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Date: Tue, 3 Nov 2015 04:24:27 +0000 (UTC)
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Subject: [jira] [Commented] (DRILL-3912) Common subexpression elimination in
 code generation
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    [ https://issues.apache.org/jira/browse/DRILL-3912?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14986654#comment-14986654 ] 

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

Github user StevenMPhillips commented on a diff in the pull request:

    https://github.com/apache/drill/pull/189#discussion_r43715223
  
    --- Diff: exec/java-exec/src/main/java/org/apache/drill/exec/expr/EvaluationVisitor.java ---
    @@ -106,19 +177,30 @@ public HoldingContainer visitFunctionCall(FunctionCall call, ClassGenerator<?> g
         @Override
         public HoldingContainer visitBooleanOperator(BooleanOperator op,
             ClassGenerator<?> generator) throws RuntimeException {
    +      HoldingContainer hc = getPrevious(op, generator.getMappingSet());
    --- End diff --
    
    Okay, I think I can make that change.


> 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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