A fetch group is a performance enhancement that allows a group of attributes of an object to be loaded on demand, which means that the data for an attribute might not loaded from the underlying data source until an explicit access call for the attribute first occurs. It avoids the wasteful practice of loading up all data of the object's attributes, in which the user is interested in only partial of them. A great deal of caution and careful system use case analysis should be use when using the fetch group feature, as the extra round-trip would well offset the gain from the deferred loading in many cases. EclipseLink fetch group support is twofold: the pre-defined fetch groups at the Entity or MappedSuperclass level; and dynamic (use case) fetch groups at the query level. Fetch groups can only be used when weaving is enabled or when individual classes that define them explicitly implement the org.eclipse.persistence.queries.FetchGroupTracker interface.