如何改善休眠查询？

Question

I have two classes which named A and B. The primary key of B is a field of tab_A. The mapping xml is like:

<hibernate-mapping>
    <class name="com.fabulous.A" table="tab_A">
        <id name="id" column="id">
            <generator class="assigned" />
        </id>
        <property name="userId" column="user_id" />

        <many-to-one name="B" column="B_id"
            class="com.fabulous.B" unique="true" lazy= "false" cascade="merge">
        </many-to-one>
    </class>
</hibernate-mapping>

And there is another mapping xml which mapping class B to table B. The query in my application is:

session.createQuery("FROM A WHERE id IN (:ids)").setParameterList("ids", ids).list();

The parameter is a list of A.id. The size of list is 100;

When lazy=true the query time is about 1s , but about 1 minute will be cost when Ab is applied. Actually B will always be required, so I close lazy loading. And the query time is about 40s.

From hibernate log, hibernate load B from table B one by one but not in a batch. I think if I extract all B.id from table A in a query and then query table B within a batch the total time will be less than 3s.

But it's not convenience, could anyone help to figure out how to improve the query with hibernate?

Thanks a lot.

Answer 1

要急切地获取查询的某些关系，只要该关系是可选的，只需执行简单的JOIN FETCH或LEFT JOIN FETCH

FROM A a JOIN FETCH a.B WHERE a.id IN (:ids)