Hibernate with Oracle sequence doesn't use it

Question

I have configured hibernate to use oracle sequence. Sequence is created with cache=20, increment=1.

All works fine, hibernate persisting entities. The id value is strange: 50,51....76,201,202...209,1008,1009,5129,5130 ....

If I ask for sequence value (select hibernate_sequence.nextval from dual) I get value like 2,3,4 ....

If I turn on hibernate sql debug, there is time to time call "select hibernate_sequence.nextval from dual" but number assigned by hibernate to ID doesn't relay on sequence!

@Id
@Column(name = "ID", insertable = false, updatable = false)
@SequenceGenerator(name = "SequenceIdGenerator", sequenceName = "HIBERNATE_SEQUENCE")
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SequenceIdGenerator")
private Long id;

Answer 1

This is because the SequenceGenerator is not really a sequence generator. It's a sequence hi-lo generator. This means that the first time it's invoked, it gets the next value from the sequence (6 for example), then multiplies this value by 50 and gives you the result (300). The next time it's invoked, it returns 301 (without going to the sequence), and so on until it reaches 349. Then it asks the sequence for the next value and obtains 7, which it multiplies by 50 again to give you 350. My algorithm description could be off by one, but you get the idea.

If you stop and start your application, it will thus have gaps. But it's more efficient than a pure sequence generator because it only makes a database call once in 50 generations.

See http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html_single/#mapping-declaration-id-enhanced-optimizers and http://docs.jboss.org/hibernate/core/3.6/reference/en-US/html_single/#mapping-declaration-id-generator for details.