<25 and
age>18.
this time timestamp is not primary skewed key, we change our parititon key to age.
In the "ListBucketing" desing, 
create table T (c1 string, c2 string, c3 string) skewed by (c1, c2) on (('x1', 'x2'), ('y1',
'y2'));
we need assume we know tables skewed by some column,but data is always skewed and we can't
list every skewed value combination.

                
> Skewed Join Optimization
> ------------------------
>
>                 Key: HIVE-3086
>                 URL: https://issues.apache.org/jira/browse/HIVE-3086
>             Project: Hive
>          Issue Type: New Feature
>            Reporter: Nadeem Moidu
>            Assignee: Nadeem Moidu
>
> During a join operation, if one of the columns has a skewed key, it can cause that particular
reducer to become the bottleneck. The following feature will address it:
> https://cwiki.apache.org/confluence/display/Hive/Skewed+Join+Optimization

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