[teiid-issues] [JBoss JIRA] (TEIID-1991) GROUP BY pushdown into subselect of UNIONs
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https://issues.jboss.org/browse/TEIID-1991?page=com.atlassian.jira.plugin...
]
Steven Hawkins commented on TEIID-1991:
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The first observation is that you should be using UNION ALL, not UNION. See also the
partitioned union logic. That alone will get you to the initial optimization of removing
the extra UNION columns:
If given (or the GROUP BY form)
SELECT DISTINCT a FROM (SELECT 'a1' as a, b FROM dm1 UNION ALL SELECT 'a2'
as a, b FROM bm2) AS t
we would produce:
SELECT DISTINCT a FROM (SELECT 'a1' as a FROM dm1 UNION ALL SELECT 'a2' as
a FROM bm2) AS t
We currently see it as unnecessary then to replace one literal value with another
('a1' vs. 1), although I'm guessing that you are looking for 1 in your
translator.
GROUP BY pushdown into subselect of UNIONs
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Key: TEIID-1991
URL: https://issues.jboss.org/browse/TEIID-1991
Project: Teiid
Issue Type: Bug
Components: Query Engine
Affects Versions: 7.7
Reporter: Mark Addleman
Assignee: Steven Hawkins
Attachments: efficient_rewrite.txt, plan.txt, plan.txt, simplified_plan.txt
If have a query of the form
SELECT a FROM (
SELECT a FROM (
SELECT a FROM ds.t1
) u
) t GROUP BY a
Teiid correctly optimizes the inner SELECT to SELECT 1 FROM ds.t1. However, if the inner
subselect is a union:
SELECT a FROM (
SELECT a FROM (
SELECT a FROM ds.t1 UNION
SELECT a FROM ds.t2
) u
) t GROUP BY a
Teiid does not optimize to
SELECT 1 FROM ds.t1 UNION
SELECT 1 FROM ds.t2
as I would expect.
See attached plan
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