如何避免出现警告“使用集合获取指定的firstResult/maxResults；在内存中应用！”使用Hibernate时？

Question

我在服务器日志中收到一个警告“firstResult/max结果指定集合获取；在内存中应用！”。然而一切正常。但我不想要这个警告。

public employee find(int id) {
    return (employee) getEntityManager().createQuery(QUERY).setParameter("id", id).getSingleResult();
}

QUERY = "from employee as emp left join fetch emp.salary left join fetch emp.department where emp.id = :id"

Answer 1

要避免此警告，必须将调用getSingleResult更改为getResultList（）。获取（0）

Answer 2

此警告的原因是，当使用获取连接时，结果集中的顺序仅由所选实体的ID定义（而不是由获取的连接定义）。

如果内存中的这种排序导致问题，请不要将firsResult/maxResults与JOIN FETCH一起使用。

Answer 3

虽然您得到了有效的结果，但SQL查询获取了所有数据，但它并没有达到应有的效率。

所以，你有两个选择。

解决此问题的最简单方法是执行两个查询：

.第一个查询将获取与提供的过滤条件匹配的根实体标识符。.第二个查询将使用之前提取的根实体标识符来获取父实体和子实体。

此方法非常容易实现，如下所示：

List<Long> postIds = entityManager
.createQuery(
    "select p.id " +
    "from Post p " +
    "where p.title like :titlePattern " +
    "order by p.createdOn", Long.class)
.setParameter(
    "titlePattern",
    "High-Performance Java Persistence %"
)
.setMaxResults(5)
.getResultList();
 
List<Post> posts = entityManager
.createQuery(
    "select distinct p " +
    "from Post p " +
    "left join fetch p.comments " +
    "where p.id in (:postIds) "  +
    "order by p.createdOn", Post.class)
.setParameter("postIds", postIds)
.setHint(
    "hibernate.query.passDistinctThrough", 
    false
)
.getResultList();

第二种方法是对符合筛选条件的父实体和子实体的结果集使用SDENSE\u RANK，并仅限制前N个post条目的输出。

SQL查询可以如下所示：

@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.id as \"pc.id\", " +
        "               pc.created_on AS \"pc.created_on\", " +
        "               pc.review AS \"pc.review\", " +
        "               pc.post_id AS \"pc.post_id\" " +
        "        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"),
            }
        )
    }
)

@NamedNativeQuery获取与提供的标题匹配的所有Post实体及其关联的PostComment子实体。DENSE_RANK窗口函数用于为每个Post和PostComment加入的记录分配排名，以便我们以后可以仅过滤我们有兴趣获取的Post记录的数量。

SqlResultSetMap提供SQL级列别名和需要填充的JPA实体属性之间的映射。

现在，我们可以按如下方式执行PostWithCommentByRank

List<Post> posts = entityManager
.createNamedQuery("PostWithCommentByRank")
.setParameter(
    "titlePattern",
    "High-Performance Java Persistence %"
)
.setParameter(
    "rank",
    5
)
.unwrap(NativeQuery.class)
.setResultTransformer(
    new DistinctPostResultTransformer(entityManager)
)
.getResultList();

现在，默认情况下，像PostWithCommentByRank这样的本机SQL查询将获取同一JDBC行中的Post和PostComment，因此我们最终将得到一个包含这两个实体的对象[]。

但是，我们想将表格Object[]数组转换为父子实体的树，因此，我们需要使用HibernateResultTransex。

DistinctPostResultTransformer如下所示：

public class DistinctPostResultTransformer
        extends BasicTransformerAdapter {
 
    private final EntityManager entityManager;
 
    public DistinctPostResultTransformer(
            EntityManager entityManager) {
        this.entityManager = entityManager;
    }
 
    @Override
    public List transformList(
            List list) {
             
        Map<Serializable, Identifiable> identifiableMap =
            new LinkedHashMap<>(list.size());
             
        for (Object entityArray : list) {
            if (Object[].class.isAssignableFrom(entityArray.getClass())) {
                Post post = null;
                PostComment comment = null;
 
                Object[] tuples = (Object[]) entityArray;
 
                for (Object tuple : tuples) {
                    if(tuple instanceof Identifiable) {
                        entityManager.detach(tuple);
 
                        if (tuple instanceof Post) {
                            post = (Post) tuple;
                        }
                        else if (tuple instanceof PostComment) {
                            comment = (PostComment) tuple;
                        }
                        else {
                            throw new UnsupportedOperationException(
                                "Tuple " + tuple.getClass() + " is not supported!"
                            );
                        }
                    }
                }
 
                if (post != null) {
                    if (!identifiableMap.containsKey(post.getId())) {
                        identifiableMap.put(post.getId(), post);
                        post.setComments(new ArrayList<>());
                    }
                    if (comment != null) {
                        post.addComment(comment);
                    }
                }
            }
        }
        return new ArrayList<>(identifiableMap.values());
    }
}

差异后结果转换器必须分离正在获取的实体，因为我们正在覆盖子集合并且我们不希望它作为实体状态转换传播：

post.setComments(new ArrayList<>());