[jira] [Commented] (TRAFODION-2418) Allow group by push-down to a fact table

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/TRAFODION-2418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15773542#comment-15773542 ] 

ASF GitHub Bot commented on TRAFODION-2418:
-------------------------------------------

GitHub user zellerh opened a pull request:

    https://github.com/apache/incubator-trafodion/pull/893

    [TRAFODION-2418] Allow group by push-down to a fact table

    Trafodion has a rule that pushes a groupby down over a join when possible, but it will not push the groupby to the left child of a join - it relies on join commutativity. This might have been ok 20 years ago when this was coded (by me...), but it isn't good for some situations today. Example:
    
        select d.y, count(f.a), sum(f.b)
        from big_fact f join small_dim d on f.x=d.y
        where d.a = 1
        group by d.y
    
    The plan we would like is a hash join with the group by on big_fact as the left child. To do this, we need to remove the heuristic that prevents this form of push-down.

You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/zellerh/incubator-trafodion bug/2418

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/incubator-trafodion/pull/893.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #893
    
----
commit 7894cc88b4ffe5ecdfdbf353453e0e0315d26b01
Author: Hans Zeller <hz...@apache.org>
Date:   2016-12-23T19:25:47Z

    [TRAFODION-2418] Allow group by push-down to a fact table

----


> Allow group by push-down to a fact table
> ----------------------------------------
>
>                 Key: TRAFODION-2418
>                 URL: https://issues.apache.org/jira/browse/TRAFODION-2418
>             Project: Apache Trafodion
>          Issue Type: Improvement
>          Components: sql-cmp
>            Reporter: Hans Zeller
>            Assignee: Hans Zeller
>             Fix For: 2.1-incubating
>
>
> Trafodion has a rule that pushes a groupby down over a join when possible, but it will not push the groupby to the left child of a join - it relies on join commutativity. This might have been ok 20 years ago when this was coded (by me...), but it isn't good for some situations today. Example:
> {noformat}
> select d.y, count(f.a), sum(f.b)
> from big_fact f join small_dim d on f.x=d.y
> where d.a = 1
> group by d.y
> {noformat}
> The plan we would like is a hash join with the group by on big_fact as the left child. To do this, we need to remove the heuristic that prevents this form of push-down.



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