如何避免出现警告"firstResult/maxResults specified with collection fetch；applying in memory!“什么时候使用Hibernate？
我在服务器日志"firstResult/maxResults specified with collection fetch；applying in memory!"中收到警告。但是，一切都运行正常。但我不想要这个警告。
我的代码是
public employee find(int id) {
return (employee) getEntityManager().createQuery(QUERY).setParameter("id", id).getSingleResult();
}我的问题是
QUERY = "from employee as emp left join fetch emp.salary left join fetch emp.department where emp.id = :id"回答 7
Stack Overflow用户
发布于 2012-07-11 19:45:04
出现此警告的原因是，当使用fetch join时，结果集中的order仅按所选实体的ID定义(而不是按join fetched)定义。
如果内存中的这种排序导致了问题，请不要将firsResult/maxResults用于JOIN FETCH。
Stack Overflow用户
发布于 2017-09-13 18:54:49
尽管您获得了有效的结果，但SQL查询会获取所有数据，并且效率并不像它应该的那样高。
因此，您有两个选择。
修复了可以以读写模式获取实体的两个SQL查询的问题
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所提供的过滤条件匹配的根实体标识符。。第二个查询将使用先前提取的根实体标识符来获取父实体和子实体。
这种方法非常容易实现，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();修复一个只能在只读模式下获取实体的SQL查询的问题
第二种方法是对符合过滤条件的父实体和子实体的结果集使用SDENSE_RANK，并仅限制前N个post条目的输出。
SQL查询可能如下所示：
@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获取与所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连接记录分配排名，以便我们以后可以只过滤我们感兴趣的Post记录的数量。
SqlResultSetMapping提供了SQL级列别名和需要填充的JPA实体属性之间的映射。
现在，我们可以像这样执行PostWithCommentByRank @NamedNativeQuery：
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，在缺省情况下，像PostWithCommentByRank查询这样的原生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Object[]。
但是，我们希望将表格Object[]数组转换为父子实体树，因此，我们需要使用Hibernate ResultTransformer。
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());
}
}DistinctPostResultTransformer必须分离正在获取的实体，因为我们正在重写子集合，而我们不希望将其作为实体状态转换传播：
post.setComments(new ArrayList<>());Stack Overflow用户
发布于 2012-08-08 20:45:59
要避免此警告，必须将call getSingleResult更改为getResultList().get(0)
https://stackoverflow.com/questions/11431670
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