XMLPARSE Oddities

Question

Server version: *10.2.0.1.0*
I am seeing a strange error when trying to encode CLOB output to xml using XMLPARSE... The issue is that rows that have a NULL in the CLOB column end up pulling in data from the last row that wasn't NULL.
-- Create a tmp table
CREATE GLOBAL TEMPORARY TABLE gtt_Test (
ID NUMBER NOT NULL,
Val CLOB NULL
) ON COMMIT PRESERVE ROWS;
-- Insert a few rows
INSERT INTO gtt_Test VALUES (1, TO_CLOB('Row 1 has a value'));
INSERT INTO gtt_Test VALUES (2, NULL);
INSERT INTO gtt_Test VALUES (3, NULL);
INSERT INTO gtt_Test VALUES (4, TO_CLOB('Row 4 has a value'));
-- Build XML from the data
SELECT
-- We can use XMLCDATA here, because the type can be converted to a string type
XMLELEMENT("Cell", XMLELEMENT("Data", XMLATTRIBUTES('String' AS "ss:Type"), XMLCDATA(ID))),
-- We can't use XMLCDATA here, because it will either error out or truncate (if we cast)
-- So, we generate our own CDATA element and stuff our CLOB in it
XMLELEMENT("Cell",
XMLELEMENT("Data", XMLATTRIBUTES('String' AS "ss:Type"), XMLCONCAT(
XMLPARSE(CONTENT '<![CDATA[' WELLFORMED),
                      XMLPARSE(CONTENT Val WELLFORMED),
                      XMLPARSE(CONTENT ']]>' WELLFORMED)
FROM gtt_Test;
Note in the output from the above SELECT query that the 'Row 1 has a value' text ends up in rows 1, 2, and 3. Now, compare the output of the above to something like:
SELECT * FROM gtt_Test;
Notice that this time, rows 2 and 3 properly ended up with NULLs in the Val column. Also, I can reduce the error producing query down to:
SELECT ID, XMLPARSE(CONTENT Val)
FROM gtt_Test;
My first query shows the issue in the context that I am actually using XMLPARSE.
Another interesting example is to compare the following two queries:
SELECT ID,
XMLPARSE(CONTENT
CASE
WHEN Val IS NOT NULL THEN Val
ELSE TO_CLOB('')
END
WELLFORMED)
FROM gtt_Test;
SELECT ID,
XMLPARSE(CONTENT
CASE
WHEN Val IS NOT NULL THEN Val
ELSE TO_CLOB('empty')
END
WELLFORMED)
FROM gtt_Test;
Notice that even with the CASE statement, XMLPARSE in the first example insists on using the last non-NULL value... while in the 2nd example we end up with "empty" for rows 2 and 3.
Any ideas?

Answer 1

Ok, I worked around the issue by moving the XMLPARSE inside a case statement, and using XMLCDATA('') in the case where Val is null.
I am still interested in knowing why XMLPARSE is reusing values from prior rows - but, for now I have a workaround.
SELECT
-- We can use XMLCDATA here, because the type can be converted to a string type
XMLELEMENT("Cell", XMLELEMENT("Data", XMLATTRIBUTES('String' AS "ss:Type"), XMLCDATA(ID))),
-- We can't use XMLCDATA here, because it will either error out or truncate (if we cast)
-- So, we generate our own CDATA element and stuff our CLOB in it
XMLELEMENT("Cell",
XMLELEMENT("Data", XMLATTRIBUTES('String' AS "ss:Type"),
CASE
WHEN Val IS NOT NULL THEN
XMLCONCAT(
XMLPARSE(CONTENT '<![CDATA[' WELLFORMED),
                             XMLPARSE(CONTENT Val WELLFORMED),
                             XMLPARSE(CONTENT ']]>' WELLFORMED)
ELSE XMLCDATA('')
END
FROM gtt_Test;
-----