Using a Value index on XMLIndex path table.

Question

I am trying to understand how to capitalize on the VALUE index created on a XMLIndex path table. So far, I have not been able to successfully use it. I am not sure whether I am misunderstanding the way it is intended to be used? Or am making a mistake somewhere else along the way. Here is a use-case where I think that it should be getting picked up by the optimizer:
CREATE TABLE TEST_TABLE
    (ID VARCHAR2(128) NOT NULL,
    XML_DATA XMLTYPE NOT NULL,
    CONSTRAINT TEST_PK PRIMARY KEY (ID))
XMLTYPE COLUMN XML_DATA STORE AS BINARY XML;
-- Populate the table with some random data.  Here, our single attribute should be
-- unique for each row.
INSERT INTO TEST_TABLE (
  SELECT dbms_random.string('A', 15), XMLElement("Test", XMLAttributes(dbms_random.string('A', 15) as "someAttr"))
  FROM DUAL
  CONNECT BY LEVEL <= 20000 );
CREATE INDEX test_xmlindex_ix ON TEST_TABLE(XML_DATA) INDEXTYPE IS XDB.XMLIndex
  PARAMETERS('
    PATHS (Include (/Test/@someAttr))
    PATH TABLE      test_path_table
    PATH ID INDEX   test_path_id_ix
    ORDER KEY INDEX test_key_ix
    VALUE INDEX     test_value_ix ');
-- Not sure if I need these or not?
CALL DBMS_STATS.GATHER_TABLE_STATS ('MYSCHEMA', 'TEST_TABLE');
CALL DBMS_STATS.GATHER_TABLE_STATS ('MYSCHEMA, 'TEST_PATH_TABLE');
-- Now, I would expect this query to use the Value index that was created.
-- Instead, the Explain plan indicates that TEST_PATH_ID_IX was used..
SELECT xml_data
FROM   test_table v
WHERE  extractValue(v.xml_data, '/Test/@someAttr') = ?
Here are the operations listed in the explain plan for the query:
SELECT STATEMENT
FILTER
TABLE ACCESS(FULL) MYSCHEMA.TEST_TABLE
TABLE ACCESS(BY INDEX ROWID) MYSCHEMA.TEST_PATH_TABLE
INDEX(RANGE SCAN) MYSCHEMA.TEST_PATH_ID_IXIf I understand everything correctly, here, there is no reason why the PATH ID index should have been used. Isn't this index strictly for the XML tags themselves? Why wouldn't it use the Value index?
Thanks,
Jon

Answer 1

PATH_TABLE
+=======================================================+
| PathID |  RID  | Order_Key | Locator |     Value      |
|-------------------------------------------------------|
|  RAW   | rowid |    RAW    |   RAW   | varchar2(4000) |
|-------------------------------------------------------|
|        |       |           |         |                |
|        |       |           |         |                |
|        |       |           |         |                |
|        |       |           |         |                |
|        |       |           |         |                |
|        |       |           |         |                |
|        |       |           |         |                |
+=======================================================+
XMLIndex
1) PATH  INDEX: (PathID, RID)              - BTree
2) ORDER INDEX: (RID, ORDER_KEY)           - BTree
3) VALUE INDEX: (substr("VALUE",1,1599))   - Function Based
Locator: Pointer to XML Fragments
XMLIndex - Namespace issue...
I believe that (although I don't know the exact mechanics) that it will use the path index first to find the needed
tree node / path, then will look for a locator that matches the request and if not will do an full table scan
afterwards, if not successful, trying to limit the request with the filter at hand, so see if it can be used, to
minimize the sub-results.
Message was edited by:
Marco Gralike