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From "Lianhui Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-4506) use one map reduce to join multiple small tables
Date Tue, 07 May 2013 02:19:16 GMT

    [ https://issues.apache.org/jira/browse/HIVE-4506?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13650371#comment-13650371
] 

Lianhui Wang commented on HIVE-4506:
------------------------------------

Fern, can you provide your sql?
if these tables used the same column in join clause, it used one mr.
example:
explain
SELECT /*+mapjoin(src2,src3)*/ src1.key, src3.value FROM src src1 JOIN src src2 ON (src1.key
= src2.key) JOIN src src3 ON (src1.key = src3.key);


                
> use one map reduce to join multiple small tables 
> -------------------------------------------------
>
>                 Key: HIVE-4506
>                 URL: https://issues.apache.org/jira/browse/HIVE-4506
>             Project: Hive
>          Issue Type: Wish
>    Affects Versions: 0.10.0
>            Reporter: Fern
>            Priority: Minor
>
> I know we can use map side join for small table.
> by my test, if I use HQL like this
> ------
> select /*+mapjoin(b,c)*/...
> from a
> left join b
> on ...
> left join c
> on ...
> -------
> b and c are both small tables, I expect do the join in one map reduce using map side
join. Actually, it would generate two map-reduce jobs by sequence.
> Sorry, currently I am just a user of hive and not dig into the code, so this is what
I expect but I have no idea about how to improve now. 

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