Hibernate:复合元素导致每次提交时delete+insert

<hibernate-mapping package="trx.domain">
    <class name="Master" table="master" dynamic-update="true" dynamic-insert="true">

        <set name="attributes" table="attribute" lazy="true"
            cascade="all" batch-size="10">
            <cache usage="nonstrict-read-write" />
            <key>
                <column name="master_id" />
            </key>
            <composite-element class="Attribute">
                <many-to-one name="type" class="AttributeType"
                     not-null="true" column="attribute_type_id" lazy="false" />
                <property name="value">
                    <column name="value" />
                </property>
            </composite-element>
        </set>
    </class>
</hibernate-mapping>

如果我只是扫描 attributes 在没有任何更新的情况下,Hibernate仍然会执行一批 delete 和 insert

Hibernate: delete from attribute where master_id=? and attribute_type_id=?
Hibernate: delete from attribute where master_id=? and attribute_type_id=?
Hibernate: insert into attribute (master_id, attribute_type_id, value) values (?, ?, ?)
Hibernate: insert into attribute (master_id, attribute_type_id, value) values (?, ?, ?)

为什么会这样?如何预防?