Doing a pl/sql GROUP BY on an XML field gives "ORA-22806: not an object or REF"

Question

Here's the sample SQL:

SELECT xml_data.field
FROM
  (SELECT sys.XMLType(source_table.data).extract('//source_node/text()') AS field
  FROM source_table
  )xml_data
GROUP BY to_clob(xml_data.field)

The error only appears when I use GROUP BY to do a count. When it is removed I get the error

ORA-22806: not an object or REF
22806. 00000 - "not an object or REF"
*Cause: An attempt was made to extract an attribute from an
        item that is neither an object nor a REF.
*Action: Use an object type or REF type item and retry the operation.
Vendor code 22804Error at Line:1

Is GROUP BY somehow mutating the type of the XML field returned in the sub query? Or (more than likely) am I just missing something?

It is worth noting that the source_table.data column is a CLOB.

Answer 1

SELECT  field
FROM    (
        SELECT  SYS.XMLType.getstringval(SYS.XMLType('<source_node>test</source_node>').extract('//source_node/text()')) AS field
        FROM    dual
        ) xml_data
GROUP BY
        field

Answer 2

the following worked for me in 9.2

SELECT  SYS.XMLType.getstringval(SYS.XMLType('<source_node>test</source_node>').extract('//source_node/text()')) AS field
FROM    dual      
group by SYS.XMLType.getstringval(SYS.XMLType('<source_node>test</source_node>').extract('//source_node/text()'))
 ;

Answer 3

I see the same error:

SQL> select * from v$version;

BANNER
-------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.1.0 - Production
PL/SQL Release 9.2.0.1.0 - Production
CORE    9.2.0.1.0       Production
TNS for 32-bit Windows: Version 9.2.0.1.0 - Production
NLSRTL Version 9.2.0.1.0 - Production

I did some rearranging of the function calls in the select statement, this causes the error to disappear.

create table source_table (data clob);
insert into source_table values ('<source_node>test</source_node>');
commit;

select xmltype.getstringval((field).extract('//source_node/text()'))
from
(
  select  xmltype(data) field
  from    source_table
)
group by xmltype.getstringval((field).extract('//source_node/text()'))
/

You have to use ((field).extract... instead of (field.extract...