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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-3981) Improve distribution of hash codes in SQLBinary and SQLChar
Date Tue, 09 Dec 2008 11:54:44 GMT
Improve distribution of hash codes in SQLBinary and SQLChar

                 Key: DERBY-3981
                 URL: https://issues.apache.org/jira/browse/DERBY-3981
             Project: Derby
          Issue Type: Improvement
          Components: Newcomer, Performance, SQL
    Affects Versions:
            Reporter: Knut Anders Hatlen
            Priority: Minor

SQLBinary.hashCode() and SQLChar.hashCode() use a very simple algorithm that just takes the
sum of the values in the array. This gives a poor distribution of hash values because similar
values will have a higher probability of mapping to the same hash code, and the higher bits
won't be used unless the array is very long. We should change these methods so that they use
an algorithm similar to the one used in java.lang.String.hashCode(), described here: <URL:http://java.sun.com/javase/6/docs/api/java/lang/String.html#hashCode()>.
This may have a positive effect on the performance of hash scans as it will reduce the likelihood
of collisions in the hash table.

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