Oracle Sequence Transactionality

Oracle guarantees sequence numbers will be different. Even if your transaction is rolled back, the sequence is 'used' and not reissued to another query.

Edit: Adding additional information after requirements around "no gaps" were stated in comments by Cris

If your requirements are for a sequence of numbers without gaps then oracle sequences will probably not be a suitable solution, as there will be gaps when transactions roll back, or when the database restarts or any other number of scenarios.

Sequences are primarily intended as a high performance generation tool for unique numbers (e.g. primary keys) without regard to gaps and transaction context constraints.

If your design / business / audit requirements need to account for every number then you would need instead to design a solution that uses a predetermined number within the transaction context. This can be tricky and prone to performance / locking issues in a multi-threaded environment. It would be better to try to redefine your requirement so that gaps don't matter.

Unfortunately you have to implement you're 'own wheel' - transactional sequence. It is rather simple - just create the table like sequence_name varchar2, value, min_value number, max_value number, need_cycle char and mess around 'select value into variable from your sequence table for update wait (or nowait - it depends from your scenario)'. After it issue update set value = variable from previous step + 1 where sequence_name = the name of your sequence and issue the commit statement from client side. That's it.

The answer is NO.

Oracle guarantees that numbers generated by sequence are different. Even if parallel requests are issued, RAC environment or rollback and commits are mixed.

Sequences have nothing to do with transactions.

See here the docs:

Use the CREATE SEQUENCE statement to create a sequence, which is a database object from which multiple users may generate unique integers. You can use sequences to automatically generate primary key values.

When a sequence number is generated, the sequence is incremented, independent of the transaction committing or rolling back. If two users concurrently increment the same sequence, then the sequence numbers each user acquires may have gaps, because sequence numbers are being generated by the other user. One user can never acquire the sequence number generated by another user. After a sequence value is generated by one user, that user can continue to access that value regardless of whether the sequence is incremented by another user.

Sequence numbers are generated independently of tables, so the same sequence can be used for one or for multiple tables. It is possible that individual sequence numbers will appear to be skipped, because they were generated and used in a transaction that ultimately rolled back. Additionally, a single user may not realize that other users are drawing from the same sequence.

sequence.nextval never returns the same value (before cycled) for the concurrent request. Perhaps you should check the following URL:

http://docs.oracle.com/cd/B19306_01/server.102/b14220/schema.htm#sthref883