问题
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?
回答1:
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
回答2:
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;
}
}
回答3:
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() ;
}
}
回答4:
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;
回答5:
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();
}
}
回答6:
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();
回答7:
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 + ")");
回答8:
Spring 5 has some builtin helper classes for that: org/springframework/jdbc/support/incrementer
回答9:
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);
回答10:
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".
来源：https://stackoverflow.com/questions/6386888/get-next-sequence-value-from-database-using-hibernate