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From Jacques Nadeau <jacq...@apache.org>
Subject Re: ProjectMergeRule
Date Tue, 21 Jul 2015 18:18:39 GMT
I'm a little nervous about that for Drill.  Despite the goal to do full
testing to make sure we weren't accidentally using field names anywhere, we
haven't yet gotten very far.  We know we're not supposed to depend on
anything but ordinal but as a name based system, it is likely something
depends on something there.

On Tue, Jul 21, 2015 at 11:04 AM, Julian Hyde <jhyde@apache.org> wrote:

> ProjectMergeRule currently refuses to reduce identity projects if the
> fields have different names.
>
> For instance suppose you have a table Dept (deptno, name) and the algebra
>
>   2: Project($1 as X, $0 as Y)
>     1: Project($1, $0)
>       0: Scan(Dept)
>
> Observe that if you combine projects #1 and #2 you end up with
>
>   3: Project($0 as X, $1 as Y)
>     0: Scan(Dept)
>
> Although the new project (#3) is an identity, it renames the fields.
> ProjectMergeRule will return the new project (#3), but it could return
> Scan(Dept) (#0).
>
> Does anyone think they will break if I make it return #0?
>
> Julian
>

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