Optimizing aggregate function in Oracle - sql

Question

I have a query for pulling customer information, and I'm adding an max() function to find the most recent order date. Without the aggregate the query takes .23 seconds to run, but with it it takes 12.75 seconds.
Here's the query:
SELECT U.SEQ, MAX(O.ORDER_DATE) FROM CUST_MST U
INNER JOIN ORD_MST O ON U.SEQ = O.CUST_NUM
WHERE U.SEQ = :customerNumber
GROUP BY U.SEQ;
ORD_MST is a table with 890,000 records.
Is there a more efficient way to get this functionality?
EDIT: For the record, there's nothing specifically stopping me from running two queries and joining them in my program. I find it incredibly odd that such a simple query would take this long to run. In this case it is much cleaner/easier to let the database do the joining of information, but it's not the only way for me to get it done.
EDIT 2: As requested, here are the plans for the queries I reference in this question.
With Aggregate
Without Aggregate

Answer 1

the problem with your query is that you join both tables completely, then the max function is executed against the whole result, and at last the where statement filters your rows.
you have improve the join, by just joining the rows with the certain custid instead of the full tables, should look like this:
SELECT U.SEQ, MAX(O.ORDER_DATE) FROM
(SELECT * FROM CUST_MST WHERE SEQ = :customerNumber ) U
INNER JOIN
(SELECT * FROM ORD_MST WHERE CUST_NUM = :customerNumber) O ON U.SEQ = O.CUST_NUM
GROUP BY U.SEQ;
Another option is to use an order by and filter the first rownum. its not rly the clean way. Could be faster, if not you will also need a subselect to not order the full tables. Didnt use oracle for a while but it should look something like this:
SELECT * FROM
(
SELECT U.SEQ, O.ORDER_DATE FROM CUST_MST U
INNER JOIN ORD_MST O ON U.SEQ = O.CUST_NUM
WHERE U.SEQ = :customerNumber
GROUP BY U.SEQ;
ORDER BY O.ORDER_DATE DESC
)
WHERE ROWNUM = 1
Are you forced to use the join for some reason or why dont you select directly from ORD_MST without join?
EDIT
One more idea:
SELECT * FROM
(SELECT CUST_NUM, MAX(ORDER_DATE) FROM ORD_MST WHERE CUST_NUM = :customerNumber GROUP BY CUST_NUM) O
INNER JOIN CUST_MST U ON O.CUST_NUM = U.SEQ
if the inner select just takes one second, then the join should work instant.

Answer 2

Run this commands:
Explain plan for
SELECT U.SEQ, MAX(O.ORDER_DATE) FROM CUST_MST U
INNER JOIN ORD_MST O ON U.SEQ = O.CUST_NUM
WHERE U.SEQ = :customerNumber
GROUP BY U.SEQ;
select * from table( dbms_xplan.display );
and post results here.
Whithout knowing an execution plan we can only guess what really happens.
Btw. my feeling is that adding composite index for ORD_MST table with columns cust_num+order_date could solve the problem (assuming that SEQ is primary key for CUST_MST table and it has already an unique index). Try:
CREATE INDEX idx_name ON ORD_MST( cust_num, order_date );
Also, after creating the index refresh statistics with commands:
EXEC DBMS_STATS.gather_table_stats('your-schema-name', 'CUST_MST');
EXEC DBMS_STATS.gather_table_stats('your-schema-name', 'ORD_MST');
try your query.