[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=12681790#comment-12681790 ] 

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

When there is a single SELECT in the inline view
SELECT u.datamart_name 
FROM (
	SELECT 'abc' AS DATAMART_NAME, a.evttypecode 
	FROM SECURITY_DM_DATACOM_DEV_R140_XE59_CMGRD1.ADMACCOUNT AS a 
) AS u
GROUP BY u.datamart_name

It appears that Teiid understands it only needs to know if there is *any* data coming from the ADMACCOUNT table, so it issues SELECT 1 FROM SECURITY_DM_DATACOM_DEV_R140_XE59_CMGRD1.ADMACCOUNT instead.

I would expect Teiid to apply the same optimization if the inline view contained a UNION.  
                
> 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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