How can I reduce the index size of a hibernate collection (set with composite-element)?

I need to reduce index-sizes because of mssql limit of 900 bytes.

I have a class which has a collection declared as a set. Because of this, the primary key consists of all notnull columns including the foreign key. An index is created out of this primary key. I do not need the index to be over all these columns.

Is there a way to reduce the index size without changing the whole setup of the data structure?

Here is the current configuration of the collection inside of the surrounding class definition:

  <set cascade="save-update,persist,merge,refresh,replicate,evict,delete,delete-orphan" fetch="select" lazy="true" table="mySubsetTable" batch-size="1000" name="attributes">
    <key foreign-key="FK_Mothertable">
      <column name="number"/>
      <column name="data"/>
    </key>
    <composite-element class="MySubsetElement">
      <property name="type" length="200" not-null="true" type="class"/>
      <property name="attribute" length="2000" column="attrValue" not-null="false"/>
      <property name="myboolean" type="boolean">
        <column name="myboolean"/>
      </property>
      <property name="anotherAttribute" length="200"/>
      <property name="evenAnotherAttribute" length="200" not-null="true"/>
      <property name="evenOneMoreAttribute" not-null="true">
        <type name="SomeClass">
          <param name="enumClass">someEnumClass</param>
        </type>
      </property>
    </composite-element>
  </set>

I am currently using hibernate 3.3.1 with xdoclet annotations:

  /**
   * Attributes of this matchable
   * 
   * @hibernate.set table="mySubsetTable" cascade="save-update,persist,merge,refresh,replicate,evict,delete,delete-orphan" lazy="true"
   *                batch-size="1000" fetch="select"
   * @hibernate.key foreign-key="FK_Mothertable"
   * @hibernate.key-column name="number"
   * @hibernate.key-column name="data"
   * @hibernate.composite-element class="MySubsetElement"
   */
   public Set<MySubsetElement> getSubsetElements() { ... }

Thanks a lot for your suggestions!

(And please do not refer me to http://docs.jboss.org/hibernate/ I already found this.)

EDIT I cannot reduce the size of all properties to fit the size limits. An index consisting of the foreign keys would suffice. Also I would really like a solution which does not change the underlying datastructure as I am working on a product which is already in use.

here is how I realized Jimmy's suggestion:

<hibernate-mapping>
    <class name="MyParent" ....>
      ...
      <set cascade="save-update,persist,merge,refresh,replicate,evict,delete,delete-orphan" fetch="select" lazy="true" table="SubsetTable" batch-size="1000" name="attributes">
        <key foreign-key="FK_ParentTable" not-null="true">
          <column name="number"/>
          <column name="data"/>
        </key>
        <one-to-many class="MySubset" entity-name="MySubsetentity"/>
      </set>
      ...
    </class>

    <class name="MySubset" ....>
          <id name="id" type="long">
            <column name="id"/>
            <generator class="MyIdGeneratorClass">
              <param name="sequence">mySequence</param>
            </generator>
          </id>
          <property name="type" length="200" not-null="true" type="class"/>
          <property name="attribute" length="2000" column="attrValue" not-null="false"/>
          <property name="myboolean" type="boolean">
            <column name="myboolean"/>
          </property>
          <property name="anotherAttribute" length="200"/>
          <property name="evenAnotherAttribute" length="200" not-null="true"/>
          <property name="evenOneMoreAttribute" not-null="true">
            <type name="SomeClass">
              <param name="enumClass">someEnumClass</param>
            </type>
          </property>   
    </class>
</hibernate-mapping>

The important part is the not-null="true" inside the key tag of the Parent subset definition. This enables the Subset to remain ignorant of the parent.