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From "Aman Sinha (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-4119) Skew in hash distribution for varchar (and possibly other) types of data
Date Mon, 30 Nov 2015 16:48:10 GMT

    [ https://issues.apache.org/jira/browse/DRILL-4119?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15032062#comment-15032062
] 

Aman Sinha commented on DRILL-4119:
-----------------------------------

[~mehant] would it make sense to open a separate JIRA for the underlying XXHash.hash64 implementation
?  I feel that for hash32, we would still want to avoid down casting and instead use the mixing
as proposed in this JIRA.  If you agree, I can merge in my patch. 

> Skew in hash distribution for varchar (and possibly other) types of data
> ------------------------------------------------------------------------
>
>                 Key: DRILL-4119
>                 URL: https://issues.apache.org/jira/browse/DRILL-4119
>             Project: Apache Drill
>          Issue Type: Bug
>          Components: Functions - Drill
>    Affects Versions: 1.3.0
>            Reporter: Aman Sinha
>            Assignee: Aman Sinha
>             Fix For: 1.4.0
>
>
> We are seeing substantial skew for an Id column that contains varchar data of length
32.   It is easily reproducible by a group-by query: 
> {noformat}
> Explain plan for SELECT SomeId From table GROUP BY SomeId;
> ...
> 01-02          HashAgg(group=[{0}])
> 01-03            Project(SomeId=[$0])
> 01-04              HashToRandomExchange(dist0=[[$0]])
> 02-01                UnorderedMuxExchange
> 03-01                  Project(SomeId=[$0], E_X_P_R_H_A_S_H_F_I_E_L_D=[castInt(hash64AsDouble($0))])
> 03-02                    HashAgg(group=[{0}])
> 03-03                      Project(SomeId=[$0])
> {noformat}
> The string id happens to be of the following type: 
> {noformat}
> e4b4388e8865819126cb0e4dcaa7261d
> {noformat}



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