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From "A B (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-681) Eliminate the parser's rewriting of the abstract syntax tree for queries with GROUP BY and/or HAVING clauses
Date Tue, 13 Mar 2007 15:38:09 GMT

    [ https://issues.apache.org/jira/browse/DERBY-681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12480437
] 

A B commented on DERBY-681:
---------------------------

Actually, it turns out that just switching to "visibleSize()" does *not* solve the problem.
 I made that change to a clean codeline and the error still occurs.  When I printed out the
values of "size()" and "visibleSize()", they *both* showed "2".

I don't know if it's relevant, but if you add a group by clause to the query, it runs fine:

select distinct vc, x from t1 as myt1
      where d <= (select max(myt1.d) from t1 as myt2
          where myt1.vc = myt2.vc and myt1.d <= myt2.d
          group by d
          having count(distinct d) <= 3); 

Manish, would you be willing to look into this a bit more?

> Eliminate the parser's rewriting of the abstract syntax tree for queries with GROUP BY
and/or HAVING clauses
> ------------------------------------------------------------------------------------------------------------
>
>                 Key: DERBY-681
>                 URL: https://issues.apache.org/jira/browse/DERBY-681
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>            Reporter: Rick Hillegas
>         Assigned To: Manish Khettry
>         Attachments: 681.patch.txt, 681.patch2.txt, 681.patch3.txt, notes.txt
>
>
> If a query contains a GROUP BY or HAVING clause, the parser rewrites the abstract syntax
tree, putting aggregates into a subselect and treating the HAVING clause as the WHERE clause
of a fabricated outer select from the subquery. This allows the compiler to re-use some machinery
since the HAVING clause operates on the grouped result the way that the WHERE clause operates
on the from list. Unfortunately, this rewriting creates an explosion of special cases in the
compiler after parsing is done. The rewriting is not systematically handled later on in the
compiler. This gives rise to defects like bug 280. We need to eliminate this special rewriting
and handle the HAVING clause in a straightforward way. This is not a small bugfix but is a
medium sized project.

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