Trying to understnd transactions better...

Question

I was reading an article concerning how oracle handles transactions and the article said the following...
Transaction Isolation
The database ensures that no uncommitted data is visible to other sessions and that the data set remains
consistent for the lifetime of a transaction, depending on the isolation level chosen. Oracle supports two
transaction isolation levels:
• Read Committed — The session is able to query data that has been committed during the lifetime of the
transaction but the data set remains stable for the life of a statement. This means that the same query may
return different results if issued twice in the same transaction if data has been changed.
• Serializable — The data set is stable for the life of a transaction. A query will always return the same results
within the same transaction even if data has been changed and committed.
so My question is "how do you choose an isolation level"? Is this a parameter that I would set for oracle? How do I know if I am using etiher
"read commited" or "serializable"?
thx.

Answer 1

Changes you make with 'set transaction' have transaction duration only. So, 'set transaction isolation level serializable' would only affect that new transaction of your session.
(Note that 'set transaction ...' actually creates a new transaction, and, as such, must be the first statement in a transaction. If you try to run it after a transaction has begun, you'll get an error.
MBOBAK@inbound1> create table test_trans(a number);
Table created.
MBOBAK@inbound1> insert into test_trans values(1);
1 row created.
MBOBAK@inbound1> set transaction isolation level serializable;
set transaction isolation level serializable
ERROR at line 1:
ORA-01453: SET TRANSACTION must be first statement of transaction
MBOBAK@inbound1> commit;
Commit complete.
MBOBAK@inbound1> set transaction isolation level serializable;
Transaction set.Hope that helps,
-Mark