ORA-31088: object XYZ depends on the schema

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This error can be seen when you are trying to delete an XML schema from the repository using the DELETE_RESTRICT or DELETE_CASCADE option in DBMS_XMLSCHEMA.DELETE_SCHEMA , but there are still XMLType tables or XMLType columns referencing it.

A slightly subtler situation involves the recycle bin of Oracle 10G and above.
Supposing the recycle bin is enabled, if you drop a table referencing an XML schema but you don't specify PURGE and you don't flush the recycle bin, then you'll get ORA-31088, which may sound difficult to understand at first.

You can reproduce the problem by executing the scripts i used in a previous posting, then performing the following additional steps:

DROP TABLE purchaseorder_as_column;

BEGIN
DBMS_XMLSCHEMA.deleteSchema(
SCHEMAURL => 'http://xmlns.oracle.com/xdb/documentation/purchaseOrder.xsd',
delete_option => DBMS_XMLSCHEMA.DELETE_CASCADE);
END;

ORA-31088: object "TEST"."PURCHASEORDER_AS_COLUMN" depends on the schema
ORA-06512: at "XDB.DBMS_XMLSCHEMA_INT", line 82
ORA-06512: at "XDB.DBMS_XMLSCHEMA", line 102

So, the cleanest workaround is to ensure that no depending objects are still present, including those that may have been moved to the recycle bin and then re-execute deleteSchema.
You may execute the following query, for instance:

select *
 from user_dependencies d, user_xml_schemas x
where d.referenced_type = 'XML SCHEMA'
and d.referenced_name = x.int_objname
and x.schema_url = 'http://xmlns.oracle.com/xdb/documentation/purchaseOrder.xsd';

Alternatively you can try to specify either of the two remaining options for the deleteSchema call: DELETE_INVALIDATE or DELETE_CASCADE_FORCE.

See message translations for ORA-31088 and search additional resources.