Can create view, but cannot create table / materialized view: Xpath is null

Question

Hi all,
We recently moved some XML documents into the database in an XMLType column and want to query the data. I've been writing some queries and turning them into materialized views. I got to one query, and something really strange is happening. My query returns the expected results, but I am getting an error when I try to create a materialized view out of it. Even stranger, I can create a view out of it, and I can manually insert its data into an existing table, but I cannot create a materialized view out of it and I cannot create a table out of it. Here is a brief summary, please let me know if anyone has suggestions.
Issuing the following commands fails in SQLDeveloper
-- Creating a materialized view out of the query fails:
create materialized view element REFRESH COMPLETE ON DEMAND as [query];
Error at Command Line:1 Column:1 SQL Error: ORA-31063: XPath compilation failed: Xpath is null.
-- Creating a table out of the query with the following shortcut fails:
create table element as [query];
Error at Command Line:1 Column:1 SQL Error: ORA-31063: XPath compilation failed: Xpath is null.
Issuing the following commands in SQLDeveloper works fine:
-- Creating a view out of the query works:
create or replace view element as [query];
-- Creating a blank table from the query and then inserting data works:
create table element as select * from [query] where 1 = 2;
insert into element select * from [query];
Here is a simplified version of the query...
I have changed the names around, and cut the query down so maybe it will be a little easier to understand. I did confirm that this query is also having the same symptoms described above. Since I changed the names, executing the query returns no results. However creating a materialized view out of the query still fails with the 'Xpath is null' error.
create materialized view element REFRESH COMPLETE ON DEMAND as
select
m.resource_id,
xml.*
from metadata_sources m,
xmltable(
'for $i in /metadata/app//*[(self::elem1 or self::elem2) and (parent::form or parent::subform)]
let $formName := if($i/parent::subform) then $i/../../@name else $i/../@name
let $subformName := if($i/parent::subform) then $i/../@name else ""
return <data
appId="{$i/ancestor::app[1]/idField}"
formName="{$formName}"
subformName="{$subformName}"
elemName="{$i/@name}"></data>' passing m.xml_content
columns
app_id NUMBER path '@appId',
form_name VARCHAR2(50 char) path '@formName',
subform_name VARCHAR2(50 char) path '@subformName',
elem_name VARCHAR2(50 char) path '@elemName'
) xml;
Edited by: user11949534 on Feb 22, 2013 1:55 PM

Answer 1

As far as creating a structured index, I was under the impression that I would need to then register an XSD. No, you can use it without an XML schema.
How about a regular relational view, with an underlying xml index?
That way you also eliminate the need for an explicit refresh step as you would then be querying real-time data, as if it were relational data.
For example :
Connected to:
Oracle Database 11g Express Edition Release 11.2.0.2.0 - Production
SQL> create table tmp_xml of xmltype ;
Table created.
SQL> insert into tmp_xml values (
  2    xmlparse(document '<root><item id="1">ABC</item><item id="2">DEF</item></root>')
  3  ) ;
1 row created.
SQL> insert into tmp_xml values (
  2    xmlparse(document '<root><item id="3">GHI</item><item id="4">JKL</item></root>')
  3  ) ;
1 row created.
SQL> create or replace view tmp_xml_v as
  2  select x.item_id, x.item_val
  3  from tmp_xml
  4     , xmltable(
  5         '/root/item' passing object_value
  6         columns item_id  number      path '@id'
  7               , item_val varchar2(3) path '.'
  8       ) x
  9  ;
View created.
SQL> create index tmp_xml_sxi on tmp_xml (object_value)
  2  indextype is xdb.xmlindex
  3  parameters (q'#
  4  XMLTABLE tmp_xml_xtb '/root/item'
  5  COLUMNS item_id  number      path '@id'
  6        , item_val varchar2(3) path '.' #'
  7  ) ;
Index created.
SQL> exec dbms_stats.gather_table_stats(user, 'TMP_XML');
PL/SQL procedure successfully completed.
SQL> set autotrace on explain
SQL> set lines 200
SQL> select * from tmp_xml_v ;
   ITEM_ID ITE
         3 GHI
         4 JKL
         1 ABC
         2 DEF
Execution Plan
Plan hash value: 4168126828
| Id  | Operation                    | Name                   | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT             |                        |     4 |   164 |     3   (0)| 00:00:01 |
|   1 |  NESTED LOOPS                |                        |       |       |            |          |
|   2 |   NESTED LOOPS               |                        |     4 |   164 |     3   (0)| 00:00:01 |
|   3 |    INDEX FULL SCAN           | SYS_C009273            |     2 |    34 |     1   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | SYS30366_30367_OID_IDX |     2 |       |     0   (0)| 00:00:01 |
|   5 |   TABLE ACCESS BY INDEX ROWID| TMP_XML_XTB            |     2 |    48 |     1   (0)| 00:00:01 |
Predicate Information (identified by operation id):
   4 - access("TMP_XML"."SYS_NC_OID$"="SYS_SXI_0"."OID")