PostgreSQL SERIAL column and Hibernate IDENTITY generator

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Introduction

When using PostgreSQL, it’s tempting to use a SERIAL or BIGSERIAL column type to auto-increment Primary Keys.

However, this article will show you that this is not a very good idea when using JPA and Hibernate.

SERIAL or BIGSERIAL

If you’ve been using MySQL, you know that AUTO_INCREMENT is a very popular choice. When migrating to PostgreSQL, you will notice that SERIAL or BIGSERIAL column types can be used just like AUTO_INCREMENT in MySQL.

SERIAL is an auto-incremented integer column that takes 4 bytes while BIGSERIAL is an auto-incremented bigint column taking 8 bytes. Behind the scenes, PostgreSQL will use a sequence generator to generate the SERIAL column values upon inserting a new ROW.

Domain model

Now, assuming we have the following post table:

CREATE TABLE post (
    id  SERIAL NOT NULL, 
    title VARCHAR(255), 
    PRIMARY KEY (id)
)

For this table, PostgreSQL creates a sequence called post_id_seq that is associated with the id SERIAL column.

So, when inserting a post row, the INSERT statement can simply omit the id column:

INSERT INTO post (title) 
VALUES ('High-Performance Java Persistence')

The id column is also the Primary Key of the post table, and it uses a SERIAL column type. The id column will be automatically be assigned the next value of the underlying post_id_seq sequence generator.

To map the post table, we need a Post entity class that looks as follows:

The Post entity id property uses the GenerationType.IDENTITY generator because the SERIAL type acts as AUTO_INCREMENTED column.

@Entity(name = "Post")
@Table(name = "post")
public class Post {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    private String title;

    //Getters and setters omitted for brevity
}

Now, to see how the post_id_seq is used, consider the following test case:

Post post1 = new Post();
post1.setTitle(
    "High-Performance Java Persistence, Part 1"
);

entityManager.persist(post1);

Post post2 = new Post();
post2.setTitle(
    "High-Performance Java Persistence, Part 2"
);

entityManager.persist(post2);

entityManager.flush();
assertEquals(
    2,
    (
        (Number) entityManager
        .createNativeQuery(
            "select currval('post_id_seq')")
        .getSingleResult()
    ).intValue()
);

After inserting 2 Post entities and flushing the Persistence Context, the current value of the post_id_seq database sequence is going to be 2, and the next sequence value is going to be 3.

#JDBC batch inserts

As convenient as it might look, using the IDENTITY with Hibernate is not without issues.

If we enable JDBC batching:

<property name="hibernate.jdbc.batch_size" value="5"/>

And persist 3 Post entities:

for (int i = 0; i < 3; i++) {
    Post post = new Post();
    post.setTitle(
        String.format(
            "High-Performance Java Persistence, Part %d", 
            i + 1
        )
    );
    entityManager.persist(post);
}

Hibernate is going to generate the following SQL INSERT statements:

INSERT INTO post (title) 
VALUES ('High-Performance Java Persistence, Part 1')

INSERT INTO post (title) 
VALUES ('High-Performance Java Persistence, Part 2')

INSERT INTO post (title) 
VALUES ('High-Performance Java Persistence, Part 3')

So, batching is going to be disabled when inserting entities.

This is because, when persisting the entity, Hibernate needs to know the entity identifier in order to generate the key under which the entity is stored in the currently running Persistence Context.

To know the identifier, Hibernate needs to execute the INSERT statement, so by the time the Persistence Context is flushed, all inserts have been already executed. Therefore, Hibernate can no longer batch the INSERT statements.

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Conclusion

Although convenient, and even suggested in many PostgreSQL book, the SERIAL and BIGSERIAL column types are not a very good choice when using JPA and Hibernate. Using a SEQUENCE generator is a better alternative since the identifier can be generated prior to executing the INSERT statement.

Behind the scenes, the SERIAL and BIGSERIAL column types use a database sequence anyway, so the only difference is that the SEQUENCE generator calls the sequence is a separate database roundtrip. However, this can also be optimized with the pooled and pooled-lo optimizers.

If the database server is close to the application servers and networking is fast, the extra database roundtrip is not going to be a performance bottleneck. For all these reasons, you should prefer using the SEQUENCE generator over IDENTITY no matter if you use PostgreSQL, Oracle or SQL Server.

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2 Comments on “PostgreSQL SERIAL column and Hibernate IDENTITY generator

  1. Confused by your concluding statement “…the SERIAL and BIGSERIAL column types are not a very good choice when using JPA and Hibernate.” Is the data type the issue, or the generation type? Can’t you still use a named sequence and the SEQUENCE generation type with SERIAL and BIGSERIAL? If we used SERIAL/BIGSERIAL when creating our table and Postgre automatically created post_id_seq, can’t we still specify “post_id_seq” via sequenceName in @SequenceGenerator?

    I see the presence of a natively named sequence as a good thing, especially when working on the database side. Having a sequence named hibernate_sequence is not unique to a table and does us no good when working in the database? Right?

    • Yes, you can use the PostgreSQL sequences created automatically, and there’s no reason to use just the hibernate_sequence. In fact you would not ask the question if you used Flyway to manage your schema. Check out my High-Performance Java Persistence book for more details.

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