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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