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