Hibernate生成不必要的查询

Question

I'm using Hibernate 3.3.1.
I have a table in db (2 fields: id , name ). Using Hibernate I have created a class for this table.

@Entity
@Table(name = "table1")
public class QTable1 implements Serializable {

    public QTable1() {
    }

    @Id
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.AUTO) //<- modification: to comment this line
    private Long id;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @Column(name = "name")
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }   

}

After making changes I want to save them using session.saveorUpdate and it saves ALL rows, that was loaded form db. Even if they wasn't changed:

update table1 set name='a' where id=1
update table1 set name='b' where id=2
update table1 set name='cc' where id=3 // only this row changed

Code, modifying the entities:

final QTable1 item = (QTable1) listTable1.getSelectedValue();
if (item != null) {
    item.setName(table1Name.getText());
}

Transaction code:

final Session session = Commons.getSessionFactory().openSession();
session.beginTransaction();
for (Object o : pool.getTable1List().toArray()) {
    final QTable1 item = (QTable1) o;
    session.saveOrUpdate(item);
}
session.getTransaction().commit();

Why it saves all the data?

If I change the implementation of id (delete @GeneratedValue(strategy=GenerationType.AUTO) ):

@Id
@Column(name = "id")
private Long id;

It saves only affected rows. And that is what I expect. But without @GeneratedValue if I try to add new row to the table I have to specify id manually, that is not good.

While debugging I saw that using @GeneratedValue I don't have my entities in session PersistanceContext in entitySnapshotsByKey so it considers that my entities are new and must by flushed to db.

How to solve that problem?

UPDATE:

If we use @GeneratedValue annotation session.saveOrUpdate(item) generates only update or insert queries. But if we don't use @GeneratedValue annotation session.saveOrUpdate(item) generates 1) select for table1 queries 2) compares if the objects are equal or not 3) decide if we need to generate update or insert queries.

Now I can't understand why @GeneratedValue prevents quering the values. Still I don't understand why it makes select queries if hibernate have a copy of selected rows in memory ( entitySnapshotsByKey ).

But I found a solvation that fits me just fine. All I need is just to replace session.saveOrUpdate(item) with session.merge(item) . It quaranties that select query will be made and the values will be compared (using @GeneratedValue or not). The only drawback - is the potential large number of queries.

Thanks to all. Will be glad if you help me to prevent generating select queries.

Answer 1

Somewhere in lower 3-rd version hibernate changed behaviour - it does not anymore require explicit save of loaded entities (of course, without proper documentation - this had biten a big chunk off my ass ) if it deems that they were changed. You may also play with unsaved value setting.

In case you like to prevent saving of persisten entities, you have to detach them from session.

Answer 2

This is expected and documented behavior. saveOrUpdate takes a detached (with an ID) or transient entity (without an ID), and then updates it (if it's detached) or saves it (if it's transient).

saveOrUpdate doesn't know if some field has changed or not. It just takes an entity and writes it to the database.

You could use merge , which fetches the entity with the given ID from the database, and then copy all the fields from the detached entity to the attached one (and then writes the entity to the database at flush time if something has changed). But this would need a select to fetch the database and be able to compare the new state with the old one.