Answer 2.1
SELECT DISTINCT O.Order_ID, O.Order_Date, C.Name, L.Quantity, I.Name
FROM Orders O, Order_Lines
L, Items I, Customers C
WHERE
O.Customer_ID = :B1
AND L.Order_ID =
O.Order_ID
AND I.Item_ID =
L.Item_ID
AND C.Customer_ID =
O.Customer_ID;
.1 SELECT
STATEMENT
..2 SORT
UNIQUE
...3 NESTED LOOPS
....4 NESTED LOOPS
.....5 NESTED LOOPS
......6
TABLE ACCESS BY INDEX ROWID 4*CUSTOMERS
.......7
INDEX UNIQUE SCAN CUSTOMERS_U1: customer_id
......6
TABLE ACCESS BY INDEX ROWID 1*ORDERS
.......7 INDEX RANGE
SCAN ORDERS_N4: customer_id
.....5 TABLE ACCESS BY
INDEX ROWID 2*ORDER_LINES
......6
INDEX RANGE SCAN ORDER_LINES_N1: order_id
....4 TABLE ACCESS BY
INDEX ROWID 3*ITEMS
.....5 INDEX RANGE SCAN ITEMS_U1: item_id,org_id
WHERE clause is missing condition on I.Org_ID that would make the join to Items unique.