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From StevenMPhillips <...@git.apache.org>
Subject [GitHub] drill pull request: DRILL-3912: Common subexpression elimination
Date Wed, 07 Oct 2015 21:52:38 GMT
GitHub user StevenMPhillips opened a pull request:


    DRILL-3912: Common subexpression elimination

    In EvaluationVisitor, map each logical expression subtree to the HoldingContainer produced
when the subtree is evaluated, and store it in a HashMap.
    Tow new classes, HashVisitor and EqualityVisitor are used to determine the hashcode and
equality for the map. Two logical expression trees are equal if all child nodes are equal,
and the root nodes are the same type and have the same fields.
    Only expressions that are in scope are available in the map. When entering a new scope,
the current map is copied into a new map. The current map is pushed onto a stack, and the
new map becomes the current map. When that scope is left, the previous map is popped off the
stack and becomes the current scope. Thus previously evaluated expression in the outer scope
are available while in the inner scope, but any new expressions will not be available when
in the outer scope.
    A new scope is entered when evaluating a boolean expression, or when evaluating either
branch of an If Expression.

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

    $ git pull https://github.com/StevenMPhillips/drill drill-3912

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


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

    This closes #189
commit f13b4eab7ea1cb61efcda0e137006bc983c9a339
Author: Steven Phillips <smp@apache.org>
Date:   2015-10-07T10:44:10Z

    DRILL-3912: Common subexpression elimination


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