[teiid-issues] [JBoss JIRA] (TEIID-1991) GROUP BY pushdown into subselect of UNIONs

[Mark Addleman (JIRA)]

    [ https://issues.jboss.org/browse/TEIID-1991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12681810#comment-12681810 ] 

Mark Addleman commented on TEIID-1991:
--------------------------------------

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
> ------------------------------------------
>
>                 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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