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From "Rick Hillegas (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, 19 May 2009 13:12:45 GMT

    [ https://issues.apache.org/jira/browse/DERBY-681?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12710725#action_12710725
] 

Rick Hillegas commented on DERBY-681:
-------------------------------------

Hi Kathey,

I don't have any smart ideas, short of borrowing a query generator from some other project,
generating a lot of queries (not just ones involving aggregates and GROUP BY), and comparing
the results between Derby and some other database. Eliminating the parser's rewriting of the
AST was a significant change to Derby's SQL interpreter. 8 years of code had piled up on top
of the assumption that the AST was rewritten in the parser. The assumptions weren't linked
to one another in any systematic way and I don't know of any systematic way to track them
all down. So far, we have seen bugs related to GROUP BY, sort order, subqueries, unions, and
column aliasing. The bugs are peppered all across the language.

> 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
>            Assignee: Manish Khettry
>             Fix For: 10.3.1.4
>
>         Attachments: 681.patch.txt, 681.patch2.txt, 681.patch3.txt, followup.patch.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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