I have an entity that has an NON-ID field that must be set from a sequence. Currently, I fetch for the first value of the sequence, store it on the client's side, and compute from that value.
However, I'm looking for a "better" way of doing this. I have implemented a way to fetch the next sequence value:
public Long getNextKey()
{
Query query = session.createSQLQuery( "select nextval('mySequence')" );
Long key = ((BigInteger) query.uniqueResult()).longValue();
return key;
}
However, this way reduces the performance significantly (creation of ~5000 objects gets slowed down by a factor of 3 - from 5740ms to 13648ms ).
I have tried to add a "fake" entity:
@Entity
@SequenceGenerator(name = "sequence", sequenceName = "mySequence")
public class SequenceFetcher
{
@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "sequence")
private long id;
public long getId() {
return id;
}
}
However this approach didn't work either (all the Ids returned were 0).
Can someone advise me how to fetch the next sequence value using Hibernate efficiently?
Edit: Upon investigation, I have discovered that calling Query query = session.createSQLQuery( "select nextval('mySequence')" ); is by far more inefficient than using the @GeneratedValue- because of Hibernate somehow manages to reduce the number of fetches when accessing the sequence described by @GeneratedValue.
For example, when I create 70,000 entities, (thus with 70,000 primary keys fetched from the same sequence), I get everything I need.
HOWEVER , Hibernate only issues 1404 select nextval ('local_key_sequence') commands. NOTE: On the database side, the caching is set to 1.
If I try to fetch all the data manually, it will take me 70,000 selects, thus a huge difference in performance. Does anyone know the internal functioning of Hibernate, and how to reproduce it manually?
Here is what worked for me (specific to Oracle, but using scalar seems to be the key)
Long getNext() {
Query query =
session.createSQLQuery("select MYSEQ.nextval as num from dual")
.addScalar("num", StandardBasicTypes.BIG_INTEGER);
return ((BigInteger) query.uniqueResult()).longValue();
}
Thanks to the posters here: springsource_forum
You can use Hibernate Dialect API for Database independence as follow
class SequenceValueGetter {
private SessionFactory sessionFactory;
// For Hibernate 3
public Long getId(final String sequenceName) {
final List<Long> ids = new ArrayList<Long>(1);
sessionFactory.getCurrentSession().doWork(new Work() {
public void execute(Connection connection) throws SQLException {
DialectResolver dialectResolver = new StandardDialectResolver();
Dialect dialect = dialectResolver.resolveDialect(connection.getMetaData());
PreparedStatement preparedStatement = null;
ResultSet resultSet = null;
try {
preparedStatement = connection.prepareStatement( dialect.getSequenceNextValString(sequenceName));
resultSet = preparedStatement.executeQuery();
resultSet.next();
ids.add(resultSet.getLong(1));
}catch (SQLException e) {
throw e;
} finally {
if(preparedStatement != null) {
preparedStatement.close();
}
if(resultSet != null) {
resultSet.close();
}
}
}
});
return ids.get(0);
}
// For Hibernate 4
public Long getID(final String sequenceName) {
ReturningWork<Long> maxReturningWork = new ReturningWork<Long>() {
@Override
public Long execute(Connection connection) throws SQLException {
DialectResolver dialectResolver = new StandardDialectResolver();
Dialect dialect = dialectResolver.resolveDialect(connection.getMetaData());
PreparedStatement preparedStatement = null;
ResultSet resultSet = null;
try {
preparedStatement = connection.prepareStatement( dialect.getSequenceNextValString(sequenceName));
resultSet = preparedStatement.executeQuery();
resultSet.next();
return resultSet.getLong(1);
}catch (SQLException e) {
throw e;
} finally {
if(preparedStatement != null) {
preparedStatement.close();
}
if(resultSet != null) {
resultSet.close();
}
}
}
};
Long maxRecord = sessionFactory.getCurrentSession().doReturningWork(maxReturningWork);
return maxRecord;
}
}
I found the solution:
public class DefaultPostgresKeyServer
{
private Session session;
private Iterator<BigInteger> iter;
private long batchSize;
public DefaultPostgresKeyServer (Session sess, long batchFetchSize)
{
this.session=sess;
batchSize = batchFetchSize;
iter = Collections.<BigInteger>emptyList().iterator();
}
@SuppressWarnings("unchecked")
public Long getNextKey()
{
if ( ! iter.hasNext() )
{
Query query = session.createSQLQuery( "SELECT nextval( 'mySchema.mySequence' ) FROM generate_series( 1, " + batchSize + " )" );
iter = (Iterator<BigInteger>) query.list().iterator();
}
return iter.next().longValue() ;
}
}
If you are using Oracle, consider specifying cache size for the sequence. If you are routinely create objects in batches of 5K, you can just set it to a 1000 or 5000. We did it for the sequence used for the surrogate primary key and were amazed that execution times for an ETL process hand-written in Java dropped in half.
