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From "Namit Jain (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-870) semi joins
Date Mon, 09 Nov 2009 05:24:32 GMT

    [ https://issues.apache.org/jira/browse/HIVE-870?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12774862#action_12774862

Namit Jain commented on HIVE-870:

Ignore my earlier comment, i didnt see it is a outer join, and not a semi join

> semi joins
> ----------
>                 Key: HIVE-870
>                 URL: https://issues.apache.org/jira/browse/HIVE-870
>             Project: Hadoop Hive
>          Issue Type: New Feature
>            Reporter: Ning Zhang
>            Assignee: Ning Zhang
>         Attachments: Hive-870.patch, Hive-870_2.patch
> Semi-join is an efficient way to unnest an IN/EXISTS subquery. For example,
> select * 
> from A
> where A.id IN 
>    (select id
>     from B
>     where B.date> '2009-10-01');
> returns from A whose ID is in the set of IDs found in B, whose date is greater than a
certain date. This query can be unnested using a INNER join or LEFT OUTER JOIN, but we need
to deduplicate the IDs returned by the subquery on table B. The semantics of LEFT SEMI JOIN
is that as long as there is ANY row in the right-hand table that matches the join key, the
left-hand table row will be emitted as a result w/o necessarily looking further in the right-hand
table for further matches. This is exactly the semantics of the IN subquery. 

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