休眠'加入'获取奇怪的行为

Question

我有一个用户和一个消息表。用户到消息是一对多的关系，消息到用户是多对一的关系。我已将多对一中的一个标记为提取连接。当我得到'单个消息时，Hibernate运行一个连接查询，但是当我获取所有消息时，Hibernate运行select查询而不是连接。可能是什么原因？以下是详细内容：它们之间

关系：

用户

<set name="messagesForFromUserUid" lazy="true" table="message" inverse="true" cascade="save-update"> 
     <key> 
      <column name="from_user_uid" not-null="true" /> 
     </key> 
     <one-to-many class="repository.Message" /> 
    </set> 
    <set name="messagesForToUserUid" lazy="true" table="message" fetch="select"> 
     <key> 
      <column name="to_user_uid" not-null="true" /> 
     </key> 
     <one-to-many class="repository.Message" /> 
    </set> 

消息

<many-to-one name="userByFromUserUid" class="repository.User" fetch="join" lazy="false"> 
     <column name="from_user_uid" not-null="true" /> 
    </many-to-one> 
    <many-to-one name="userByToUserUid" class="repository.User" fetch="select" lazy="proxy"> 
     <column name="to_user_uid" not-null="true" /> 
    </many-to-one> 

当我取一个单一的消息对象时，Hibernate会如期运行一个连接查询：

Message m = (Message) s.get(Message.class, 2); 

Hibernate: 
select 
    message0_.message_uid as message1_1_1_, 
    message0_.from_user_uid as from2_1_1_, 
    message0_.to_user_uid as to3_1_1_, 
    message0_.message_text as message4_1_1_, 
    message0_.created_dt as created5_1_1_, 
    user1_.user_uid as user1_0_0_, 
    user1_.user_name as user2_0_0_, 
    user1_.user_password as user3_0_0_, 
    user1_.email as email0_0_, 
    user1_.first_name as first5_0_0_, 
    user1_.last_name as last6_0_0_, 
    user1_.created_dt as created7_0_0_ 
from 
    hello.message message0_ 
inner join 
    hello.user user1_ 
     on message0_.from_user_uid=user1_.user_uid 
where 
    message0_.message_uid=? 

但是，当我在一个拿来的全部信息，Hibernate的运行SELECT查询来代替：

List<Message> l = s.createQuery("from Message").list(); 

Hibernate: 
select 
    message0_.message_uid as message1_1_, 
    message0_.from_user_uid as from2_1_, 
    message0_.to_user_uid as to3_1_, 
    message0_.message_text as message4_1_, 
    message0_.created_dt as created5_1_ 
from 
    hello.message message0_ 
Hibernate: 
select 
    user0_.user_uid as user1_0_0_, 
    user0_.user_name as user2_0_0_, 
    user0_.user_password as user3_0_0_, 
    user0_.email as email0_0_, 
    user0_.first_name as first5_0_0_, 
    user0_.last_name as last6_0_0_, 
    user0_.created_dt as created7_0_0_ 
from 
    hello.user user0_ 
where 
    user0_.user_uid=? 

Hibernate: 
select 
    user0_.user_uid as user1_0_0_, 
    user0_.user_name as user2_0_0_, 
    user0_.user_password as user3_0_0_, 
    user0_.email as email0_0_, 
    user0_.first_name as first5_0_0_, 
    user0_.last_name as last6_0_0_, 
    user0_.created_dt as created7_0_0_ 
from 
    hello.user user0_ 
where 
    user0_.user_uid=? 

Answer 1

看起来好像Hibernate并不总是使用在映射中为HQL或Criteria查询定义的获取策略。它们通常用于获取/加载。在这里找到一个参考：https://forum.hibernate.org/viewtopic.php?f=1&t=957561

Answer 2

我不知道，但是这可能是原因。如果您运行单个查询即联接或子查询。没关系，单个查询可能没有任何性能差异。但是，如果您正在运行多个查询（在您的情况下是多条消息），则可能会引发性能问题。我肯定会选择简单的select查询，而不是join/subqueries。如果我们考虑性能，这绝对有意义。这就是Hibernate所做的。