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