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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4241) Improve transition from read-only to writable Clob representation
Date Fri, 22 May 2009 07:34:45 GMT
Improve transition from read-only to writable Clob representation
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                 Key: DERBY-4241
                 URL: https://issues.apache.org/jira/browse/DERBY-4241
             Project: Derby
          Issue Type: Sub-task
          Components: JDBC
    Affects Versions: 10.5.1.1, 10.6.0.0
            Reporter: Kristian Waagan
            Assignee: Kristian Waagan
            Priority: Minor


When a store stream Clob is going to be modified, it will be written out to the temporary
area of Derby and represented as a TemporaryClob.
The transfer of the data is done in a sub-optimal manner for two reasons;
 o for transfer of the complete Clob, the copy method operates on the byte level and we're
not able to save the character length.
 o for transfer of parts of the Clob (i.e. truncation), we have to first decode the UTF-8
encoding to find the byte count and then transfer the same bytes.

I intend to do the following two changes;
 1) Add a getCharLengthIfKnow-method to InternalClob.
 2) Add a UTF-8 aware copy method to LOBStreamControl.

When a complete Clob is to be copied, code like this will be executed;
  cachedCharLength = internalClob.getLengthIfKnown();
  if (cachedCharLength > 0)
      // use existing byte-oriented copy method for best performance (copy until EOF)
  else
      cachedCharLength = control.copyUTF8Data()

When parts of a Clob is to be copied, we always use the UTF-8 aware copy method, but we also
do a cheap range check.
  cachedCharLength = internalClob.getLengthIfKnown();
  if (cachedCharLength > 0 && requestedLength > cachedCharLength)
      throw EOFException();
  if (cachedCharLength == requestedLength)
     // use existing byte-oriented copy method for best performance (copy until EOF)
  else
      cachedCharLength = control.copyUTF8Data(requestedLength);

Adding the UTF-8 aware copy method was started under DERBY-4023, including comments on the
first revision of a patch.

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