Question

I need for a particular business scenario to set a field on an entity (not the PK) a number from a sequence (the sequence has to be a number between min and max

I defined the sequence like this :

CREATE SEQUENCE MySequence
  MINVALUE 65536 
  MAXVALUE 4294967296 
  START WITH 65536
  INCREMENT BY 1
  CYCLE
  NOCACHE
  ORDER;

In Java code I retrieve the number from the sequence like this :

select mySequence.nextval from dual

My question is :

If I call this “select mySequence.nextval from dual” in a transaction and in the same time in another transaction same method is called (parallel requests) it is sure that the values returned by the sequence are different ?

Is not possible to have like read the uncommitted value from the first transaction ?

Cause let’s say I would have not used sequence and a plain table where I would increment myself the sequence, then the transaction 2 would have been able to read same value if the trasactinalitY was the default “READ COMMITTED”.

Answer 1

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.