Hibernate分页机制

Question

我试图使用Hibernate分页查询(PostgreSQL)

我为我的sql查询设置了setFirstResult(0),setMaxResults(20).我的代码如下:

Session session = getSessionFactory().getCurrentSession();
session.beginTransaction();
Query query = session.createQuery("FROM Customers");
query.setFirstResult(0);
query.setMaxResults(20);
List<T> entities = query.list();
session.getTransaction().commit();

但是在查看SQL hibernate日志时,我仍然看到完整的SQL查询:

Hibernate: select customer0_.id as id9_, customer0_.customer_name as dst2_9_, customer0_.addres as dst3_9_ from tbl_customers customer0_  

为什么查询Hibernate分页SQL日志时没有LIMIT OFFSET？

有谁知道Hibernate分页机制？

我猜Hibernate将选择所有数据,将数据放入Resultset,然后在Resultset中进行分页,对吧？

Answer 1

我在查询和休眠回叫中使用。两者都按预期工作。Hibernate Query执行的结果介于给定的First和Max大小之间。在这里，好像您传递了SQL而不是HQL进行查询。如果是的话，那应该行不通。

-在这里查看我的代码。

        Query query = this.getSession().createQuery("FROM QueryType");
        query.setFirstResult(0);
        query.setMaxResults(20);
        List toDelete = query.list();

并在日志中：

select * from（从MY_TBL_NAME querytype0_选择（选择-所有列名称。（不想在此处共享。））rownum <=？

Answer 2

有很多方法可以分页.

HQL和setFirstResult,setMaxResults API

Session session = sessionFactory.openSession();
Query query = session.createQuery("From Foo");
query.setFirstResult(0);
query.setMaxResults(10);
List<Foo> fooList = query.list();
//Total count
String countQ = "Select count (f.id) from Foo f";
Query countQuery = session.createQuery(countQ);
Long countResults = (Long) countQuery.uniqueResult();
//Last Page
int pageSize = 10;
int lastPageNumber = (int) ((countResult / pageSize) + 1);

HQL和ScrollableResults API

String hql = "FROM Foo f order by f.name";
Query query = session.createQuery(hql);
int pageSize = 10;

ScrollableResults resultScroll = query.scroll(ScrollMode.FORWARD_ONLY);
resultScroll.first();
resultScroll.scroll(0);
List<Foo> fooPage = Lists.newArrayList();
int i = 0;
while (pageSize > i++) {
    fooPage.add((Foo) resultScroll.get(0));
    if (!resultScroll.next())
        break;
}
//Total count
resultScroll.last();
int totalResults = resultScroll.getRowNumber() + 1;

只需Criteria API

Criteria criteria = session.createCriteria(Foo.class);
criteria.setFirstResult(0);
criteria.setMaxResults(pageSize);
List<Foo> firstPage = criteria.list();
//Total count
Criteria criteriaCount = session.createCriteria(Foo.class);
criteriaCount.setProjection(Projections.rowCount());
Long count = (Long) criteriaCount.uniqueResult();

baeldung用例子列出了所有这些.

Answer 3

您可以将 JPA 分页用于实体查询和本机 SQL。

为了限制底层查询的ResultSet大小，JPAQuery接口提供了setMaxResults方法.

导航下一页需要将结果集定位在上一页结束的位置。为此，JPAQuery接口提供了setFirstResult方法。

JPQL

List<Post> posts = entityManager.createQuery("""
    select p
    from Post p
    order by p.createdOn
    """, Post.class)
.setFirstResult(10)
.setMaxResults(10)
.getResultList();

DTO 投影查询

JPA 查询分页不限于仅返回实体的实体查询。您也可以将它用于 DTO 投影。

List<PostCommentSummary> summaries = entityManager.createQuery("""
    select new
       com.vladmihalcea.book.hpjp.hibernate.fetching.PostCommentSummary(
           p.id, p.title, c.review
       )
    from PostComment c
    join c.post p
    order by c.createdOn
    """)
.setMaxResults(10)
.getResultList();

本机 SQL 查询

JPA 查询分页不限于实体查询，例如 JPQL 或 Criteria API。您也可以将它用于本机 SQL 查询。

List<Post> posts = entityManager.createQuery("""
    select p
    from Post p
    left join fetch p.comments
    where p.title like :titlePattern
    order by p.createdOn
    """, Post.class)
.setParameter("titlePattern", "High-Performance Java Persistence %")
.setMaxResults(5)
.getResultList();

JOIN FETCH 和分页

但是，如果我们尝试JOIN FETCH在实体查询中使用子句，同时也使用 JPA 分页：

List<Post> posts = entityManager.createQuery("""
    select p
    from Post p
    left join fetch p.comments
    where p.title like :titlePattern
    order by p.createdOn
    """, Post.class)
.setParameter("titlePattern", "High-Performance Java Persistence %")
.setMaxResults(5)
.getResultList();

Hibernate 将发出以下警告消息：

HHH000104: firstResult/maxResults specified with collection fetch; applying in memory!

并且执行的 SQL 查询将缺少分页子句：

SELECT p.id AS id1_0_0_,
       c.id AS id1_1_1_,
       p.created_on AS created_2_0_0_,
       p.title AS title3_0_0_,
       c.created_on AS created_2_1_1_,
       c.post_id AS post_id4_1_1_,
       c.review AS review3_1_1_,
       c.post_id AS post_id4_1_0__,
       c.id AS id1_1_0__
FROM post p
LEFT OUTER JOIN post_comment c ON p.id=c.post_id
WHERE p.title LIKE :titlePattern
ORDER BY p.created_on

这是因为 Hibernate 想要按照JOIN FETCH子句的指示完全获取实体及其集合，而 SQL 级别的分页可能会截断ResultSet可能留下集合中Post元素较少的父实体comments。

HHH000104警告的问题在于 Hibernate 将获取Post和PostComment实体的乘积，并且由于结果集的大小，查询响应时间将很重要。

为了解决此限制，您必须使用窗口函数查询：

@NamedNativeQuery(
    name = "PostWithCommentByRank",
    query = """
        SELECT *
        FROM (
            SELECT
                *,
                DENSE_RANK() OVER (
                    ORDER BY "p.created_on", "p.id"
                ) rank
            FROM (
                SELECT
                    p.id AS "p.id", p.created_on AS "p.created_on",
                    p.title AS "p.title", pc.post_id AS "pc.post_id",
                    pc.id as "pc.id", pc.created_on AS "pc.created_on",
                    pc.review AS "pc.review"
                FROM  post p
                LEFT JOIN post_comment pc ON p.id = pc.post_id
                WHERE p.title LIKE :titlePattern
                ORDER BY p.created_on
            ) p_pc
        ) p_pc_r
        WHERE p_pc_r.rank <= :rank
        """,
    resultSetMapping = "PostWithCommentByRankMapping"
)
@SqlResultSetMapping(
    name = "PostWithCommentByRankMapping",
    entities = {
        @EntityResult(
            entityClass = Post.class,
            fields = {
                @FieldResult(name = "id", column = "p.id"),
                @FieldResult(name = "createdOn", column = "p.created_on"),
                @FieldResult(name = "title", column = "p.title"),
            }
        ),
        @EntityResult(
            entityClass = PostComment.class,
            fields = {
                @FieldResult(name = "id", column = "pc.id"),
                @FieldResult(name = "createdOn", column = "pc.created_on"),
                @FieldResult(name = "review", column = "pc.review"),
                @FieldResult(name = "post", column = "pc.post_id"),
            }
        )
    }
)

有关使用窗口函数解决HHH000104问题的更多详细信息以及DistinctPostResultTransformer.