1. Derby
  2. DERBY-4241

Improve transition from read-only to writable Clob representation


    • Type: Improvement Improvement
    • Status: Closed
    • Priority: Minor Minor
    • Resolution: Fixed
    • Affects Version/s:,
    • Fix Version/s:
    • Component/s: JDBC
    • Labels:
    • Bug behavior facts:


      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)
      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)
      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.

      1. better.txt
        4 kB
        Kristian Waagan
      2. derby-4241-1a-InternalClob.getLengthIfKnown.diff
        3 kB
        Kristian Waagan
      3. derby-4241-2a-utf8AwareCopy.diff
        11 kB
        Kristian Waagan
      4. derby-4241-2b-utf8AwareCopy.diff
        12 kB
        Kristian Waagan
      5. derby-4241-32core-cmt.txt
        5 kB
        Kristian Waagan

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            • Assignee:
              Kristian Waagan
              Kristian Waagan
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