Hibernate为@ManyToOne JPA带注释的属性创建N + 1个查询

问题描述

我有这些课程:

@Entity
public class Invoice implements Serializable {
    @Id
    @Basic(optional = false)
    private Integer number;

    private BigDecimal value;

    //Getters and setters
}

@Entity
public class InvoiceItem implements Serializable {
    @EmbeddedId
    protected InvoiceItemPK invoiceItemPk;

    @ManyToOne
    @JoinColumn(name = "invoice_number", insertable = false, updatable = false)
    private Invoice invoice;

    //Getters and setters
}

当我运行此查询时:

session.createQuery("select i from InvoiceItem i").list();

它执行一个查询以从InvoiceItem中选择记录，如果我有10000个发票项目，它会生成10000个附加查询以从每个InvoiceItem中选择发票.

It executes one query to select the records from InvoiceItem, and if I have 10000 invoice items, it generates 10000 additional queries to select the Invoice from each InvoiceItem.

我认为，如果所有记录都可以在单个sql中提取，那就更好了.实际上，我发现为什么它不是默认行为很奇怪.

I think it would be a lot better if all the records could be fetched in a single sql. Actually, I find it weird why it is not the default behavior.

那我该怎么办?

推荐答案

尝试

session.createQuery("select i from InvoiceItem i join fetch i.invoice inv").list();

它应该使用联接在单个SQL查询中获取所有数据.

It should get all the data in a single SQL query by using joins.

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