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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DRILL-3912) Common subexpression elimination in code generation
Date Tue, 03 Nov 2015 02:28:32 GMT

    [ https://issues.apache.org/jira/browse/DRILL-3912?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14986538#comment-14986538
] 

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

Github user jacques-n commented on the pull request:

    https://github.com/apache/drill/pull/189#issuecomment-153219893
  
    This is a net gain. Agree with @jinfengni to track the SR replacement separately. +1


> Common subexpression elimination in code generation
> ---------------------------------------------------
>
>                 Key: DRILL-3912
>                 URL: https://issues.apache.org/jira/browse/DRILL-3912
>             Project: Apache Drill
>          Issue Type: Improvement
>            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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