SQL Query Optimization

Question

I have a query related to how the CBO and RBO actually works.
I understand that Oracle uses CBO in 10g and was using both RBO and CBO from version 7 to 9i.
Lets say I have a query
Select * from X where A = &A and B = &B and C = &C
Index is created on A and B.
Please correct me if I am wrong, internally oracle maintains a hash for the fields A and B.
When the above query is executed, oracle needs to search in the hash.
If I change the query like
Select * from X where B = &B and A = &A and C = &C, will it (order of cols in where clause) make any diff. to the oracle when it searches in the hash with respect to performance.
On similar lines lets say i have a query (no index this time)
select * from X where A = &A and B = &B
Lets assume that the first condition A=&A will greatly reduce the list and the second condition B=&B will almost always be a subset of the result of first condition (if executed separately).
As a develop I know about the above condition, so will it help if the query is written like this or even if I write something like select * from X where B = &B and A = &A, it will give the same performance.
Please advise.

Answer 1

Oracle does not maintain a hash internally, it uses B-Tree indexes. Essentially, the actual value of each indexed column plus the rowid (a pointer to the physical location of the row) is kept for each index entry. As long as you have the leading columns of the index in the where clause in any order, Oracle will be able to use an index on those columns. Under the RBO, it would certainly use the index. Under the CBO, Oracele may or may not use the index. The choice would depend on a number of factors such as the statistics on the table and index, and various initialization parameters.
In your second example with no indexes, Oracle would do a full table scan examining every record. There is no documented order to the evaluation of predicates (at least none that I have ever seen), and I doubt very much whether it would have much of an effect on the performance of a full table scan if the predicates were evaluated in optimal order versus worst possible order.
TTFN
John