When XML attacks!

At some point in your xml wrangling career you will hit an node whose data is too big for Oracle's EXTRACTVALUE (I think the upper limit is 4000 characters) and get this lovely message.

ORA-01706: user function result value was too large
01706. 00000 -  "user function result value was too large"

To mitigate this problem, you need to switch to XMLTABLE and it's useful PASSING attribute:

    fix.biggie AS notorious
    source_table_with_xmltype_column o,
    XMLTABLE ( '/*'
        PASSING o.xml_field COLUMNS
            biggie VARCHAR2(4000) PATH 'substring(/path/to/offending/item/text(),1,3999)'
    ) fix;

The gist of the fix is to use xpath to truncate the text value before it's handed off to Oracle. Trying to cast the xmltype column as a varchar2(4000) will not fix it because the problem happens as extractvalue is parsing the node.

Items of note

XML ( '/*' - how to specify the root of an xml document

biggie VARCHAR2(4000) PATH 'substring(/path/to/offending/item/text(),1,3999)'