Hibernate sequence generator confused by sharing it for multiple entities

Question

My classes use an ID like

@Id @Generated(GenerationTime.INSERT) @GeneratedValue private Integer id;

This works perfectly for H2 (supporting sequences) and gets interpreted for MySql by creating a helper table hibernate_sequence. Using this answer, everything looks the way I want, especially using a single sequence for all tables.

One thing seems to be wrong: There are multiple rows in the helper table. My id is declared in a @MappedSuperclass and during initialization, for each concrete class this line gets executed:

insert into hibernate_sequence values ( 1 )

This is obviously wrong: there is a line per table there and each contains the same value (initially one; when changed, they all change in the same way, as the SQL is update hibernate_sequence set next_val=? where next_val=?, so it effects all the rows in the same way).

It's harmless, but I wonder: Is it a bug or am I doing something wrong?

Answer 1

If you want to make it works you need to use other strategy for now:

@GenericGenerator(
        name = "table_generator",
        strategy = "org.hibernate.id.enhanced.TableGenerator"
)
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "table_generator")

What about org.hibernate.id.enhanced.SequenceStyleGenerator:

I believe there is an issue in how hibernate initializes shared single row sequence table.

For hibernare 5.0.6.Final Source of the issue lies in org.hibernate.boot.internal.InFlightMetadataCollectorImpl class in

private void processExportableProducers(MetadataBuildingContext buildingContext) {
    // for now we only handle id generators as ExportableProducers

    final Dialect dialect = getDatabase().getJdbcEnvironment().getDialect();
    final String defaultCatalog = extractName( getDatabase().getDefaultNamespace().getName().getCatalog(), dialect );
    final String defaultSchema = extractName( getDatabase().getDefaultNamespace().getName().getSchema(), dialect );

    for ( PersistentClass entityBinding : entityBindingMap.values() ) {
        if ( entityBinding.isInherited() ) {
            continue;
        }

        // ***************************************************************************
        // For Instance, it does not filter out the same entityBinding.getIdentifier()
        // and make initialization multiple time
        // ***************************************************************************
        handleIdentifierValueBinding(
                entityBinding.getIdentifier(),
                dialect,
                defaultCatalog,
                defaultSchema,
                (RootClass) entityBinding
        );
    }

    for ( Collection collection : collectionBindingMap.values() ) {
        if ( !IdentifierCollection.class.isInstance( collection ) ) {
            continue;
        }

        handleIdentifierValueBinding(
                ( (IdentifierCollection) collection ).getIdentifier(),
                dialect,
                defaultCatalog,
                defaultSchema,
                null
        );
    }
}

Answer 2

Same Sequence Generator

Presume there is a Base class annotated with @MappedSuperclass. There are also A and B classes extending Base class; If you annotate Id field in Base class with @SequenceGenerator, all subclasses of Base share the same sequence generator and will increament/use the same sequence in the database for their ids. This is ofcourse harmless but results in ugly numbers for ids :

@MappedSuperclass
public class Person {    
  @Id
  @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="id_gen")
  @SequenceGenerator(name="id_gen", sequenceName="a_seq", allocationSize=1)
  private Long id;
}

@Entity
public class A extends Base {
}

@Entity
public class B extends Base {
}

This is what their ids look like after adding A, then B, then A :

A{id=1, name='...'}
B{id=2, name='...'}
A{id=3, name='...'}

Distinct Sequence Generator

A nicer way would be having a fresh sequence for each table which can be implemented through assigning different sequence generators for different subclasses :

@MappedSuperclass
public class Person {    
    @Id
    @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="id_gen")
    private Long id;
}

@Entity
@SequenceGenerator(name="id_gen", sequenceName="a_seq", allocationSize=1)
public class A extends Base {
}

@Entity
@SequenceGenerator(name="id_gen", sequenceName="b_seq", allocationSize=1)
public class B extends Base {
}

Their ids will look like this after adding A, then B, then A :

A{id=1, name='...'}
B{id=1, name='...'}
A{id=2, name='...'}

Answer 3

A class annotated with @MappedSuperclass has no table of it's own. Only the entities that inherit that class have tables. So according to that the initialization has no bug, because, the sequence should be different for each concrete entity. So this is not a bug, rather it's the expected behavior.

Regarding update, if all three rows get updated when you insert a record in a SINGLE table then there is definitely a bug. But I am not sure if this is the scenario that you are experiencing.