Can this oracle query be tuned anymore? - sql

Question

I'm trying to minimize the cost of this query as much as possible without creating any indexes.
This is the original query with a cost of 599:
SELECT DISTINCT OL.PRODUCT_ID
FROM ORDERS O JOIN ORDER_LINES OL ON (O.ORDER_ID = OL.ORDER_ID)
JOIN PRODUCTS P ON (OL.PRODUCT_ID = P.PRODUCT_ID)
JOIN CUSTOMERS C ON (C.CUSTOMER_ID = O.CUSTOMER_ID)
WHERE C.CUSTOMER_ID = 474871
OR UPPER(C.FIRST_NAME) = 'EDGAR';
This is what I've done so far. The cost is now 344:
SELECT OL.PRODUCT_ID
FROM ORDER_LINES OL
WHERE EXISTS
(SELECT ORDER_ID
FROM ORDERS
WHERE CUSTOMER_ID = 474871
AND ORDER_ID = OL.ORDER_ID)
OR EXISTS
(SELECT ORDER_ID
FROM ORDERS
WHERE CUSTOMER_ID IN
(SELECT CUSTOMER_ID
FROM CUSTOMERS
WHERE UPPER(FIRST_NAME) = 'EDGAR')
AND ORDER_ID = OL.ORDER_ID);
Is there anything that stands out that I may try to drive down the cost more?
Here is a screen shot of the explain plan:
Screenshot of ERD:

Answer 1

Looking at the cost is misleading and may lead you to make changes that aren't actually beneficial. To quote Tom Kyte, "You cannot compare the cost of 2 queries with each other. ... they might as well be random numbers."
The best way to check query performance is to actually time the query, ideally with realistic data. You should also be wary of premature optimization. Your first query is pretty straight-forward; I would stick with it unless a performance issue manifests.

Answer 2

SELECT OL.PRODUCT_ID
FROM ORDER_LINES OL
WHERE OL.ORDER_ID IN
(SELECT ORDER_ID FROM ORDERS
WHERE CUSTOMER_ID IN (SELECT CUSTOMER_ID FROM CUSTOMERS
WHERE CUSTOMER_ID = 474871 OR UPPER(FIRST_NAME) = 'EDGAR')
);
I suppose there is an index on OL.ORDER_ID (now you have FULL SCAN of ORDER_LINES)

Answer 3

I'm curious if the engine is smart enough to apply the where clause before the joins... If it's doing it after the join, then the results it has to scan are larger than they need to be... What happens if you move the limiting criteria to the join so it HAS to be evaluated before the join occurs. (fully expect this to be 599 or less. just don't know if it will be less...
SELECT OL.PRODUCT_ID
FROM CUSTOMERS C
INNER JOIN ORDERS O
ON (C.CUSTOMER_ID = O.CUSTOMER_ID)
AND (C.customer_ID = 47871 OR upper(C.First_name) = 'EDGAR')
INNER JOIN ORDER_LINES OL ON (O.ORDER_ID = OL.ORDER_ID)
INNER JOIN PRODUCTS P ON (OL.PRODUCT_ID = P.PRODUCT_ID)
GROUP BY OL.Product_ID
I wonder if the OR is causing the problem....
if you run it w/o the or how much cost is reduced
and then what happens if you union the two sets instead of using an or.
SELECT OL.PRODUCT_ID
FROM CUSTOMERS C
INNER JOIN ORDERS O
ON (C.CUSTOMER_ID = O.CUSTOMER_ID)
INNER JOIN ORDER_LINES OL ON (O.ORDER_ID = OL.ORDER_ID)
INNER JOIN PRODUCTS P ON (OL.PRODUCT_ID = P.PRODUCT_ID)
WHERE C.customer_ID = 47871
UNION
SELECT OL.PRODUCT_ID
FROM CUSTOMERS C
INNER JOIN ORDERS O
ON (C.CUSTOMER_ID = O.CUSTOMER_ID)
INNER JOIN ORDER_LINES OL ON (O.ORDER_ID = OL.ORDER_ID)
INNER JOIN PRODUCTS P ON (OL.PRODUCT_ID = P.PRODUCT_ID)
WHERE upper(C.First_name) = 'EDGAR')