JPA and PostgreSQL with GenerationType.IDENTITY

Question

I have a question about Postgres and GenerationType.Identity vs Sequence

In this example...

@Id
@SequenceGenerator(name=\"mytable_id_seq\",
                  


        

Answer 1

From "Pro JPA2" book:

"Another difference, hinted at earlier, between using IDENTITY and other id generation strategies is that the identifier will not be accessible until after the insert has occurred. Although no guarantee is made about the accessibility of the identifier before the transaction has completed, it is at least possible for other types of generation to eagerly allocate the identifier. But when using identity, it is the action of inserting that causes the identifier to be generated. It would be impossible for the identifier to be available before the entity is inserted into the database, and because insertion of entities is most often deferred until commit time, the identifier would not be available until after the transaction has been committed."

Answer 2

I think it can be helpful if you are using the same sequence for more than one table (for example you want a unique identifier for many types of bills) ... also If you want to keep track of the sequence away from the auto generated key

Answer 3

If you have a column of type SERIAL, it will be sufficient to annotate your id field with:

@Id @GeneratedValue(strategy=GenerationType.IDENTITY)

This is telling Hibernate that the database will be looking after the generation of the id column. How the database implements the auto-generation is vendor specific and can be considered "transparent" to Hibernate. Hibernate just needs to know that after the row is inserted, there will be an id value for that row that it can retrieve somehow.

If using GenerationType.SEQUENCE, you are telling Hibernate that the database is not automatically populating the id column. Instead, it is Hibernate's responsibility to get the next sequence value from the specified sequence and use that as the id value when inserting the row. So Hibernate is generating and inserting the id.

In the case of Postgres, it happens that defining a SERIAL column is implemented by creating a sequence and using it as a default column value. But it is the database that is populating the id field so using GenerationType.IDENTITY tells Hibernate that the database is handling id generation.

These references may help:

http://docs.jboss.org/hibernate/orm/5.2/userguide/html_single/Hibernate_User_Guide.html#identifiers-generators

https://www.postgresql.org/docs/8.1/static/datatype.html#DATATYPE-SERIAL