Hibernate sequence on oracle, @GeneratedValue(strategy = GenerationType.AUTO)

Question

I'm usign @GeneratedValue(strategy = GenerationType.AUTO) to generate and id on my entity.

I don't now how it works, but on my chils table, generates id values, that follow the parent sequence.

//parent table
    @Entity
    @Table(name="parent")
    public class Parent {

        @Id 
        @GeneratedValue(strategy = GenerationType.AUTO)
        @Column(name="id")
        private long id;

            @OneToMany(cascade = {CascadeType.ALL},fetch = FetchType.LAZY)
            @JoinColumn(name=parentId")
            @ForeignKey(name="FKparent")
            private List<child> child;

    //child table
    @Entity
    @Table(name="child")
    public class Child {

    @Id 
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="id")
    private long id;

The inserted id values on parent, updates the sequence. The inserted id values on child, updates the sequence. On next insert of parent, the sequence... uses values updated by child insertions...

This Annotations, are no creating two sequences, only one. This is correct/expected?

I insert my entities with my dao service only using entityManager.persist(parent);

Answer 1

Yes this is correct and expected.

You can create individual sequences for each table, but IMHO that is just extra code with no actual benefit.

Answer 2

These Annotations are no creating two sequences, only one. Is this correct/expected?

That's the expected behavior. When using @GeneratedValue(strategy = GenerationType.AUTO), the JPA provider will pick an appropriate strategy for the particular database. In the case of Oracle, this will be SEQUENCE and, since you did not specify anything, Hibernate will use a single global sequence called hibernate_sequence.

Is this correct? Well, I don't know, it depends on your needs. Just in case, the default maximum value for an Oracle sequence is 1E+27, or 1,000,000,000,000,000,000,000,000,000. That's enough for many.

Now, it is possible to use GenerationType.AUTO and still control the name of the sequence when the database uses sequences:

@Id
@GeneratedValue(strategy=GenerationType.AUTO, generator="my_entity_seq_gen")
@SequenceGenerator(name="my_entity_seq_gen", sequenceName="MY_ENTITY_SEQ")
private Long id;