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https://issues.apache.org/jira/browse/HIVE-591?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12752808#action_12752808
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Emil Ibrishimov commented on HIVE-591:
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To support the Hive1 syntax, we will add a PRESERVE keyword to specify which
tables to preserve.
For example, to preserve tables A and C:
FROM UNIQUEJOIN A(a.c1,a.c2) PRESERVE, B(b.c1,b.c2), C(c.c1,c.c2) PRESERVE...
The sources of UNIQUEJOINs can only be tables (no subquery support for now)
> create new type of join ( 1 row for a given key from multiple tables)
> ---------------------------------------------------------------------
>
> Key: HIVE-591
> URL: https://issues.apache.org/jira/browse/HIVE-591
> Project: Hadoop Hive
> Issue Type: New Feature
> Components: Query Processor
> Reporter: Namit Jain
> Assignee: Emil Ibrishimov
> Attachments: HIVE-591.1.patch
>
>
> It will be useful to support a new type of join:
> say:
> select .. from JOIN TABLES (A,B,C) WITH KEYS (A.key, B.key, C.key) where ....
> The semantics are that for a given key only 1 row is created - nulls are
> present for the the tables which do not contain a row for that key.
> There is no limit on the number of tables, the number of keys should be the
> same as the number of tables.
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