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