how the display-entity work?

[guo weizhan]

in the ListContent section of ContentForms.xml, why the length of
contentName is more than 20 will cause SQLSTATE: 22001 error in DB2?
 <field name="contentName" widget-style="tabletext">
            <display-entity entity-name="Content"
description="${contentName}" key-field-name="contentId">
                <sub-hyperlink target="editContent?contentId=${contentId}"
description="[${contentId}]" link-style="buttontext"/>
            </display-entity>
 </field>