XML Query Performance on Nested Tables

Question

I am running an Xpath query on an XMLTYPE column extracting rows from a nested table which is two levels down from the root element:
e.g.
select extractValue(x.xmldata,'/rootElement/VersionNumber') VerNumber
, etc...
, etc.
, extractValue(value(m),'/Member/Name/Ttl') Member_title
, to_char(extractValue(value(e),'/Event/EventDate'),'yyyy-mm-dd') Event_Date
, extractValue(value(e),'/Event/Type') Event_type
from xml_table x,
table(XMLSequence(extract(x.xmldata,'/rootElement/Member'))) m
,table(XMLSequence(extract(value(m),'/Member/Event'))) e
The query is taking around 7 minutes to process 2000 XML docs and the result returns 61,000 rows.
Statspack reports indicate that the main resource used is the LOBSEGMENT which supports the SYS_XDBPD$ column (system generated) in the Member nested table - result was over 100 million Logical Reads and Consistent Gets.
Is there any way to influence the Schema Registration/XML Table creation to result in the XDBPD$ column being stored as a Table or VARRAY - the underlying data type for XDBPD$ is:
XDB.XDB$RAW_LIST_T VARRAY(1000) OF RAW(2000)
OR
Should I spend time tuning access to the LOBSEGMENT?
Anyone tuned this area of XDB before??
Thanks
Dave

Answer 1

Do you need DOM Fidelity at all. DOM Fidelity is typically only needed in a document orientated application or where items like processing instructions and comments are present and need to be maintaned.
Here are some notes from some training ppt slides that may help you decide
Dom Fidelity requires that XML DB track document level meta data
It creates overhead when inserting and retrieving XML due to processing as well as adding additional storage requirements. Dom Fidelity can be turned on and off at the 'Type' Level Each SQL Type corresponding to a complexType where DOM is enabled includes a System Binary attribute called SYS_XDBPD$. The SYS_XDBPD$ attribute is referred to as the Positional Descriptor (PD)
The PD is used to manage the instance level meta data. The format of the PD is not documented The PD is stored as a (in-line) LOB.
The PD is used to track
Comments and Processing Instructions
Location and Prefixes in Namespace declarations
Empty Vs Missing Nodes
Presence of Default Values
Use of Substitutable elements
Ordering of elements in all and choice models
XMLSchemaInstance attributes
xsi:nil, xsi:Type
Mixed content –
Text() nodes that are interspaced with elements
Disabling DOM Fidelity improves performance
Reduces storage requirements
Reduces elapsed time for insert and retrieval operations
DOM Fidelity is disabled on a Type by Type basis
Use annotation xdb:maintainDOM="false“ on complexType definition
Specifying xdb:maintainDOM="false“
Removes the PD attribute from the SQLType.
Causes the information managed in the PD to be discarded when shredding the XML document
In general you can disable DOM Fidelity when
All information is represented as simple content
Elements with text() node children and or attribute values
XML does not make use Mixed Text
No Comments or Processing Instructions
Applications do not need to differentiate between empty and missing elements
Applications do not differentiate between default Vs missing values
Order of elements in repeating Sequence, All and Choice models is not meaningful
Typically Data-Centric XML does not require DOM Fidelity while Document Centric XML does. With DOM Fidelity disabled retrieved documents will still be valid per the XML Schema
Order is preserved when repetition is specified at element or complexType level
Use annotation xdb:maintainOrder=“false” for slight performance improvement in this case that order is not required in these cases.
Order is not preserved when repetition is specified at the ‘model’ level, eg on a Choice, Sequence or All containing multiple child elements.
Take the following document
<?xml version="1.0" encoding="UTF-8"?>
<!--Sample XML file generated by XMLSPY v2004 rel. 4 U (http://www.xmlspy.com)-->
<dft:root xmlns:dft="domFidelityTest"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="domFidelityTest domFidelityExample">
<?SomePI SomeValue?>
     <xxx:choice xmlns:xxx="domFidelityTest">
     <!--SomeComment-->
          <xxx:A>String</xxx:A>
          <xxx:B>String</xxx:B>
          <?Another PI SomeOtherValue?>
          <xxx:C>String</xxx:C>
          <xxx:A>String</xxx:A>
          <!--Another Comment-->
          <xxx:B>String</xxx:B>
          <xxx:C xsi:nil="true"/>
</xxx:choice>
</dft:root>
With DOM Fidelity disabled it would come back as
<?xml version="1.0" encoding="WINDOWS-1252"?>
<!--Sample XML file generated by XMLSPY v2004 rel. 4 U (http://www.xmlspy.com)-->
<dft:root xmlns:dft="domFidelityTest“ testAttr="NotInOriginal">
<dft:choice>
<dft:A>String</dft:A>
<dft:A>String</dft:A>
<dft:B>String</dft:B>
<dft:B>String</dft:B>
<dft:C>String</dft:C>
<dft:C/>
</dft:choice>
</dft:root>
In general you can disable DOM Fidelity and see reduced storage requirements and increased throughout...
Hope this helps...