Poor performance when xmlquery contains attribute matching XPATH

Question

We send a select xmlquery to Oracle 11g database, if the query contains an XPATH that has an attribute name test in it, the performance is significantly worse than if indexing is used. Here are the two queries:
1)
select xmlquery('declare default element namespace "namespace"; for $e at $pos in $c/services/service let $result := if ($pos >= 2) then "|||||ERROR|||||" else $e return $result' passing document as "c" RETURNING CONTENT).getStringVal(), "LASTUPDATE" from services where id = :1
2)
select xmlquery('declare default element namespace "namespace"; for $e at $pos in $c/services/service[1] let $result := if ($pos >= 2) then "|||||ERROR|||||" else $e return $result' passing document as "c" RETURNING CONTENT).getStringVal(), "LASTUPDATE" from services where id = :1
The only difference is the two XPATHS:
1)$c/services/service
2)$c/services/service[1]
The second query performs 7X to 10X better than the first. More specifically, the Oracle DB CPU will reach 80-100% with only 40 tps of the first query but the second could hit 500 tps.
We would expect the first to perform slightly worse than the second, because Oracle would need to compare the uri values in the doc to the query. But, the documents being searched only contain one service/@uri.
Does anyone know why the performance is so bad with the first query? Is there anything we can do to tune the DB to help in this area. We need to be able to do the first query but the current CPU utilization is extremely too much.

Answer 1

sorry, number 1 should be $services/service(@uri="test"). That should be [ instead of (, the forum wasn't displaying it correctly above.  It should be that way in the query as well.
Edited by: user6668496 on Mar 10, 2009 1:50 PM