生成Hibernate_sequence表

Question

I have id column with generated strategy AUTO, I'm wondering, why MySql generate hibernate_sequence table? I supposed that hibernate will pick IDENTITY id generating strategy

<mapped-superclass class="com.cl.xlp.model.data.Identity">
    <attributes>
        <id name="id">
            <column name="id" />
            <generated-value strategy="AUTO" />
        </id>
    </attributes>
</mapped-superclass>

Hibernate properties

hibernate.dialect=org.hibernate.dialect.MySQL5Dialect
hibernate.hbm2ddl.auto=update

Mysql connector version

version.mysql.connector>5.1.39</version.mysql.connector>

Mysql server version is 5.6.12

Answer 1

The way Hibernate interprets AUTO generation type has changed starting with Hibernate version 5.0.

When using Hibernate v 4.0 and Generation Type as AUTO , specifically for MySql, Hibernate would choose the IDENTITY strategy (and thus use the AUTO_INCREMENT feature) for generating IDs for the table in question.

Starting with version 5.0 when Generation Type is selected as AUTO, Hibernate uses SequenceStyleGenerator regardless of the database. In case of MySql Hibernate emulates a sequence using a table and is why you are seeing the hibernate_sequence table. MySql doesn't support the standard sequence type natively.

References

• http://docs.jboss.org/hibernate/orm/5.0/userguide/html_single/Hibernate_User_Guide.html#identifiers-generators-auto
• https://www.thoughts-on-java.org/5-things-you-need-to-know-when-using-hibernate-with-mysql/

Answer 2

If you use strategy="AUTO" , Hibernate will generate a table called hibernate_sequence to provide the next number for the ID sequence. You may have forgotten to add the AutoIncrement feature to your table's PK.

You may use generation strategy strategy="IDENTITY" to enforce using the AutoIncrement feature available in MySql and avoid creating a table.