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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Resolved: (DERBY-3280) Poor distribution of hash values from RecordId.hashCode()
Date Mon, 17 Dec 2007 10:36:43 GMT

     [ https://issues.apache.org/jira/browse/DERBY-3280?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Knut Anders Hatlen resolved DERBY-3280.
---------------------------------------

       Resolution: Fixed
    Fix Version/s: 10.4.0.0
       Derby Info:   (was: [Patch Available])

Committed revision 604818.

> Poor distribution of hash values from RecordId.hashCode()
> ---------------------------------------------------------
>
>                 Key: DERBY-3280
>                 URL: https://issues.apache.org/jira/browse/DERBY-3280
>             Project: Derby
>          Issue Type: Improvement
>          Components: Performance, Services, Store
>    Affects Versions: 10.4.0.0
>            Reporter: Knut Anders Hatlen
>            Assignee: Knut Anders Hatlen
>             Fix For: 10.4.0.0
>
>         Attachments: d3280.diff, TestClient.java
>
>
> The hash values returned by RecordId.hashCode() are constructed by performing bitwise
xor on the 32 least significant bits of the record number, the page number, the container
id and the segment id. Since all of these values tend to be relatively small positive integers,
the hash values also tend to be clustered in a very small range. This leads to a higher frequency
of hash collisions in the lock table, which makes the hash tables work less efficiently and
thereby reduces the performance.
> As an example, the simple join load in the test client attached to DERBY-1961 uses two
tables, TENKTUP and ONEKTUP, with 10000 rows and 1000 rows, respectively. The RecordIds for
these 11000 rows map to less than 900 distinct hash codes.

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