XQuery performance in Oracle 10gR2

Question

Hello
I'm actually trying to measure the performances of XQuery FLOWR queries on Oracle 10gR2. For that, I've created a simple table with one integer field (ID) and one XMLType field. This XMLType field contains 10000 documents. The size of these documents varies between 2Kb and 14Kb approximately.
A simple XQuery like below (without "WHERE" or complexe "RETURN" clause) runs quite well and I get the query results in a resonable time.
SELECT xtab.COLUMN_VALUE
FROM contractXDraft, XMLTABLE(
     'declare namespace ctxCD="contractX/contractXDraft";for $x in /ctxCD:contractXDraft
     return
     <response>
          Hello
     </response>'
PASSING OBJECT_VALUE) xtab;
On the other hand, if a add a "WHERE" clause to filter the results, the query execute time become very long (~hours...) and I must always abort the query execution by killing the "oracle" process because the page file memory used by this process increase linearly ! Here below is represented a such query :
SELECT xtab.COLUMN_VALUE
FROM contractXDraft, XMLTABLE(
     'declare namespace ctxCD="contractX/contractXDraft";for $x in /ctxCD:contractXDraft
where $x/ctxCD:ReferenceDetails/ctxCD:ContractHeaderDetails/ctxCD:ContractNumber = "19163-contract657-2.xml"
     return
     <response>
          Hello
     </response>'
PASSING OBJECT_VALUE) xtab;
These above queries are executed on a server dedicated for these tests. This server is a Pentium IV 3.2 GHz with 1GB of RAM and Windows 2003 Server Enterprise SP1 is installed on it.
I want to know if someone can give me an explanation about this huge difference of execution time between the two queries above ?? Is it a syntax mistake ? Is it an hardware problem ?
Thank you very much for your help !!
Regards
MD

Answer 1

MD,
Sounds like your XMLType field is using unstructured storage (i.e., LOB-based) instead of the structured storage (i.e., O-R based). You can check out the XML DB online doc (http://download-west.oracle.com/docs/cd/B19306_01/appdev.102/b14259/xdb01int.htm#BABECDCF) to learn more about the differences between the two. Essentially, without any associated indexes, using XQuery over unstructured storage will result in a full table scan, which can be very slow when there are large number of rows.
Regards,
Geoff