I could not paste formatted code into comment. Here's the sequence DDL:
create sequence seq_mytable_sid
minvalue 1
maxvalue 999999999999999999999999999
increment by 1
start with 1
cache 1000
order
nocycle;
To get the new id, all you have to do is flush the entity manager. See getNext() method below:
@Entity
@SequenceGenerator(name = "sequence", sequenceName = "mySequence")
public class SequenceFetcher
{
@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "sequence")
private long id;
public long getId() {
return id;
}
public static long getNext(EntityManager em) {
SequenceFetcher sf = new SequenceFetcher();
em.persist(sf);
em.flush();
return sf.getId();
}
}
POSTGRESQL
String psqlAutoincrementQuery = "SELECT NEXTVAL(CONCAT(:psqlTableName, '_id_seq')) as id";
Long psqlAutoincrement = (Long) YOUR_SESSION_OBJ.createSQLQuery(psqlAutoincrementQuery)
.addScalar("id", Hibernate.LONG)
.setParameter("psqlTableName", psqlTableName)
.uniqueResult();
MYSQL
String mysqlAutoincrementQuery = "SELECT AUTO_INCREMENT as id FROM information_schema.tables WHERE table_name = :mysqlTableName AND table_schema = DATABASE()";
Long mysqlAutoincrement = (Long) YOUR_SESSION_OBJ.createSQLQuery(mysqlAutoincrementQuery)
.addScalar("id", Hibernate.LONG)
.setParameter("mysqlTableName", mysqlTableName)
.uniqueResult();
Interesting it works for you. When I tried your solution an error came up, saying that "Type mismatch: cannot convert from SQLQuery to Query". --> Therefore my solution looks like:
SQLQuery query = session.createSQLQuery("select nextval('SEQUENCE_NAME')");
Long nextValue = ((BigInteger)query.uniqueResult()).longValue();
With that solution I didn't run into performance problems.
And don't forget to reset your value, if you just wanted to know for information purposes.
--nextValue;
query = session.createSQLQuery("select setval('SEQUENCE_NAME'," + nextValue + ")");
Here is the way I do it:
@Entity
public class ServerInstanceSeq
{
@Id //mysql bigint(20)
@SequenceGenerator(name="ServerInstanceIdSeqName", sequenceName="ServerInstanceIdSeq", allocationSize=20)
@GeneratedValue(strategy=GenerationType.SEQUENCE, generator="ServerInstanceIdSeqName")
public Long id;
}
ServerInstanceSeq sis = new ServerInstanceSeq();
session.beginTransaction();
session.save(sis);
session.getTransaction().commit();
System.out.println("sis.id after save: "+sis.id);
Your idea with the SequenceGenerator fake entity is good.
@Id
@GenericGenerator(name = "my_seq", strategy = "sequence", parameters = {
@org.hibernate.annotations.Parameter(name = "sequence_name", value = "MY_CUSTOM_NAMED_SQN"),
})
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "my_seq")
It is important to use the parameter with the key name "sequence_name". Run a debugging session on the hibernate class SequenceStyleGenerator, the configure(...) method at the line final QualifiedName sequenceName = determineSequenceName( params, dialect, jdbcEnvironment ); to see more details about how the sequence name is computed by Hibernate. There are some defaults in there you could also use.
After the fake entity, I created a CrudRepository:
public interface SequenceRepository extends CrudRepository<SequenceGenerator, Long> {}
In the Junit, I call the save method of the SequenceRepository.
SequenceGenerator sequenceObject = new SequenceGenerator(); SequenceGenerator result = sequenceRepository.save(sequenceObject);
If there is a better way to do this (maybe support for a generator on any type of field instead of just Id), I would be more than happy to use it instead of this "trick".
Spring 5 has some builtin helper classes for that: org/springframework/jdbc/support/incrementer
来源：https://stackoverflow.com/questions/6386888/get-next-sequence-value-from-database-using-hibernate