Oracle：在@GeneratedValue 注释的情况下如何生成主键id

Question

So I have a repository class in which I can see the primary key field has a annotation

@Id
@GeneratedValue

So, what I know that when the strategy is not explicitly defined the default strategy set is auto. And in the case of auto strategy the undermine database decides the strategy from: Sequence, Table, Identity.

So, my database is Oracle, in case of Oracle, sequence is the most preferred strategy.

Correct me if I am wrong.

My question is how I can know which strategy is used by my table, is it sequence and if sequence, is it table specific or universal.

And is this sequence ensuring that the new id is greater than max id.

I am quite new in this field, and not able to find any useful resource for understanding this.

Thanks for your help in advance. :)

Answer 1

I'm quite sure it is table specific sequences if you configured ddl auto. Search for JPA in Google to find a bunch of articles about the subject like :

https://www.objectdb.com/java/jpa/entity/generated

The Sequence Strategy

The sequence strategy consists of two parts - defining a named sequence and using the named sequence in one or more fields in one or more classes. The @SequenceGenerator annotation is used to define a sequence and accepts a name, an initial value (the default is 1) and an allocation size (the default is 50). A sequence is global to the application and can be used by one or more fields in one or more classes. The SEQUENCE strategy is used in the @GeneratedValue annotation to attach the given field to the previously defined named sequence:

@Entity
// Define a sequence - might also be in another class:
@SequenceGenerator(name="seq", initialValue=1, allocationSize=100)
public class EntityWithSequenceId {
    // Use the sequence that is defined above:
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="seq")
    @Id long id;
}

Unlike AUTO and IDENTITY, the SEQUENCE strategy generates an automatic value as soon as a new entity object is persisted (ie before commit). This may be useful when the primary key value is needed earlier. To minimize round trips to the database server, IDs are allocated in groups. The number of IDs in each allocation is specified by the allocationSize attribute. It is possible that some of the IDs in a given allocation will not be used. Therefore, this strategy does not guarantee there will be no gaps in sequence values.

The Table Strategy

The TABLE strategy is very similar to the SEQUENCE strategy:

@Entity
@TableGenerator(name="tab", initialValue=0, allocationSize=50)
public class EntityWithTableId {
    @GeneratedValue(strategy=GenerationType.TABLE, generator="tab")
    @Id long id;
}

ORM-based JPA providers (such as Hibernate, TopLink, EclipseLink, OpenJPA, JPOX, etc.) simulate a sequence using a table to support this strategy. ObjectDB does not have tables, so the TABLE and SEQUENCE strategies are almost identical.

A tiny difference is related to the initial value attribute. Whereas the SEQUENCE strategy maintains the next sequence number to be used the TABLE strategy maintains the last value that was used. The implication for the initialValue attribute is that if you want sequence numbers to start with 1 in the TABLE strategy initialValue=0 has to be specified in the @SequenceGenerator annotation.

or : https://vladmihalcea.com/why-you-should-never-use-the-table-identifier-generator-with-jpa-and-hibernate/

Even this article that explains that sequence strategy have far best performances with benchmarked code snippets : https://vladmihalcea.com/why-you-should-never-use-the-table-identifier-generator-with-jpa-and-hibernate/

If your stack is spring-data-jpa they have a very good documentation on pivotal. https://spring.io/projects/spring-data-jpa