我对 hibernate 和 Spring-data-jpa 很陌生。我想做的就是使用分页获取一个实体及其所有相关实体(大约有 4000 万条记录)。即一个查询急切地获取根实体及其所有映射实体/集合 = 1 个查询
但是我遇到了 (n+1) 个问题:根实体的一个查询 + 每个根实体的相关映射实体/集合的一个查询 = (n+1) 个查询 这确实会影响性能。
我也想知道我在实体中的映射是否正确。
有人可以指导我吗?谢谢
应用程序栈:
<java-version>1.7</java-version>
<spring.framework.version>4.0.2.RELEASE</spring.framework.version>
<hibernate-core>4.3.4.Final</hibernate-core>
<hibernate-validator>5.1.0.Final</hibernate-validator>
<hibernate-entitymanager>4.3.4.Final</hibernate-entitymanager>
<spring-data-jpa>1.5.1.RELEASE</spring-data-jpa>
我有2个实体类(客户和订单)如下:
客户实体
@Entity
@Table(name = "view_customerOrders_to_process")
public class Customer implements Serializable {
private Long id;
private List<Order> Orders;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "customer_id", updatable = false)
public Long getId() {
return id;
}
........
@NotFound(action = NotFoundAction.IGNORE)
@OneToMany(fetch = FetchType.EAGER, mappedBy = "customer")
//Do not cascade any changes to child entities.
@Where(clause = "order_status = 'PENDING'")
public List<Order> getOrders() {
return order;
}
}
尝试使用 batchSize,但由于某种原因它不起作用(同样的 n+1 问题)。
//@Fetch(FetchMode.SELECT)
//@BatchSize(size=25)
订单实体:
@Table(name = "tbl_orders")
public class Order implements Serializable {
private Long id;
private int customerId;
private Customer customer;
private OrderType orderType;
..........
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "order_id", updatable = false)
public Long getId() {
return this.id;
}
@OneToOne
@NotFound(action = NotFoundAction.IGNORE)
@JoinColumn(name = "order_type", nullable = true, unique = false, updatable = false)
public OrderType getOrderType() {
return this.orderType;
}
@NotFound(action = NotFoundAction.IGNORE)
@ManyToOne(fetch = FetchType.LAZY, optional = true, targetEntity = Customer.class)
@Fetch(FetchMode.JOIN)
@JoinColumns({
@JoinColumn(updatable = false, insertable = false,
name="customer_id", referencedColumnName = "customer_id", nullable=false),
. . . . .
})
public Customer getCustomer() {
return customer;
}
}
服务类:
@Service
public class MetsGeneratorService {
. . .
@Autowired
private CustomerRepository customerRepository;
public List<Customer> run() {
PageRequest pageRequest = new PageRequest(0, batchSize);
List<Customer> customers = customerRepository.findOrdersUnprocessed(pageRequest);
return customers;
}
. . .
}
存储库:
public interface CustomerRepository extends JpaRepository<Customer, Long> {
@Query("SELECT e FROM Customer e")
List<Customer> findOrdersUnprocessed(Pageable pageable);
//Tried using the below one which seems to eliminate paging
//and understandably getting out of memory.
//@Query(value = "SELECT e FROM Customer e left join fetch e.orders")
}
试过改成
@Query(value = "SELECT e FROM Customer e left join fetch e.orders",
countQuery = "select count(e) from Customer e")
List<Customer> findOrdersUnprocessed(Pageable pageable);
如果您看到它正在处理的日志,则执行“select *”而不是分页。
日志消息是 WARN org.hibernate.hql.internal.ast.QueryTranslatorImpl - HHH000104: firstResult/maxResults 用集合获取指定;在内存中申请!
最佳答案
你可以使用
@Fetch(FetchMode.SUBSELECT)
去除(n+1)个查询问题。
关于Spring-data-jpa 渴望获取加入和使用分页不起作用,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/26901010/