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From "Gopal V (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HIVE-11638) ExprNodeDesc hashMap accidentally degrades into O(N) instead of O(1)
Date Tue, 25 Aug 2015 06:11:45 GMT
Gopal V created HIVE-11638:
------------------------------

             Summary: ExprNodeDesc hashMap accidentally degrades into O(N) instead of O(1)
                 Key: HIVE-11638
                 URL: https://issues.apache.org/jira/browse/HIVE-11638
             Project: Hive
          Issue Type: Bug
          Components: Logical Optimizer
    Affects Versions: 1.2.1, 1.1.1, 1.0.1, 0.14.0, 1.3.0, 2.0.0
            Reporter: Gopal V
            Assignee: Gopal V


Due to the absence of ExprNodeDesc::equals() & the presence of a hashCode(), any code
which computes a Set or HashMap of identical objects ends up with a skewed hashmap which degrades
column lookups from O(1) to O(N) during optimizer passes.

So, several hundred references to a single column desc will be stored in a single hashbucket,
but since they default to Object::equals(), they are not equal to each other.

Finalize the equals() method, so that the approach from HIVE-10215 carries over safely.



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