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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-3912) Common subexpression elimination in code generation
Date Wed, 07 Oct 2015 23:23:26 GMT

    [ https://issues.apache.org/jira/browse/DRILL-3912?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14947797#comment-14947797
] 

ASF GitHub Bot commented on DRILL-3912:
---------------------------------------

Github user jaltekruse commented on the pull request:

    https://github.com/apache/drill/pull/189#issuecomment-146363523
  
    Do you want to go ahead and add these comments as javadocs on the classes? This is the
kind of information that would help a future dev understand and extend these classes in the
future.


> Common subexpression elimination in code generation
> ---------------------------------------------------
>
>                 Key: DRILL-3912
>                 URL: https://issues.apache.org/jira/browse/DRILL-3912
>             Project: Apache Drill
>          Issue Type: Bug
>            Reporter: Steven Phillips
>            Assignee: Jinfeng Ni
>
> Drill currently will evaluate the full expression tree, even if there are redundant subtrees.
Many of these redundant evaluations can be eliminated by reusing the results from previously
evaluated expression trees.
> For example,
> {code}
> select a + 1, (a + 1)* (a - 1) from t
> {code}
> Will compute the entire (a + 1) expression twice. With CSE, it will only be evaluated
once.
> The benefit will be reducing the work done when evaluating expressions, as well as reducing
the amount of code that is generated, which could also lead to better JIT optimization.



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