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From "Steven Phillips (JIRA)" <j...@apache.org>
Subject [jira] [Created] (DRILL-3912) Common subexpression elimination
Date Wed, 07 Oct 2015 21:42:27 GMT
Steven Phillips created DRILL-3912:
--------------------------------------

             Summary: Common subexpression elimination
                 Key: DRILL-3912
                 URL: https://issues.apache.org/jira/browse/DRILL-3912
             Project: Apache Drill
          Issue Type: Bug
            Reporter: Steven Phillips


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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