Oracle stores XMLType data either in LOB or object-relational columns, based on XMLSchema information and how you specify the storage clause. The XMLDATA pseudocolumn lets you access the underlying LOB or object relational column to specify additional storage clause parameters, constraints, indexes, and so forth.
Example The following statements illustrate the use of this pseudocolumn. Suppose you create a simple table of XMLType with one CLOB column:
CREATE TABLE xml_lob_tab of XMLTYPE XMLTYPE STORE AS CLOB;
To change the storage characteristics of the underlying LOB column, you can use the following statement:
ALTER TABLE xml_lob_tab MODIFY LOB (XMLDATA) (STORAGE (MAXSIZE 2G) CACHE);
Now suppose you have created an XMLSchema-based table like the xwarehouses table created in "Using XML in SQL Statements". You could then use the XMLDATA column to set the properties of the underlying columns, as shown in the following statement:
ALTER TABLE xwarehouses ADD (UNIQUE(XMLDATA."WarehouseId"));