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

[Steven Hawkins (JIRA)]

    [ https://issues.jboss.org/browse/TEIID-1991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12681792#comment-12681792 ] 

Steven Hawkins commented on TEIID-1991:
---------------------------------------

Yes, it makes a difference if the column is a projected literal.  That optimization would be done on top of what I was mentioning before - otherwise you'd still be left with extra union columns.  The non-union case is much simpler as we are able to remove the view and propagate the constant. 

What you're suggesting only works since the projected value is the same for all union branches.  This also seems contrived as you are grouping on a constant column, is that intended?
                
> 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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