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
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.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
