Hibernate use of PostgreSQL sequence does not affect sequence table

Question

I've configured Hibernate to use PostgreSQL sequence (via annotations) to generate values for primary key id column as follows:

@Id 
@SequenceGenerator(name="pk_sequence",sequenceName="entity_id_seq")
@GeneratedValue(strategy=GenerationType.SEQUENCE,generator="pk_sequence")
@Column(name="id", unique=true, nullable=false)
public int getId() {
    return this.id;
}

What I see with this configuration is that hibernate is already assigning id values > 3000 on persisting, whereas the query on used sequence shows the following:

database=# select last_value from entity_id_seq;
last_value 
------------
     69

(1 row)

Questions:
Is there anything wrong or not?
Should hibernate sync with the sequence table?
If not, where does it store the last generated id?

Thank you.

Solution 1

I had the same problem. It is related to the id allocating strategies of Hibernate. Whe n you choose GenerationType.SEQUENCE, Hibernate uses HiLo strategy which allocates IDs in blocks of 50 by default. So you can explicitly set allocationSize value like this:

@Id 
@SequenceGenerator(name="pk_sequence",sequenceName="entity_id_seq", allocationSize=1)
@GeneratedValue(strategy=GenerationType.SEQUENCE,generator="pk_sequence")
@Column(name="id", unique=true, nullable=false)
public int getId() {
    return this.id;
}

Though, I've also heard opinions that using HiLo strategy with allocationSize=1 is not a good practice. Some people recommend to use GenerationType.AUTO instead when you have to deal with database-managed sequences

Update: I did end up going with allocationSize=1, and things seem to work as I expect now. My application is such that I don't really need blocks of IDs anyway, so YMMV.

Solution 2

DO NOT USE GenerationType.SEQUENCE for Postgres sequences!

It's completely counter-intuitive, but the Hibernate folks completely messed up on this. You must use GenerationType.AUTO or Hibernate will demolish your sequences if you have to restart/rebuild your DB. It's almost criminally negligent that they would allow this code to go into a production build, but the Hibernate team is rather famous for their bull-headed stances towards flatly-wrong positions (check out their position on LEFT JOINs, for instance).

Solution 3

First, you have to determine which version of Hibernate you are using. In terms of hibernate-core versions, 3.2 onwards introduced more consistent support for id generators especially in regards to defined in annotations. See http://in.relation.to/Bloggers/New323HibernateIdentifierGenerators for a discussion.

Next 3.6 introduced a setting ('hibernate.id.new_generator_mappings') which makes the generators discussed in that blog the default way JPA-annotations are handled. The setting is false by default because Hibernate has to maintain backwards compatibility with older versions. If you want the new behavior (which is completely recommended) then simply set that setting to true.

How GenerationType is handled depends on which version you are using and whether you have 'hibernate.id.new_generator_mappings' set to true. I will assume you are using 3.6+ (since anything older is, well, old) and do have 'hibernate.id.new_generator_mappings' set to true (since that is the recommendation for new apps):

1. GenerationType.AUTO -> treated as GenerationType.SEQUENCE
2. GenerationType.SEQUENCE -> maps to the org.hibernate.id.enhanced.SequenceStyleGenerator class discussed in the blog
3. GenerationType.TABLE -> maps to the org.hibernate.id.enhanced.TableGenerator class discussed in the blog