Derived tables and Order by clause

Question

Hi,
I have a query that looks as below.
The execution time of the query (10 sec) is fine when order by clause is removed from the query. But with order by clause, the execution time is 40 times more(400 sec). How to optimize this query so that it will execute within a minute, with ORDER BY clause included?
select X.a1, X.a2, X.a3, X.a4, Y.b1, Y.b2, Y.b3
from (select A.a1 a1, A.a2 a2, A.a3 a3, A.a4 a4
from A_table A
where A.a1 = 'something') X,
(select B.b1 b1, B.b2 b2, B.b3 b3
from B_table B
where B.b1 = 'some') Y
where
X.a1 = Y.a1 (+) AND (some other conditions)
order by X.a1, X.a2, Y.b1, Y.b2, X.a3, X.a4
thanks in advance,
Venkat

Answer 1

You do need the inline view Y if you are outer joining it because you must apply the
where B.b1 = 'some'
condition first.
However, the inline view X is not needed. Still, I'm not sure that getting rid of it is going to improve performance. Try it and see.
Incidentally, the outer join condition
where X.a1 = Y.a1 (+)
refers to Y.a1, but the inline view Y has no column named a1. Did you mean Y.b1?
What are the "some other conditions"? Do any of them involve the inline view Y alone (that is, are not join conditions)? If so, it may be that either: a) you don't need an outer join, or, b) those conditions should be moved into the inline view Y. In either case, this may improve performance.
Are you sure the execution time without the order by clause is really 10 seconds? Or, is that just the elapsed time to return the first row? With the order by clause, Oracle must identify all the rows before it can return the first row.