问使用flushMode=autoHibernate查询要慢得多，直到CLEAR()被调用？

Place place = em.createQuery("from Place where name = :name", Place.class)
    .setParameter("name", name)
    .setFlushMode(FlushModeType.COMMIT)  // <-- yay!
    .getSingleResult();

@Entity @Table(name="place") @Immutable
public class Place {
    private Long _id;
    private String _name;

    @Id @GeneratedValue
    public Long getId() { return _id; }
    public void setId(Long id) { _id = id; }

    @Basic(optional=false) @Column(name="name", length=700,
        updatable=false, nullable=false, unique=true,
        columnDefinition="varchar(700) character set 'ascii' not null")
    public String getName() { return _name; }
    public void setName(String name) { _name = name; }

    @Override
    public boolean equals(Object o) { /* ... */ }

    @Override
    public int hashCode() { return getName().hashCode(); }
}

Place place = em.createQuery(
    "select p from Place p where p.name = :name", Place.class)
    .setParameter("name", name)
    .getSingleResult();

PreparedStatement ps = c.prepareStatement(
    "select id, name from place where name = ?");
ps.setString(1, name);
ResultSet rs = ps.executeQuery();
while (rs.next()) {
    Place place = new Place();
    place.setId(rs.getLong(1));
    place.setName(rs.getString(2));
}
rs.close();
ps.close();

Seconds/Query    Approach
0.000160s        JDBC
0.000286s        Hibernate calling clear() after import and every 100 queries
0.000653s        Hibernate calling clear() once after the import
0.012533s        Hibernate w/o calling clear() at all
0.000292s        Hibernate w/o calling clear(), and with flush-mode COMMIT