Improve distribution of hash codes in SQLBinary and SQLChar
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Key: DERBY-3981
URL: https://issues.apache.org/jira/browse/DERBY-3981
Project: Derby
Issue Type: Improvement
Components: Newcomer, Performance, SQL
Affects Versions: 10.4.2.0
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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