Question

我有一个用户和一个消息表。 User to Message是一对多关系，Message to User是多对一关系。我已将多对一中的一个标记为fetch join。当我得到一条消息时，Hibernate会运行一个连接查询，但是当我获取所有消息时，Hibernate会运行select查询而不是连接。可能是什么原因？以下是详细信息：

他们之间的关系：

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>

当我获取单个消息对象时，Hibernate按预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，当我一次性获取所有消息时，Hibernate会运行选择查询：

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

对于HQL或Criteria查询，Hibernate看起来并不总是使用映射中定义的获取策略。它们通常用于获取/加载。在此处找到了参考：https://forum.hibernate.org/viewtopic.php?f=1&t=957561

Answer 2

我不确定但这可能是原因。如果运行单个查询，即连接或子查询。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（在您的情况下是多个消息），可能会引发性能问题。我肯定会选择简单的选择查询而不是连接/子查询。如果我们考虑性能，这肯定是有道理的。这就是Hibernate所做的。

Hibernate'加入'获取奇怪的行为

2 个答案: