How to tune the performance of Oracle SQL/XML query?

Question

Hi all,
I am running Oracle 9i and like to run the following Oracle SQL/XML query. It takes about 3+ hour and still not finish. If I get rid all the XML stuffs it only take minutes to run. Does anybody know how to what's the reason of this slow and how to tune it?
SELECT XMLElement("CUSTOMER",
XMLForest(C_CUSTKEY "C_CUSTKEY", C_NAME "C_NAME", C_ADDRESS "C_ADDRESS", C_PHONE "C_PHONE", C_MKTSEGMENT "C_MKTSEGMENT", C_COMMENT "C_COMMENT"),
(SELECT XMLAgg(XMLElement("ORDERS",
XMLForest(O_ORDERKEY "O_ORDERKEY", O_CUSTKEY "O_CUSTKEY", O_ORDERSTATUS "O_ORDERSTATUS", O_ORDERPRIORITY "O_ORDERPRIORITY", O_CLERK "O_CLERK", O_COMMENT "O_COMMENT"),
(SELECT XMLAgg(XMLElement("LINEITEM",
XMLForest(L_ORDERKEY "L_ORDERKEY", L_RETURNFLAG "L_RETURNFLAG", L_LINESTATUS "L_LINESTATUS", L_SHIPINSTRUCT "L_SHIPINSTRUCT", L_SHIPMODE "L_SHIPMODE", L_COMMENT "L_COMMENT")
FROM LINEITEM
WHERE LINEITEM.L_ORDERKEY = ORDERS.O_ORDERKEY)
FROM ORDERS
WHERE ORDERS.O_CUSTKEY = CUSTOMER.C_CUSTKEY)
FROM CUSTOMER ;
Thanks very much in advance for your time,
Jinghao Liu

Answer 1

ajallen wrote:
Why not something more like
SELECT *
FROM fact1 l,
FULL OUTER JOIN fact1 d
ON l.company = d.company
AND l.transactiontypeid = 1
AND d.transactiontypeid = 2;
Because this is not an equivalent of the original query.
drop table t1 cascade constraints purge;
drop table t2 cascade constraints purge;
create table t1 as select rownum t1_id from dual connect by level <= 5;
create table t2 as select rownum+2 t2_id from dual connect by level <= 5;
select * from (select * from t1 where t1_id > 2) t1 full outer join t2 on (t1_id = t2_id);
select * from t1 full outer join t2 on (t1_id = t2_id and t1_id > 2);
     T1_ID      T2_ID
         3          3
         4          4
         5          5
                    6
                    7
     T1_ID      T2_ID
         1
         2
         3          3
         4          4
         5          5
                    6
                    7