[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...
]
Mark Addleman commented on TEIID-1991:
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Actually, it is not contrived. Our use case is a union of named extracts of data (we call
these data marts). This is the moral equivalent of Teiid's multi-source model. The
constant column indicates the data mart name. We create a view for each of these unions.
The goal is to get a distinct list of data mart names by querying the view.
It seems to me that the following query
SELECT t.a FROM (
SELECT 'dm1' AS a, dm1.b FROM dm1
UNION
SELECT 'dm2' AS a, dm2.b FROM ds2
) t GROUP BY a
can be rewritten as
SELECT t.a FROM (
SELECT 'dm1' AS a FROM (SELECT 1 FROM dm1.b) t
UNION
SELECT 'dm2' AS a FROM (SELECT 1 FROM dm2.b) t
) t GROUP BY a
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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